The Questions and Answers have been split into a number of parts, and are
available on separate pages to keep each page to a reasonable size.
In-Depth Articles
Projects and Articles under development
Part 1 - Common Questions
Where is the latest online documentation ?
The latest online documentation can be found at ...
Where is the latest software download ?
The latest software for downloading can be found at ...
When will the PICAXE-08M be available ?
The PICAXE-08M became an officially available product in mid-July 2004.
Why doesn't Program Download always work ?
When the PICAXE is running, it monitors its Serial In line to determine if a new
download has been initiated using the Programming Editor. When you initiate the
download, the PICAXE sees the event, responds to the download request, and
the PICAXE and Programming Editor manage the download between them, with the
PICAXE executing the program once it is completely downloaded.
Because the PICAXE monitors Serial In, it is necessary to ensure that Serial In
leg is connected to 0V for reliable operation when the download interface is not
used in your PICAXE circuit.
There are a number of commands which require the complete attention of the
PICAXE and it will not monitor the Serial In line while executing these; this
means that the initiation of a new download is not recognised by the PICAXE
and the Programming Editor will report that it was unable to download to the
PICAXE.
The commands which will prevent the PICAXE recognising a new download are ...
- SERIN
- INFRAIN and INFRAIN2
- PAUSE or WAIT longer than 5 seconds
Download may also not be recognised if a corrupt program is being executed
by the PICAXE. This can occur when a download has failed because the
download cable was removed or power failed during downloading. It is also
possible to corrupt the program when using the WRITE command on PICAXE-08,
08M, 18 and 18A where EEPROM data shares the same space as program code; a
write to a location which contains program code will corrupt it. It may also
be possible to corrupt the program by using PEEK and POKE outside of the
approved address ranges.
When the PICAXE is executing one of the commands which prevents it seeing a
new download or the program code has become corrupted, it will be necessary
to reset the PICAXE when the download is attempted.
A push-to-make 'Reset Button' can be fitted to all PICAXE's except the
PICAXE-08 and 08M between the Reset pin and 0v; pushing the button will reset
the PICAXE.
Note that the Reset pin should be connected to the PICAXE '+V' power supply line
through a 4K7 resistor as below ..
+V -----------.---------------.-----------
| |
.|. |
4K7 | | .-----^-----.
|_| | +V |
| | |
| | |
}---------| Reset |
| | |
| O | |
"RESET" || | 0V |
| O `-----.-----'
| |
| |
0V -----------^---------------^-----------
A PICAXE that is not responding to downloads should be reset when the progress
bar appears after download starts, or, for even better reliability, the reset
button should be held down, the download initiated, and the button released when
the download progress bar appears.
How to reset the PICAXE-08 and 08M is described in a later section.
Note that when resetting a PICAXE-08, 08M, 18, 18A or 18X in this way, the 4MHz
operating speed will always be expected by the PICAXE, so 4MHz operation must
be selected in the View/Options menu.
Why can't I do a Program Download at all ?
If your PICAXE is not allowing downloads, even when it is being held in reset
until the download progress bar appears, there may be some other fault.
Unfortunately the reason, or even reasons, for the problem may vary depending
upon whether you are using your own designed hardware or manufacturer's supplied
hardware such as a PICAXE Starter Kit or CyberPet board.
The following are some of the reasons why program download may not be working
although not all may be applicable to your own situation ...
- Wrong Serial Port selected in Programming Editor
- Download cable plugged in to wrong Serial Port on PC
- Wrong PICAXE operating speed selected ( View/Options menu )
- Broken download cable
- Wrongly wired download cable
- Wrongly wired download socket / connector
- Missing or wrong components in the Serial In circuit
- Solder shorts or track breaks on PICAXE PCB
- PICAXE-08 Leg 7 not connected/linked to download socket
- Reset pin not connected to +V via 4K7 resistor
- Faulty PICAXE
- PICAXE plugged into socket wrongly
- No socket to PICAXE pin contact
- No power to PICAXE
- Wrong power supply voltage
- PICAXE outputs drawing too much current
- Incompatible PC Serial Port
- PC Serial Port Socket not wired to motherboard
- Wrong PC Serial Port D25 to D9 converter used
If you are using manufacturer's hardware, it is highly unlikely that there are
any faults there, however check that the PICAXE is firmly seated in its socket
and that you have connected the correct power supply voltage.
It is advisable to install the Programming Editor on another PC and check if a
download can be done from there, and to check if you can download into a PICAXE
which others can download into. This will help determine whether the fault lies
with your PC or the PICAXE.
When using your own hardware, it is advantageous to be able to disconnect the
PICAXE from the hardware for programming to help identify those cases where it
is your circuit that is causing problems with download. A small
vero-board ( strip-board ) or bread-board circuit can be fairly easily
constructed for this purpose.
How do I reset a PICAXE-08 or 08M ?
