The first thing to note is that there are two distinct serial interfaces for the
PICAXE - The program Download Interface, and serial interfaces under the
control of the user's program.Understanding the Download interface, described in detail below, will make
understanding User Controlled Serial Interfacing much easier.
This article is based upon information provided by Revolution Education Ltd, the
manufacturer of the PICAXE, and from practical experience using PICAXE devices.
AXE025 |
CON038 |
AXE026 |
CON039 |
ADA010 |
Index
The Download Interface
The program Download Interface is used to download programs for the
PICAXE using the serial interface. There are two legs of all PICAXE's which are
generally dedicated to this function. Although some PICAXE's may allow at least
one of those pins to be used for other functions, we will ignore those
additional functions for now.The Download Interface uses two legs on the PICAXE which are labelled as "Serial
In" and "Serial Out" and ultimately connect to a PC's serial port. The Serial In
line receives data from the PC and Serial Out sends data to the PC; both are
required to achieve successful programming.
The legs used for Serial In and Serial Out depend on the PICAXE variant used,
and are as follows ...
| Function |
08, 08M |
18, 18A, 18X |
28, 28A, 28X |
40X |
| Serial In
| 2
| 3
| 6
| 6
|
| Serial Out
| 7
| 2
| 7
| 7
|
PICAXE Pin-Outs
Pin-outs indeed, and the first issue to address is why this article refers to
"Legs" rather "Pins" as is more traditional when discussing connections to
integrated circuits ( IC's ).
The reason "Pin" has not been used is because the PICAXE documentation refers
to "Input Pins" and "Output Pins" which are numbered in a manner which can be
confused with the pin numbering of the actual device, and these are often
referred to simply as "Pins". To avoid confusion between whatever "Pin" may mean,
we will refer to the physical pins on the device as "Legs" in the rest of this
article.
The pin-outs ( or perhaps, to be consistent, leg-outs ) of the PICAXE variants
are as shown below. The legs used for Serial In and Serial Out have been
explicitly identified as "SI" and "SO" respectively, along with all other
connections which must be made to create a minimal, working PICAXE system. The
0V and +V legs must be connected to a power supply, the two XT legs must be
connected to a crystal or resonator, and RST is the active low Reset line which
should be wired via a 4K7 resistor to +V and can be activated by shorting the
leg to 0V via a push button.
08, 08M 18, 18A, 18X 28, 28A, 28X 40X
.----..----. .----..----. .----..----. .----..----.
-| +V 0V |- -| |- -| RST |- -| RST |-
2 -| SI SO |- 7 2 -| SO |- -| |- -| |-
-| |- 3 -| SI |- -| |- -| |-
-| |- -| RST |- -| |- -| |-
`----------' -| 0V +V |- -| |- -| |-
8-Pin -| |- 6 -| SI |- 6 -| SI |-
-| |- 7 -| SO |- 7 -| SO |-
-| |- -| 0V |- -| |-
-| |- -| XT +V |- -| +V |-
`----------' -| XT 0V |- -| 0V |-
18-Pin -| |- -| +V |-
-| |- -| 0V |-
-| |- -| XT |-
-| |- -| XT |-
`----------' -| |-
28-Pin = =
-| |-
`----------'
40-Pin
The Download Cable
The following diagram shows how to connect the Download interface to a PC which
has a 9-way serial port connector, through a 3-way Molex header. Tech-Supplies
Ltd sells a "keyed" 3-way Molex Header, as part number CON038, which is ideal
for use
as it prevents an appropriately keyed Molex Plug being inserted into the header
incorrectly. Tech-Supplies Ltd, also supply a pre-built Download Cable, as part
number AXE025, which will connect to the "keyed" 3-way Molex Header wired as
below ...
.---. .---------------.
/ | .------. .----. | |
GND | O |--5------. .----| ===| 1 | O |-----------------------------| Serial Out |
| O | | | | .----| | || .------. | |
| O | .---|---|----| | ===| 2 | O ||----------.----| |------| Serial In |
| O | | | | | `----| | || | `------' | |
TX | O |--3--' `---|----| ===| 3 | O |---. _|_ 22K .--| 0v |
| O | | `------' `----' | | | | | |
RX | O |--2----------' | | | 10K | `---------------'
| O | | | | | PICAXE
| O | | `-.-' |
\ | 9-Way Female | | |
`---' Socket (DCE) --^------^---------------^-- 0v
Note that there are circumstances where plugging the Molex plug into the header
the wrong way round can lead to the PICAXE having its Serial Out line shorted
to 0v which may cause irreparable damage to the PICAXE chip. It is therefore
recommended that both "keyed" Molex plugs and headers are used to prevent this
from happening, and to take special care when using a Molex header or plug
which isn't keyed.
The "TX" and "RX" designations on the PC connector refers to the names used by
the PC serial interface. TX, "transmit" sends data from the PC and thus connects
to the PICAXE's Serial In through the resistor network, and RX, "receive"
correspondingly receives data into the PC and thus connects to the PICAXE's
Serial Out.
In some documentation and on PICAXE oriented web sites, you may hear of "Serial
Out" being referred to also as "TX" or Transmit and "Serial In" as "RX" or
"Receive" which can cause confusion as to which "TX" and "RX" being referred to.
Sometimes "Serial In" may be called "TX" because it connects to the "TX" from
the PC. This can cause further confusion because the Serial In line - implicitly
"receiving" is labelled as a "transmit" line used for sending. It is the same
when Serial Out is referred to as "RX". My recommendation is to refer to "TX
to the PC", "TX to the PICAXE", "RX from the PC" and "RX from the PICAXE" when
there is any likely confusion.
