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ARM JTAG Voltage Tolerance

Hiya. I've been messing around with an old router, which is apparently too old for OpenWRT, but I figure it's still decent enough to play around with for myself. It uses an ARM7 processor. There's a JTAG port on the board, minus headers, on which I tested voltages and it appears to be hooked up (unlike various other parts of the board, like the serial headers). But the problem is that it's 3.3v. I've seen some mixed info on whether it would be safe to hook this directly to the 5v of a parallel port. If at all possible I'd rather avoid extra hardware since it's just a little side project and not really worth spending money on. So I'm wondering if the ARM might be 5v tolerant, and if not, do you suppose I could get by just using some resistors to do voltage division?

Reply to
FyberOptic
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Reply to
Brendan Gillatt

I found a datasheet for this one, and it appears you're right, it's

3.3v +- 5%.

However, I just thought of something. The parallel port has some open- collector pins. And the cpu pulls some of the JTAG pins internally:

TCK = pulled down TMS = pulled up TDI = pulled up TDO = none nTRST = pulled up

So I should be able to connect open-collector pins from the parallel port to all of the ones pulled up and it should work fine, would it not? The only one I'd have to do anything different with that I can figure is TCK, which I could either use voltage division on a standard powered pin from the parallel, or pull it high somehow from the router's own 3.3v from an open-collector pin. And TDO should work connected straight to an input pin as you suggested.

I just looked at that, and it seems like it'd be handy. I'm not sure I've ever messed with enough ARM devices yet to warrant a purchase (mostly PIC/AVR/MCS51/Z80 stuff), but depending on how much I learn from messing with this router, that might change! Assembly just seems a lot less restrictive so far with RISC than stuff I've used before.

Reply to
FyberOptic

Build yourself a Wiggler clone using a 5V tolerant interface chip. The wiggler circuit is powered by the 'target', not the PC.

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Reply to
Nico Coesel

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