Neither the PICAXE-08 or 08M have a Reset Pin and cannot therefore have a 'Reset
Button' fitted in the same way as other PICAXE's; all Resets must be performed
by removing the power to the PICAXE and re-applying it.
If the PICAXE is in a circuit which includes capacitors across the power supply
lines ( and all circuits should have decoupling capacitors across the power
supply near each IC ), and especially power supply smoothing capacitors, it may
take a considerable time for the voltage delivered to the PICAXE to drop to a
low enough level that the re-application of power is detected as a reset - This
applies to all PICAXE's and not just the PICAXE-08 or 08M.
It is possible to use a single-pole, double-throw (SPDT) switch to create a
'Reset Switch' for the PICAXE-08 and 08M as below ...
+V In >----------O +V to PICAXE
\ O----------> and other
.---O electronics
|
.|.
| | ?R
|_| See Text
|
|
0V In >------^-------------------> 0V
The choice of value for the 'dump resistor' will depend upon the amount of
capacitance within the circuit as all stored voltage will be dumped through
the switch in a surge of current. If only decoupling capacitors, a single
PICAXE and only passive components make up the electronics which is powered
then it will normally be possible to replace the resistor with a link to 0V.
Can I use a PICAXE in a Safety Critical System ?
NO.The PICAXE may never be used in a Safety Critical System.
Can I use a PICAXE in a Mission Critical System ?
Possibly, but think long and hard before you do.
It is necessary to understand what the system which includes the PICAXE is meant
to do and the consequences should it fail or malfunction. If a failure or
malfunction may put a life in danger, or lead to a life being put in danger,
then it is a Safety Critical System and a PICAXE must not be used.
What may at first appear not to be a Safety Critical System may turn out to be
so. A
system fitted to a vehicle may not appear Safety Critical at first glance, but
even something as apparently benign as an interior light delay circuit or a
car burglar alarm in a failure mode could affect the vehicle's electrical system
and lead to the loss of power and consequently affect steering, braking and
other safety systems during driving. Such faults can obviously be life
threatening, to passengers and others, even if not the driver, and such systems
must therefore
be considered as Safety Critical by consequence. That is why you should never
fit a PICAXE system to an automobile's power supply or interface it to any
other vehicle electronics.
Some systems may not be life threatening in their own right should the system
fail, but may also be life threatening by consequence. Should a smoke detector
system designed around a PICAXE fail then it will not alert occupiers of a
fire or the smoke from it, which could obviously lead to fatalities. Such
systems are, by consequence, Safety Critical Systems.
Other systems may be harder to classify; Home Burglar Alarms are a typical case
in point. Such an alarm is obviously designed to prevent some loss which may
occur if the system fails, so it is certainly Mission Critical. The question
you must ask is - are you prepared to suffer the consequences of your system
failing or malfunctioning ? If not, or are reluctant to do so, then do not use
a system designed around a PICAXE.
Do not forget that it is usually not only the
consequences to yourself which have to be taken into account, but the
consequences to others - How do you explain when you have led your family to
believe that something you have designed and built to protect them fails to do
so, injures them, damages their valuable property or allows its loss ? Seeing
your PICAXE project burn your family home down will not get you to the top of
the popularity list.
If your burglar alarm or entry alert system is also designed to alert occupiers
when entry is made into a property, then it may well be that your system is also
Safety Critical. As can be seen; it's not a simple issue to deal with.
The rule of thumb is to consider anything and everything that could go wrong,
and assume that it will,
and what could go wrong is anything which has even the remotest likelihood
of going wrong, from mains failure to a single faulty component. In professional
terms this is "risk assessment" and "failure mode analysis", and both are
incredibly difficult tasks to undertake properly and fully without previous
experience and professional, qualified guidance.
It is absolutely essential that when you are considering what could go wrong
that you are as pessimistic, cynical and as depressive as you can be; if it can
go wrong it will, and it is necessary to know what that is and the consequences
as early as possible. One failure may lead to another, and a small insignificant
failure in its own right can have severe consequences - That's what tends to
happen even in major disasters; Chernobyl, Three Mile Island, Piper-Alpha,
even 9-11.
It is also necessary to get friends, family and colleagues to have a look at
what you are doing, or proposing to do, in order to get their opinions; they may
see something which you had never considered, or had dismissed as unimportant.
Do not forget that you are also likely to have faults in your PICAXE program
code which you may be unaware of until it is too late.
How do I use a PICAXE in an Automobile ?
Do not attempt to connect a PICAXE into an automobile's electrical system
Only use a PICAXE in an automobile when it is not connected to the vehicle
electronics or power supply in any away.
Incorrectly fitted electronics could easily lead to damage to your vehicle and
may even lead to your own death or that of others.
It is highly likely that any electronics you add to an automobile will
invalidate your insurance and make the vehicle illegal for use. The
consequences could be that you may personally face huge legal bills should you
be involved in an incident or accident ( even if it is not your fault ). You
may be prosecuted, and face a prison sentence or large fine, if such electronics
are found to be fitted by the police or other authorities.