If you want to use a 25-way Serial Port connection instead of a 9-way connector,
you can either use a 9-to-25 converter ( ADA010 ) or use the wiring as shown
below. Note that the functions of pins 2 and 3 on the 25-way connector are different to
those on the 9-way connector.
.---.
/ |
| O |
= =
GND | O |--7------. .---------------.
| O | | .------. .----. | |
| O | | .----| ===| 1 | O |-----------------------------| Serial Out |
| O | | | | .----| | || .------. | |
| O | .---|---|----| | ===| 2 | O ||----------.----| |------| Serial In |
| O | | | | | `----| | || | `------' | |
| O | | `---|----| ===| 3 | O |---. _|_ 22K .--| 0v |
| O | | | `------' `----' | | | | | |
RX | O |--3--|-------' | | | 10K | `---------------'
| O | | | | | | PICAXE
TX | O |--2--' | `-.-' |
| O | | | |
| O | --^------^---------------^-- 0v
\ | 25-Way Female
`---' Socket (DCE)
DCE or DTE ?
The definition of DCE ( Data Communication Equipment ) and DTE ( Data Terminal
Equipment ) is fairly complex, but, at the end of the day, boils down to the
type of connector used and which pins of that connector are used for the serial
input and output lines.
The two circuits above show DCE connections, using female sockets, which allows
a PC to be connected using a straight-through ( or 'one-to-one' ) cable with a
male plug on one end and a female plug on the other. The Pre-built download
connector cable is a DCE oriented connection.
If you wish to use male sockets ( DTE ) instead of female sockets ( DCE ), you
will have to use a cable with female plugs at each end, the cable will have to
be, what is known as, a 'crossover cable', and the sockets will have to be
wired up as below ...
.---.
/ |
| O |
= =
GND | O |--7----------.
| O | |
.---. | O | |
/ | | O | | .------.
| O |--5----------. | O | .---|----| ===| 1
| O | | .------. | O | | | | .----|
| O | .-------|----| ===| 1 | O | .---|---|----| | ===| 3
| O | | | | .----| | O | | | | | `----|
TX | O |--3--' .---|----| | ===| 2 RX | O |--3--' | `----| ===| 3
| O | | | | `----| | O | | `------'
RX | O |--2------' `----| ====| 3 TX | O |--2------'
| O | `------' | O |
| O | | O |
\ | 9-Way Male \ | 25-Way Male
`---' Socket (DTE) `---' Socket (DTE)
Note that the connections on the Molex now go to different pins on the
connectors but that the "TX" and "RX" designations have not changed; this is
what can make connecting serial devices so difficult, as was described
earlier - For DTE to DCE, TX goes to TX, RX goes to RX, while in DTE to DTE,
TX goes to RX, and RX goes to TX. Note that the functions of pins 2 and 3
continue to differ for 9-way and 25-way connections.
Alternative Inter-Connections
The Molex connection can be left out entirely if desired, with the connections
going directly to a suitable 9 or 25-way socket, or the Molex connection can be
replaced by using 3.5mm Jack Plugs and Sockets with the following wiring ...
1/S 2/R 3/T .---------------.
.----------. 1/S 2/R 3/T .--------------------. | |
| |---------.-----.-----. .-| .--. .--. .--. | .---------| Serial Out |
| | | | \ | | | | | | | | | | | |
| | | | / | | | | | | | | | | .------| Serial In |
| |---------^-----^-----' `-| | | | | | | | | | | |
`----------' | | | `--| |--| |--| |--' | | .---| 0v |
| | | `--' `--' `--' | | | | |
PC RX ----------' | | | | | | | | `---------------'
| | `-----|-----|--------' | | PICAXE
PC TX -----------------' | | | .-----. | |
| }-----|--| |--' |
GND --------------------------' _|_ | `-----' |
| | | 22K |
10K | | | |
|_ _| | |
| | |
--^-----^--------------^-- 0v
Tech-Supplies Ltd sell a PCB mount 3.5mm Jack Socket, as part number CON039,
which can be wired as in the circuit below. Note that the Jack Socket is shown
as being viewed from above, that is the pins of the socket shown, will be
underneath the socket when mounted on a PCB or veroboard.
.---------------.
Viewed from Above | |
.--------------------| Serial Out |
.--------------. | .-----. | |
.-| + + O |--2/R--.---|---.---| |------| Serial In |
| | .--. O |--2/R--' | | `-----' | |
| | | | O |--1/S------' | 22K .---| 0v |
| | `--' O |--3/T--. _|_ | | |
`-| + + O |--3/T--{ | | | `---------------'
`--------------' | | | 10K | PICAXE
| | | |
| `-.-' |
| | |
--^-------^------------^-- 0v
Tech-Supplies Ltd supply a pre-built Download Cable, as part number AXE026,
which has a 3.5mm Jack Plug and will connect to a 3.5mm Jack Socket wired as
above.
The wiring to the tip, ring and sleeve of the Jack Plug conforms to that used
in the pre-built Download Cable.
Note that there are circumstances where plugging this Jack Plug into the Socket
can lead to the PICAXE having its Serial Out line shorted to 0v which may cause
irreparable damage to the PICAXE chip. Additionally, the TX line from the PC
can be shorted to the Serial Output from the PICAXE which may cause irreparable
damage to the PICAXE chip, the PC's serial port, or both.
It is therefore recommended that the plug is only inserted or removed when the
PICAXE power supply is turned off, or the Jack Plug should only be inserted or
removed when the cable is disconnected from the PC's serial port.
