Multiple 08M2s on one board

[dandlion]

I am designing a board with 3 08M2s - all pre-programed. My question is:
Can I tie the unused Serial-in pins to ground via one 10K / 22K pair or do I need to tie each one separately through its own resistor pair? I'm not sure why 2 pull-down resistors are required on the serial-in pin but that seems to be what the manual is saying. (Or am I mis-understanding?) I'd rather not put 6 resistors on the board if 2 (or 1) will do.

 

[bpowell]

I'd be concerned about trying to tie 3 chips SERIN ports to ground with one resistor...as that would basically mean you're shorting all three pins to each other....that could potentially lead to some issues if your code goes bonkers and starts throwing voltage out one of those pins. Just a thought.

You should be able to tie each of them to ground individually with a 22k or 10k...the reason you need two resistors in the download circuit is you're reducing the incoming voltage to a level the PICAXE can handle...but you only need one resistor to tie the port low...I seem to recall the PICAXE will act funny if that pin is left floating...I think it'll go into "download" mode, and effectively stop working until reset.

Even with a "DISCONNECT" command, the chip will still scan for a program download at power-up...so it's important to tie that pin low.

 

[inglewoodpete]

Two resistors per PICAXE are only required when you plan to program the chips in-circuit.

The safest solution is to put a single resistor (4.7k to 33k) between the Ser In pin and 0v on each PICAXE.

 

[dandlion]

Thanks. I was worried about shorting the 3 pins together. I can live with the 3 10K res solution.

 

[westaust55]

The SerialIN pins can be used as general inputs (after use of the DISCONENCT command) but can never become outputs.

Hence you could tie all three low, even cnnected directly to 0 V but why not use at least a separate 10 kOhm resistor for each so that the input can be separately be used for each 08M2. A second resistor for each and even if no onboard socket you are part way there to being able to program them on the project board.

What is the great objection to 6 resistors?
Surely the cost and space are not big considerations (have them in vertical orientation if you do want to keep space requirements down.

 

[dandlion]

The SerialIN pins can be used as general inputs (after use of the DISCONENCT command) but can never become outputs.

Hence you could tie all three low, even cnnected directly to 0 V but why not use at least a separate 10 kOhm resistor for each so that the input can be separately be used for each 08M2. A second resistor for each and even if no onboard socket you are part way there to being able to program them on the project board.

What is the great objection to 6 resistors?
Surely the cost and space are not big considerations (have them in vertical orientation if you do want to keep space requirements down.

You're right, of course. I guess I was just being stingy. I hadn't planned on using on-board programming. Hope to get the bugs worked out on the breadboard stage. (Am I being too optimistic?)

 

[bpowell]

You're right, of course. I guess I was just being stingy. I hadn't planned on using on-board programming. Hope to get the bugs worked out on the breadboard stage. (Am I being too optimistic?)

Well, you may get all the bugs worked out...but when you throw your project into use...you may find new features you'd like to add...and not having on-board programming will make it more difficult.

 

[hippy]

The SerialIN pins can be used as general inputs (after use of the DISCONENCT command) but can never become outputs.

Connecting all input pins together then through a single resistor to 0V would protect from damage so long as only one PICAXE ever made its pin an output.

Connecting each pin to 0V via a separate resistor is a better solution because it best prevents damage no matter what happens. This would most likely be recommended by hardware designers.

Recommendations are just that and do not need to be followed if one is prepared to accept the risk that comes in not doing so and the 'adverse event' risk does seem quite low for most projects.

We would generally recommend fitting all six download resistors for a three PICAXE system plus some type of connection to enable download and/or debugging of each. It can sometimes be difficult to get a multi-PICAXE system operating as desired, hard to confirm what is and is not working, or to get help with problems if there is not a means to debug or reprogram the PICAXE chips in place.

The ability to see what each chip is doing and easily update the software can be invaluable. It can be a frustrating and difficult task without that. Cost saving by omitting the download interfaces may turn out to be a false economy though Sod's Law suggests that if you do fit them you won't need them, and if you don't you will and quickly regret not having them.