Yet Another Download Problem

JezWeston

New Member
This seems a common scenario - new circuit, Picaxe soldered in, can't download a program to it. So to save everyone's time, I've been through as much debugging as I can think of, and I think I've found the problem (or at least a problem).

Picaxe (28X1 SMT) has power, reset pin is high, resonator connected, resistors on download circuit seem fine, the download cable (AXE027) checks out, it can be used to program another Picaxe on a different test board and there's continuity from the D female socket to the pins on the Picaxe.

Now, here's what I think is the problem. With download cable and power plugged in, I read -0.5V at the serial in pin, 0V at the serial out. Reading around the forums suggests that I should have 5.5V at the serial out pin. Hippy's page on serial comms suggests that connecting the serial out to 0V can damage the Picaxe, and I certainly did that when sorting my wiring out. Given that the Picaxe is soldered in (and SMT), then I can't pull it out of the circuit and drop it into a test board.

So, my theory is that I've buggered the picaxe though connecting the serial out to 0V. How could I tell if I'm right or wrong?
 
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Technical

Technical Support
Staff member
Connecting serout to 0V basically causes a short circuit, so can permanently damage the chip.

If you have an oscilloscope (or the cheaper version - an LED +resistor to 0V) on serial out, you should see some activity when the serial pin is toggled high/low. No activity at all suggests a damaged serial out pin, which is fatal.
 

hippy

Technical Support
Staff member
Serial In at -0.5V (idle) and 5.5V ( active ) is okay for a desktop, Serial Out idling at 0V is also okay, 5V when active.

Shorting Serial Out to 0V can damage the chip so it may be dead. I cannot suggest much more than Technical have :-(
 

JezWeston

New Member
Ok, given that I can't program the chip, what can I do that should cause detectable activity on the serial out pin?

Is attempting a download guaranteed to cause activity on that pin? I ask in case there is another problem that is preventing a download from getting to the point where the serial out line is active. If there is, then seeing no activity on the serial out pin just confirms that there's a problem, rather than confirming that it is specifically this problem.

Or am I overthinking this, and 0V on serial out when trying to download always means dud chip?
 

hippy

Technical Support
Staff member
As Serial In is taken high there should be some activity seen on Serial Out, hence Technical's suggestion to use a scope or LED+R to monitor for that.
 

JezWeston

New Member
Scope gives a lovely sawtooth on serial in, nothing at all on serial out.

Ok, conclusion is it's a dud chip. Thanks for the help in pining this down.

(As it's surface mount, new bit will have to come from Oz (I'm in NZ) and the Easter break has just started, so won't be able to get a new chip for nearly two weeks, luckily there's other parts of this project that I can get on with.)
 

JezWeston

New Member
Yup, dud chip

Another 28X1 arrived, soldered it in, it works.

So, from this I've learnt that a 180 Ohm resistor on the Serial Out line is a very good idea to stop you blowing up the chip when pfaffing about with programming connectors.

(I also learnt that you can plug a two-row IDC socket onto only one row of a two-row plug and then not notice your mistake for several hours of swearing and wondering why the new chip wasn't working either...)
 
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