My personal recommendation is to use the "keyed" 3-way Molex header described
earlier, as this is much easier to install and is usually more accessible when
used on a PCB or in a veroboard design, and has none of the above risks;
providing that the Molex plug is connected to the header the correct way round.
The Download Interface can be left out of circuits where there will be no need
to download programs into the PICAXE, but to ensure correct operation of the
PICAXE, it is necessary to wire Serial In to 0v. My recommendation is to fit
the few components that are needed just in case it does become necessary to
download a later version of the program. This is particularly important if you
are soldering the PICAXE into place rather than using a socket.
The PICAXE-08 and PICAXE-08M Download Interface
The PICAXE-08 Download Interface is as described above, with the added
complication that Leg 7, Serial Out, can also be used as a user controlled pin
once the program download has been completed.
If Leg 7 is to be used as a user controlled pin, then it is best to take the
Serial Out line to a 3-way Molex so that a 2-pin Molex Link can be used to
route the Leg's output to either the PC serial interface or the electronics
it will control. Alternatively, a single pole, single throw ( SPST ) switch can
be used.
Always remember to check the Molex Link or switch setting when downloading, as
the program download will fail if the setting is wrong.
Download Cable Colour Coding Standards
It is recommended that the following colour coded wiring scheme is used when
wiring up your own Download Cables, as this conforms with the manufacturer's
standard colour scheme ...
| Colour |
Function |
Molex Pin |
Jack |
9-Way Pin |
25-Way Pin |
| Orange
| Serial Out to PC RX in
| 1
| Sleeve
| 2
| 3
|
| Red
| Serial In from PC TX out
| 2
| Ring
| 3
| 2
|
| Brown
| 0v / GND
| 3
| Tip
| 5
| 7
|
The Download Interface Resistor Network
At first glance, the 10K and 22K Resistor Network in the Serial In interface
looks wrong; surely, if it's a voltage divider to get 12V down to 5V the 10K
ought to be between the Serial In leg and 0V, not the PC end of the 22K ?
If it were a voltage divider, you would be correct. But it isn't.
Microchip PICmicro devices have clamping diodes built in which stop the voltages
applied to the pins going much above the power supply or much below 0V, and the
22K resistor is not there to drop the voltage at all, but to limit the amount
of current which will flow into the PICAXE leg.
There is a small element of voltage dividing going on between the 22K and the
internal resistance between the Serial In leg and 0V, but that is not the
primary purpose of the 22K resistor.
The 10K is there to ensure that the Serial In leg is pulled to 0V when the
PICAXE runs without a download cable connected. That is its only purpose.
When the PICAXE runs, it monitors the Serial In leg to look for programs being
downloaded, and if it were left floating, there is every possibility that it
would misinterpret what it sees on the Serial In leg and jump to its internal
program download routines rather than execute your previously downloaded
program. Pulling the Serial In line down to 0V with the 10K prevents this from
happening.
Were the 10K to be placed across the Serial In leg and 0V, then, in conjunction
with the 22K, it would act as a voltage divider, and when the circuit was used
with serial ports which only output +5V, as is the case with many Laptops, the
voltage would be dropped considerably, to a level which would not necessarily
be read correctly by the PICAXE, which would make program downloading
unreliable.
So, what looks to be a badly designed or wrongly drawn circuit at first glance,
turns out to be an ideal solution for most serial interface applications with
the PICAXE, and most PICmicro devices in general.
User Controlled Serial Interfacing
The User Controlled Serial Interface is very similar to the Download Interface
except that the legs used to receive and transmit serial data can be specified
by the user in their program.
The PICAXE-08 and 08M can use up to four of its five I/O lines for serial
input ( pins 1, 2, 3 or 4 ) or output ( pins 0, 1, 2 or 4 ), the
PICAXE-18, 18A and 18X, can support up to 5 serial inputs and 8 serial outputs,
while the PICAXE-28, 28A, 28X and 40X can support up to 8 serial inputs and up
to 8 serial outputs.
The PICAXE-08M, 18X, 28X and 40X can also support an additional serial output
line using the SERTXD command which will be dealt with later.
The Physical Interface
The nature of the PICAXE, which is actually a pre-programmed PICmicro device,
is such that serial interfacing can be accomplished, in most cases, using a
minimal number of resistors and without additional active electronics.
Such interfacing is suitable for use with most PC serial ports, and, if it
isn't, then it will not be possible to download programs into the device. In
my own experimentation, I have not found a single serial port which does not
work with this interface, and this has been tested on a number of desktop PC's,
battery powered Laptops, PDA's and other devices.
The main difference between
this interface and an industry standard RS232 one, is that the PICAXE will
only generate 0v and power supply level signals ( normally 5V ) whereas RS232
nominally has an output of -15V/+15V ( although -12V/+12V is usually used ).
This issue is discussed further in the "Effects of Power Supply Voltage"
section later.
The interface we will use allows any voltage between -15V and +15V to be sent
to the PICAXE without any problem.
If the serial port to which you connect a PICAXE will not work with this simple
interface, and requires full RS232 compatibility, then you will need to use one
of the MAX232-style interface chips to create a usable serial interface. Because
most users of the PICAXE will be interfacing to a PC's serial port, and full
RS232 compatibility adds considerable complexity to a project, this article
does not consider such interfacing, and will only deal with the simpler
passive interfacing.
This section deals only with the electrical connections required to create a
usable serial interface. Please see the PICAXE Basic Commands documentation for
full details of using the SEROUT, SERIN and SERTXD commands for serial
communications, and the later section in this article on "Baud Rate and
Over-Clocking".
