Download circuit

[johndk]

I recently build two different PCB circuits each using a 28x2. I'd been doing some testing on the first when it was time to fire up the second. To my surprise, I received the typical "Hello" from the second unit, but could not program it. Checked, and checked, and checked again and couldn't find anything wrong with board number 2. So I switched to doing some schematic work for which I had to reference the layout on board #1. I found that I had made an error on the download circuit for board #1 (the one that was working). I had bypassed the 22k resistor and connected directly to serial input. The 10k resistor to ground was in place.

The second board was wired correctly, with the 22k between the cable and the serial input. Yet the second board did not work! So what's the obvious work-around? Yep, jumper the 22k with a short length of wire. And it worked! Now both boards are happy without the 22k download resistor.

So now I need an explanation. Why?

I'm running both boards at 3.3v, but other than that, they've got pretty standard components (all rated for 3v operation). Is there a different download circuit for 3.3v? Will I cause eventual damage by running without the 22k (especially since the incoming serial voltage is higher)?

The 'conundrum king',

John

 

[srnet]

Is there a different download circuit for 3.3v?

No.

Will I cause eventual damage by running without the 22k (especially since the incoming serial voltage is higher)?

Expect the PICAXE to damaged without the recommended download circuit in place.

The standard download circuit works very reliably in my experience on 3.3V powered X2s, so if your circuit only works with the 22K shorted out, it seems likely you have connected something up incorrectly.

 

[hippy]

Which download cable are you using ?

There is risk of damage if the 22K is shorted out or made a wire link. We would recommend against applying non-current limited voltages higher than PICAXE supply into any pins.

 

[johndk]

I'm using the Picaxe download cable. The recommended download circuit has worked fine in the past. But those projects have been at 5v. I'm working with the prototype at the moment. So if I blow that up, not such a big deal. But I need to know for the production versions what the problem might be. Any ideas?

 

[rossko57]

Have you checked the resistor values?

 

[westaust55]

Are you sure that you have the 22 kOhm resistor in the right place?
The 22 kOhm does not use the 10 kOhm as a voltage divider - it should be connected between the 10 kOhm and the PICAXE SerialIn pin.
The purpose of the 22 kOhm resistor is to limit the current when programming from a higher voltage than the PICAXE supply such as a true RS232 port when voltages are often 12 V and may be as high as 25 V (by the RS232 std).

 

[johndk]

Yes, the resistor values are correct, both by imprint and by meter. And I was about to reply to Westaust "Yes, of course they're in the right place. I checked a hundred times." But, since I asked for ideas, I thought I could just check one more time. And, indeed, the 22k WAS in the wrong location. I had it between the cable and the 10k not the 10k and the serial port. Haven't tried it yet, but since I've been using the download circuit for other projects, I'm sure it will work with the correction.

Thanks Westy, for pointing that out! Can't tell you how many times I looked at it.

 

[johndk]

Confirmed. Moving the 22k solved the problem.

Just goes to show how one can overlook the obvious ... many times over.

Thanks again.

 

[erco]

Honest mistake. I actually enjoy soldering new PCBs for projects, and do one or two a week. I use reading glasses and plenty of light to build, and I keep a 10X loupe handy to check my connections. After soldering on the bottom, I flip the board over and top-test with a multimeter before applying power or inserting chip in socket. Check for shorted neighboring pins, then measure resistances from my 3-pin programming header and picaxe socket. Verify 32K from serin to ground and 10K from header pin to ground. Finally, apply power and verify voltage at Picaxe socket power pins. Power off, insert Picaxe, power up, connect to computer and smoke test.

Those resistors are so small and nearly unreadable that I usually verify values via multimeter during assembly.