If you are considering fitting a PICAXE to a vehicle which will not be used on
the public highway, you will face numerous difficulties in making the PICAXE
safe for use and in connecting the PICAXE to the vehicle electronics.
You will need to run the PICAXE from a regulated power supply, and will need to
design your circuit to work with the voltages used within the automobile.
Most automobiles are so-called "12V Negative Earth", which means that the
chassis of the car is at 0V and all power is +12V relative to the chassis. The
actual voltage is however nominally 13.8V, but the voltage may often
exceed this by a huge amount - hundreds of volts - for brief periods, and may
also drop well below that.
The extremely high voltages, low voltages, brown-outs, and general 'noise' and
interference which can be found on an automobile's power supply can make it
extremely difficult to connect any electronics into a vehicle. The power supply
needs to be protected from the massive input voltages which can destroy the
power supply and other electronics, and drops in voltage may cause the
electronics to reset or latch-up.
On top of that; any electronics you fit to an automobile may cause problems to
other electronic systems within the vehicle and may render safety and other
necessary equipment ineffective. Any flaws in your electronic design or faults
in assembly may lead to consequences which are not acceptable; fire and even
death.
Do not attempt to connect a PICAXE into an automobile's electrical system or
interface it to any other vehicle electronics.
How fast is a PICAXE ?
This is one of the most difficult questions to answer about the PICAXE in a
meaningful way. As with modern desktop PC's the speed of program execution is
not a direct reflection of the speed of its controlling oscillator.
The PICAXE range uses a 4MHz oscillator by default, with the PICmicro it is
based upon having a 1uS instruction cycle. This equates to a basic
1 MIPS ( Million Instructions per Second ) execution speed, although some
instructions do take two cycles to execute, slowing it slightly.
The PICAXE is however what's called a pre-programmed interpreter, and while
the interpreter program itself runs at 1 MIPS, the program it
interprets ( your encoded Basic Language program ) is run much slower. It is
estimated from testing that the interpreted instruction set executes about
250 times slower, giving an interpreted instruction speed of around
0.004 MIPS, but this can vary as the actual instruction time depends upon
where the instruction is placed in program code memory in the PICAXE, and how
long a particular statement takes to execute depends upon how many things it
has to do; how many instructions it converts to.
It is therefore not easy to give a clear or meaningful measure of PICAXE speed.
And while the estimated 0.004 MIPS seems incredibly low at first glance,
it is often far more important to know that a program will complete its task
in the time allocated to do it, rather than how long it does actually take. The
following code snippet takes 0.9mS to execute on a PICAXE-18 at 4MHz ...
Loop:
pins = pins ^ $FF
GOTO Loop
This equates to around 2000 Statements Per Second, but that is not reflected
accurately in the execution of other statements, some of which are quicker,
some slower. You may often get much better performance than this, but it cannot
be guaranteed, and you may also see worse performance in some cases. Some
statements such as WRITE and those accessing I2C and other peripherals can
be much slower than other statements. The execution time of the SERIN and
SEROUT statements are dependant upon the baud rate selected; the higher the
baud rate, the faster their execution. Overall execution speed will drop
massively whenever a DEBUG statement is executed, but the inclusion of
DEBUG statements does not slow the program down unless they are executed.
The PICAXE 08, 08M, 18, 18A and 18X all use an internal "RC" oscillator and do
not have reliably stable operating speeds which will vary due to manufacturing
tolerances and
with changes of temperature and operating voltages. It is not uncommon that
a particular program may run 10% faster or slower on a same variant PICAXE
under different environmental circumstances.
The PICAXE-28, 28A, 28X and 40X all use a resonator or crystal controlled
osciallator which will give much more stable and consistant operating speeds.
The PICAXE 08M, 18A and 18X can have their operating speeds doubled by use
of the SETFREQ command which will give a better, doubled, Statements per
Seconds figure, as will the PICAXE-28, 28A, 28X and 40X when an 8MHz resonator
or crystal is used. A PICAXE-28X or 40X can do even better, delivering a
quadrupled Statements per Seconds figure when a 16MHz resonator or crystal is
used.
As a rule of thumb, for guidance only; at 4MHz, you can expect a PICAXE to
execute around 2000 Statements per Second, at 8MHz that figure becomes 4000,
and at 16MHz it becomes 8000.
PICAXE is a trademark of Revolution Education Ltd.
These PICAXE pages are produced entirely independantly of Revolution Education
Limited and may not reflect the opinion of Revolution Education Limited or its
agents. The information provided is based upon and derived from information
published by Revolution Education Limited, other sources of PICAXE information
and the author's own experiments and prior experience. The views expressed by
the author do not necessarily represent those of Revolution Education Limited or
its agents. While every effort has been made to ensure that the information on
these PICAXE pages is accurate and correct, the author can accept no
responsibility for any errors or ommissions which do occur. The information
provided is used entirely at your own risk.