It is recommended that the same "keyed" 3-way Molex header configuration is
used as is described in the "Serial Download Interface" earlier, as this allows
the same cables used for downloading to be used for User Controlled Serial
Interfacing. As with the Download Interface, the Molex connection can be
replaced with 3.5mm Jack Plugs and Sockets, or can be omitted entirely with
the connections being made directly to 9-way or 25-way PC connectors.
To simplify the circuit diagrams below, only the 9-way socket and Molex
connection examples are shown.
Serial Output - SEROUT
Serial output can sent from a PICAXE under user program control by using the
SEROUT command. The SEROUT command will allow the program to send serial data
out through any of the PICAXE legs that are defined as its Digital Output Pins;
for the PICAXE-08 this can be any of its five I/O pins, and for others, any of
the the Digital Outputs.
Only normally designated Digital Outputs can be used to send serial data; which
means that Port A and Port C on the PICAXE-28X, and Port A, Port C and Port E
on the PICAXE-40X, cannot be used to transmit serial data.
The circuit and connections necessary to send serial out using SEROUT are as
shown in the diagram below ...
.---. 180R .---------------.
/ | .------. .----. .------. | |
GND | O |--5------. .----| ===| 1 | O |-----------| |----------| Digital Out |
| O | | | | .----| | || `------' | |
| O | | | | | ===| 2 | O || | |
| O | | | | `----| | || | |
| O | `---|----| ===| 3 | O |---. .--| 0v |
| O | | `------' `----' | | | |
RX | O |--2----------' | | `---------------'
| O | | | PICAXE
| O | | |
\ | 9-Way Female --^----------------------^-- 0v
`---' Socket (DCE)
The astute will have noticed that this circuit is almost exactly the same as
that used to connect Serial Out in the Download Interface, with the addition of
the 180R resistor.
Those of you who are familiar with the PICmicro, which is what a PICAXE is at
its heart, will be wondering why the 180R needs to be included when the leg
used for User Controlled Serial Interfacing is electrically no different to
the Serial Out leg.
The answer is, that I don't know, but both circuits are those specified by the
manufacturer's data sheet. Because the legs used are electrically identical,
and so is the
serial port to which they can be connected, it would be expected that if the
180R is necessary here then it would also be necessary in the Download
Interface, and if it's not necessary there, then it needn't be a necessity
here.
Because the legs are electrically identical, those who are concerned over the
discrepancy should be able to fit a 180R resistor in the Serial Out line of
the Download Interface without causing any adverse effects.
Serial Input - SERIN
Serial input can be received by a PICAXE under user program control by using the
SERIN command. The SERIN command will allow the program to receive serial data
through any of the PICAXE legs that are defined as its Digital Input Pins;
for the PICAXE-08 and 08M this can be any of its five I/O pins, and for others,
any of the Digital Inputs.
Only normally designated Digital Inputs can be used to receive serial data;
which means that Port A and Port C on the PICAXE-28X, and Port A, Port C and
Port E on the PICAXE-40X, cannot be used to receive serial data.
The circuit and connections necessary to receive serial in using SERIN are as
shown in the diagram below ...
.---.
/ | .------. .----.
GND | O |--5------. | ===| 1 | O | .---------------.
| O | | | .----| | || .------. | |
| O | .---|--------| | ===| 2 | O ||----------.----| |------| Serial In |
| O | | | | `----| | || | `------' | |
TX | O |--3--' `--------| ===| 3 | O |---. _|_ 22K .--| 0v |
| O | `------' `----' | | | | | |
| O | | | | 10K | `---------------'
| O | | | | | PICAXE
| O | | `-.-' |
\ | 9-Way Female | | |
`---' Socket (DCE) --^------^---------------^-- 0v
Serial Input to a PICAXE-08
If you require Serial Input to a PICAXE-08 and want to use Leg 3 you need
to add a small signal diode, such as a 1N4148, between Leg 3 and +V, the power
supply to the PICAXE, as shown in the diagram below ..
--.-------.-- +V
| |
__|__ + |
1N4148 / \ | .---------------.
.----. / \ | | |
1 | O | --.-- `---| +V |
| || .------. | | |
2 | O ||----------.----| |------^--------3--| Digital In |
| || | `------' | |
3 | O |---. _|_ 22K .---| 0v |
`----' | | | | | |
| | | 10K | `---------------'
| | | | PICAXE-08
| `-.-' |
| | |
--^------^--------------------------^-- 0v
This diode is required because the electrical design of Leg 3 on the PICAXE-08
is different to its other legs, and different to legs on the other PICAXE
processors.
If you are using any leg other than Leg 3 on a PICAXE-08, or using a PICAXE
other than a PICAXE-08, this diode is not necessary.
Serial Input and Output
The above descriptions and circuits show how Serial Input and Serial Output can
be done separately, but in many cases, it will be desirable to have both Serial
Input and Serial Output connections between the PC and the PICAXE. This can be
achieved by simply combining the two circuits above together, as below ...
180R
.---. .------. .---------------.
/ | .------. .----. .----| |---. | |
GND | O |--5------. .----| ===| 1 | O |----------' `------' `--| Digital Out |
| O | | | | .----| | || .------. | |
| O | .---|---|----| | ===| 2 | O ||----------.----| |------| Digital In |
| O | | | | | `----| | || | `------' | |
TX | O |--3--' `---|----| ===| 3 | O |---. _|_ 22K .--| 0v |
| O | | `------' `----' | | | | | |
RX | O |--2----------' | | | 10K | `---------------'
| O | | | | | PICAXE
| O | | `-.-' |
\ | 9-Way Female | | |
`---' Socket (DCE) --^------^---------------^-- 0v
If you require Serial Input to a PICAXE-08 and want to use Leg 3 you need
to add a small signal diode, such as a 1N4148, between Leg 3 and +V, the power
supply to the PICAXE, as described earlier.
You may notice the remarkable similarity of this circuit diagram to that of the
Download Interface, and apart from the addition of the 180R resistor in the
PC's RX connection, and the use of legs other than Serial In and Serial Out,
they are effectively the same.
Serial Output - SERTXD
In addition to the User Controlled Serial Output which can be sent through any
of the Digital Outputs using the SEROUT command, the PICAXE-08M, 18X, 28X and
40X have a SERTXD command which allows serial data to be sent, under user
program control, through the Serial Out leg part of the Download Interface.
In order to receive the data sent using SERTXD, the PC serial Port should simply
be connected to the Download Interface.
Although data can be sent through the Serial Out leg, it is not possible to
receive serial data through the Serial In leg when the user program is
running; there is no SERRXD command, and it is recommended that data is not
sent to the Serial In leg while the PC serial port is connected to the
Download Interface.
Whereas the baud rate of the serial output transmitted through a Digital Output
can be chosen as required, the baud rate of tha transmitted through the Serial
Out leg using SERTXD is fixed at 4800 for 4MHz operation, 9600 for 8MHz
operation, and at 19200 for 16MHz operation.
Serial data transmitted through the Serial Out leg still uses the same one
start bit, 8 data bits and one stop bit as all other serial transmissions from
the PICAXE use.
Data Format
Although not part of the electronic interface, the data format of serial
communications is an important part of the protocol relating to serial
interfacing.
The PICAXE can transmit and receive at a variety of baud rates, and can transmit
and receive on entirely different baud rates, however, for PC communications, it
is almost a certainty that you will want to transmit and receive at the same
baud rate when connected to a single PC serial port.
The baud rate specifications used in the SERIN and SEROUT commands also specify
the polarity of the serial output; that is, in its most simplistic form,
whether the serial output line is normally kept at 0v or at the power supply
voltage when nothing is being sent. In order to communicate with the PC the
polarity of the serial output must be set to what the PC expects, and using the
interface we have described in this article it must be one of the
"N" ( Inverted Polarity - "Not" ) baud rates; N300, N600, N1200, N2400 or N4800.
These must be used in both the SERIN and SEROUT commands, rather than the
"T", "ON" or "OT" baud rates.
While the baud rate chosen can be selected by the user, the actual data format
cannot; serial communications is always performed using 8 data bits, no parity
bit and 1 stop bit, and your PC application must be configured to use the same
settings.
The PICAXE will be able to receive data from a serial port configured to send
1, 1.5 or 2 stop bits, but you may need to add delays after every SEROUT or
SERTXD command, and send every byte individually, for the PC to receive the data
correctly.
While the SEROUT command, which sends serial data from a Digital Output Pin,
can have the baud rate to use chosen and specified, the SERTXD command, which
sends serial data through the Serial Out leg used during program download, has
its baud rate fixed and cannot be changed except by running the PICAXE at a
higher speed than normal. Please see the later section on "Baud Rate and
Over-Clocking" for further details.
Handshaking
The PICAXE does not require "handshaking" on the Download Interface as this
is all handled by the Program Editor, however connection to serial ports for
User Controlled Serial may require handshaking; signalling between each
side of the interface that there is data ready to be transmitted, and
indicating that data can be sent.
This handshaking can be done either by hardware or in software. The PICAXE
supports neither; if you want to use handshaking then you must implement it
yourself. As this requires considerable knowledge of serial interfacing and
the associated protocols, this will not be dealt with in this article.
Most serial interfaces can be configured to use no handshaking, neither hardware
nor software ( commonly called "XON/XOFF" ), however some may still require the
hardware handshaking lines to be connected; the Amstrad NC100 Notepad computer
is a particular case in point.
In order to make a serial port usable with a PICAXE, it may be necessary to
make a link between the RTS ( Ready To Send ) and CTS ( Clear To Send ) lines
on the serial port connector. It may also be necessary in some cases to
connect the DTR ( Data Terminal Ready ) line to the DSR ( Data Set Ready ) and
DCD ( Data Carrier Detect ) lines.
The additional wiring which may be required on the serial port connectors is
given in the diagram below. The connections are the same whether female ( DCE )
or male ( DTE ) sockets are used ...
.------------------.
| |
| .---. |
| / | |
| | O | |
| = = |
| | O |<-8--{ DCD
DTR `--20--| O | |
.---. | O | |
/ | | O | |
| O | | O |<-6--' DSR
| O | | O |
| O |--4--. DTR | O |<-5--. CTS
CTS .--8->| O | | | O | |
| | O | | | O |--4--' RTS
RTS `--7--| O | | | O |
| O | | | O |
DSR .--6->| O | | | O |
| | O |<-1--{ DCD | O |
| \ | | 14 | O |
| `---' | | O | 1
| | \ |
`-----------------' `---'
Although it may not be necessary to include all the hardware handshaking lines
bove, it does no harm to include them anyway, and it is recommended that all
handshaking connections are made to allow the PICAXE to connect to all types
of serial interfaces.
Without handshaking between the PICAXE and the other device, each may send data
whenever it wants to, regardless of whether the receiver is ready for it or
not. It is therefore important that both sides are ready to receive data when
it is sent, or the sent data will either be missed or incorrect data will be
received.
This is particularly important for the PICAXE which is a relatively slow device,
and may not be ready to receive more data until it has finished processing the
data it last received. Data bytes sent "back-to-back", with no delays between
the data sent, will prove particularly problematic for the PICAXE, especially
at higher baud rates.
It is usually possible to configure a PC's serial port to buffer incoming
characters, so they are not lost no matter what else the PC may be doing, but
this is not possible with the PICAXE.
The only solution to avoiding lost or corrupted data is to make sure that the PC
does not send serial data to the PICAXE until it is ready for it, either by
implementing a handshaking protocol in hardware or software, or by adding delays
between sending data from the PC.
The simplest mechanism is to have the PICAXE send a "Ready" data byte to the PC
to indicate that it is ready for a new character, with a SERIN command
immediately afterwards.
Combined Download and User Controlled Serial Interface
Sometimes it is necessary, or convenient, to use a single PC serial interface to
perform program downloads and to be used for communicating with the program
once it is running. This could be done by unplugging the serial cable from the
Download Interface and into another Serial Interface, but can often be achieved
by using a simple switch. Note that you will still need two sets of resistor
networks for both the Download Interface and the user serial interface.
.---------------.
| |
O------------------------------------------------| Serial Out |
.----O / .------. | |
| . O-------. .-------.--------------| |------| Serial In |
| . | | | 180R `------' | |
.----. | . .---|---' | .------. 22K | |
1 | O |---' . | `-----------|---| |-----------------| Digital Out |
| || . O---' | `------' .------. | |
2 | O ||--------O / .------------|---------.----| |------| Digital In |
| || O------' | | `------' | |
3 | O |---. _|_ _|_ 22K .--| 0v |
`----' | | | | | | | |
| O Download | | 10K | | 10K | `---------------'
| O / | | | | | PICAXE
| O User Serial `-.-' `-.-' |
| | | |
--^----------------------------^---------^---------------^-- 0v
If you are using SERTXD for user controlled serial output, then the interface is
even simpler ...
.---------------.
| |
.---------------------------------------------------------| Serial Out |
| .------. | |
| .---------------.--------------| |------| Serial In |
.----. | | | `------' | |
1 | O |---' | | 22K | |
| || O---' | .------. | |
2 | O ||--------O / .------------|---------.----| |------| Digital In |
| || O------' | | `------' | |
3 | O |---. _|_ _|_ 22K .--| 0v |
`----' | | | | | | | |
| O Download | | 10K | | 10K | `---------------'
| O / | | | | | PICAXE
| O User Serial `-.-' `-.-' |
| | | |
--^----------------------------^---------^---------------^-- 0v
Always remember to check the switch setting when downloading, as the program
download will fail if the setting is wrong.
Serial Interfacing for Multiple PICAXE's
A single serial output from a PC can be used to communicate with a number
of PICAXE's simply by wiring all the user serial input lines together.
Any number of PICAXE's can send user serial output to a PC by 'diode mixing'
the user serial output lines together.
Any low power signal diode, such as the 1N4148, can be used to perform this
'diode mixing', and
the technique has been proven to work, however it may not work with all PC
serial ports or those on other devices. In some cases it may be necessary to
use pull-up or pull-down resistors to make the interface work. These will
have to be carefully chosen, to ensure that damage is not caused to the
device that is being connected to.
Note that all PICAXE's must be configured to use the same baud rates and
each must be able to determine whether any incoming serial data is for itself
or another PICAXE. Every PICAXE must also make sure that it does not send out
any serial data while another PICAXE is doing so.
Preventing serial output clashes, which would lead to the PC receiving corrupt
data, can be accomplished by either using hardware arbitration to determine
which can access the serial interface - each PICAXE
requests access and some other hardware ( possible another PICAXE ) signals
that the requesting PICAXE has gained access - or by only sending data in
response to serial input received, which puts the onus on scheduling serial
transfers upon the PC program which communicates with the PICAXE's.
| | | .---------------.
| |/| | | | |
}------< |----------|---|---| Digital Out |
| |\| | | | |
| 1N4148 }---|---| Digital In |
| | | | |
| | }---| 0v |
| | | | |
| |/| | | `---------------'
}------< |------. | | PICAXE #2
180R | |\| | | | .---------------.
.----. .-----. | 1N4148 | | | | |
1 | O |----| |----' `---|---|---| Digital Out |
| || `-----' .------. | | | |
2 | O ||---------------.----| |-------^---|---| Digital In |
| || | `------' | | |
3 | O |---. _|_ 22K }---| 0v |
`----' | | | | | |
| | | 10K | `---------------'
| | | | PICAXE #1
| `-.-' |
| | |
--^-----------^-----------------------^-- 0v
Multiple Serial Interfacing with PICAXE-08's
If you are supplying serial input into Leg 3 of a PICAXE-08, you may need to
add a diode between Leg 3 and +V as described earlier in the "User Controlled
Serial Interface" section.
Only one diode will be required no matter how many PICAXE-08's are used, and
it may be left out if the serial input also goes to any other leg than Leg 3 on
any PICAXE-08 or is connected to any PICAXE other than a PICAXE-08, however, if
the PICAXE-08 using Leg 3 for serial input is ever to be the only chip installed
then a diode must be used.
The diode can be any small signal diode, such as a 1N4148, and fitting it,
even if not required, will not adversely affect the circuit's operation.
Serial Links Between PICAXE's
If you want to use serial interfacing to connect two or more PICAXE's together,
you do not need to use the circuits which are required to interface to a PC
serial port; it is a simple matter of connecting one PICAXE's Digital Output
to Digital Inputs on the ones you wish to send serial data to.
A single PICAXE can send serial data to multiple PICAXE's by simply connecting
the Digital Output from the PICAXE which is transmitting to Digital Inputs on
those PICAXE's which are receiving, as shown below ...
| | .---------------.
| | | |
}---|---| Digital In |
| | | |
| }---| 0v |
| | | |
| | `---------------'
| |
.---------------. | | .---------------.
| | | | | |
| Digital Out |--------------------------^---|---| Digital In |
| | | | |
| 0v |---. }---| 0v |
| | | | | |
`---------------' | | `---------------'
| |
--^--------------------------^-- 0v
Multiple PICAXE's can communicate serially by using 'diode mixing' as was
described earlier with multiple PICAXE's communicating with a PC serials port,
as shown in the circuit below ...
.---------------. | | | | .---------------.
| | | |\ | | | | | |
| Digital Out |---|----| >|----{ }---|---| Digital In |
| | | |/ | | | | | |
| 0v |---{ 1N4148 | | }---| 0v |
| | | | | | | |
`---------------' | | | | `---------------'
| | | |
.---------------. | | | | .---------------.
| | | |\ | | | | | |
| Digital Out |---|----| >|----^----.----^---|---| Digital In |
| | | |/ | _|_ | | |
| 0v |---{ 1N4148 | | }---| 0v |
| | | ?K | | | | |
`---------------' | | | | `---------------'
| `-.-' |
| | |
--^-----------------^--------^-- 0v
It is also possible to create a 'multi-drop' network, where every PICAXE can
communicate with every other using a single common line ...
.---------------. | | | | .---------------.
| Digital In |---|------------{ }------------|---| Digital In |
| | | |\ | | | | /| | | |
| Digital Out |---|----| >|----{ }----|< |----|---| Digital Out |
| | | |/ | | | | \| | | |
| 0v |---{ 1N4148 | | 1N4148 }---| 0v |
| | | | | | | |
`---------------' | | | | `---------------'
| | | |
.---------------. | | | | .---------------.
| | | | | | | |
| Digital In |---|------------{ }------------|---| Digital In |
| | | |\ | | | | /| | | |
| Digital Out |---|----| >|----^----.----^----|< |----|---| Digital Out |
| | | |/ | _|_ | \| | | |
| 0v |---{ 1N4148 | | 1N4148 }---| 0v |
| | | ?K | | | | |
`---------------' | | | | `---------------'
| `-.-' |
| | |
--^-----------------^-----------------^-- 0v
Baud Rates and Over-Clocking
All PICAXE's nominally operate at 4MHz, using either an on-chip "RC" oscillator
set to 4MHz, or by using an external 4MHz resonator or crystal.
At 4Mhz, the PICAXE-08, 08M, 18, 18A, 28 and 28A support baud rates of 300, 600,
1200 and 2400, and the PICAXE-18X, 28X and 40X support baud rates of 600, 1200,
2400 and 4800. The PICAXE-08M, 18X, 28X and 40X also support the SERTXD command
which transmits serial data at 4800 baud.
For those PICAXE's which can have their operating speeds increased - in the case
of the 08M, 18A and 18X by including the SETFREQ command, and for the PICAXE-28,
28A, 28X and 40X, by replacing the 4MHz resonator or crystal with an 8MHz or
16MHz part - the baud rates available are those available at 4MHz, doubled for
8MHz operation and quadrupled at 16MHz. The baud rate used for the SERTXD
command will be correspondingly increased for those PICAXE's which support it.
The following table shows the baud rates available for use with each variant
of the PICAXE range ...
|
SERIN / SEROUT
| SERTXD
|
|
4MHz |
8MHz |
16MHz |
4MHz |
8MHz |
16MHz |
| PICAXE-08
| 300, 600,
1200, 2400 |
-
| -
| -
| -
| -
|
| PICAXE-08M
| 300, 600,
1200, 2400 |
600, 1200,
2400, 4800
[1] |
-
| 4800
| 9600
[1] |
-
|
| PICAXE-18
| 300, 600,
1200, 2400 |
-
| -
| -
| -
| -
|
| PICAXE-18A
| 300, 600,
1200, 2400 |
600, 1200,
2400, 4800
[2] |
-
| -
| -
| -
|
| PICAXE-18X
| 600, 1200,
2400, 4800 |
1200, 2400,
4800, 9600
[1] |
-
| 4800
| 9600
[1] |
-
|
| PICAXE-28
| 300, 600,
1200, 2400 |
600, 1200,
2400, 4800
[3] |
1200, 2400,
4800, 9600
[3] |
-
| -
| -
|
| PICAXE-28A
| 300, 600,
1200, 2400 |
600, 1200,
2400, 4800
[3] |
1200, 2400,
4800, 9600
[3] |
-
| -
| -
|
| PICAXE-28X
| 600, 1200,
2400, 4800 |
1200, 2400,
4800, 9600
| 2400, 4800,
9600, 19200
| 4800
| 9600
| 19200
|
| PICAXE-40X
| 600, 1200,
2400, 4800
| 1200, 2400,
4800, 9600
| 2400, 4800,
9600, 19200
| 4800
| 9600
| 19200
|
[1] By using "SETFREQ"
[2] By poking OSCCON - Not recommended by manufacturer !
[3] By changing crystal/resonator - Not recommended by manufacturer !
Although it is possible to achieve relatively high baud rates by increasing
the operating speed, the manufacturers do not guarantee correct serial
communications at baud rates higher than those provided for 4MHz operation,
and add that, "A maximum of 4800 is recommended for complex serial
transactions".
The reliability of high speed serial communications depends on a number of
factors, especially the stability of the on-chip oscillator which controls
the operating speed of the PICAXE. For the PICAXE's which use resonators or
crystals, the operating speed will be relatively stable, but those parts
which are controlled by an on-chip "RC" oscillator will be less stable, and
will be prone to variations in operating speed due to fluctuation of
temperature; see the "Effects of Temperature" section below for further details.
Specifying the Baud Rate Required
Although you may have to tell the Programming Editor used to develop and
download programs into a PICAXE what the operating speed of the PICAXE is, the
operating speed is not taken into account when the SERIN and SEROUT commands
are used; specifying a baud rate of "N1200" will generate 1200 baud at 4MHz,
2400 baud at 8MHz and 4800 baud at 16MHz. To actually use 4800 baud at
16Mhz, you would have to specify a baud rate of "N1200".
To avoid problems where the program you are using may be run on PICAXE's with
different operating speeds, but you want to use the same baud rate regardless,
it is recommended that you create your own 'User Defined Symbols' which specify
the baud rate actually required, and are set to the approriate in-built baud
rate specifiers dependant upon operating speed. For example -
A PICAXE-28X operating at 4MHz could have the following definitions included ...
SYMBOL BAUD600 = N600
SYMBOL BAUD1200 = N1200
SYMBOL BAUD2400 = N2400
SYMBOL BAUD4800 = N4800
A PICAXE-28X operating at 8MHz would then have the following definitions
included ...
SYMBOL BAUD1200 = N600
SYMBOL BAUD2400 = N1200
SYMBOL BAUD4800 = N2400
SYMBOL BAUD9600 = N4800
A PICAXE-28X operating at 16MHz would then have the following definitions
included ...
SYMBOL BAUD2400 = N600
SYMBOL BAUD4800 = N1200
SYMBOL BAUD9600 = N2400
SYMBOL BAUD19200 = N4800
Any SERIN or SEROUT command which specifies its baud rate by using one of the
defined symbols will therefore use that baud rate no matter what the operating
speed is, providing that the symbol is properly defined for the operating speed.
It also means that should you change the operating speed of the PICAXE that your
program is running on, you only need to change the baud rate definitions and
will not have to search through your program checking that every SERIN and
SEROUT command has been correctly specified.
For example ...
SEROUT LCD, BAUD2400, ( 254, 1, "Hello World !" )
will always send serial data at 2400 baud, no matter what the PICAXE operating
speed is, with BAUD2400 appropriately defined.
The Effects of Power Supply Voltage
Because the PICAXE can only drive its Serial Out and Digital Output legs at
0v or at its power supply level, the ability to communicate serially with
another device depends upon that device accepting the voltage levels as
valid and meaningful.
To provide full RS232 interfacing the PICAXE would need additional electronics
to output -15V/+15V
voltage levels, but most serial ports will accept any voltage at or below 0v
to be the equivalent of -15V and a reasonable positive voltage to be equivalent
to +15V; this is what makes the simplistic interface successful in the
great majority of cases.
As the supply voltage to the PICAXE drops though, so too does the voltage which
the PICAXE can put out on Serial Out and its Digital Output legs and this may
become too low for the PC's serial port to recognise properly. This may cause
any number of apparent faults with serial communications which may also be
intermittent in nature.
The voltage at which difficulties may occur is hard to predict as it is very
much dependent upon the serial port to which the PICAXE is connected. Some
may accept very low voltages, while others may not even work when the PICAXE
is delivering a full 5V.
The Effects of Temperature
While the operating speed of the PICAXE-28, 28A, 28X and 40X is fairly stable
across a wide temperature range through the use of a resonator or crystal
controlled oscillator used for timing control, the PICAXE-08, 08M, 18, 18A and
18X are implicitly less stable because of their use of a simple internal "RC"
oscillator and will be more severely affected by changes in temperature.
The operating speed of the internal" oscillator parts will drift as the temperature
changes, whether they are changes in the ambient temperature or through
self-heating while operating. The proximity of other components which may be
warming up during operation, especially voltage regulators, may also heat up
the PICAXE. It is fairly likely that the operating speed will change when
a PICAXE is powered-up until it reaches a nominal operating temperature.
As the operating speed drifts then so will the accuracy of the timing of the
serial output which the PICAXE is generating, and its ability to accurately
read any serial input will be correspondingly affected. The problem can be
compounded if the devices at each end of the serial link are drifting in
opposite directions, possibly doubling the effect of the error.
Serial communications with a PICAXE are all done using one start bit, 8 data
bits and one stop bit; 10 bits in total, with the bits being sent in that
order. Should the PICAXE slow down by 6%, when the PC receiving this serial
data comes to be reading the last data bit the PICAXE will actually be sending
out the last but one data bit. This will result in PC accepting corrupted data
or the data being ignored if the following stop bit is not of the correct value.
The late arrival of the stop bit itself may also lead to another, non-existent,
byte of data being received or further errors, and discarded data.
The resultant affect of temperature change may therefore be that the PICAXE
no longer sends valid serial data or is unable to receive it. With large
enough drift, it may become impossible to use serial data no matter what
baud rate is selected.
It should be noted that this problem affects not just User Controlled Serial
Interfaces but also the Program Download Interface.
PICAXE is a trademark of Revolution Education Ltd.
PICmicro is a registered trademark of Microchip Inc.
