ARM JTAG Voltage Tolerance

[FyberOptic]

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?

 

[Brendan Gillatt]

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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?

I very, very, very much doubt it has 5v safe inputs, check the datasheet
for the part if you can find it.

You can use a voltage divider to turn 5v into 3.3v and, if your hardware
is properly TTL compatible (2.5v threshold), you can simply wire the
outputs through.

FWIW, I would grab yourself an ARM-USB-TINY from Olimex for doing any
work - using a 'Wobbler' from the parallel port will take AGES to
re-write the whole firmware. They're cheap enough and are useful for
having around.

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
PGP Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
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[FyberOptic]

I very, very, very much doubt it has 5v safe inputs, check the datasheet
for the part if you can find it.

You can use a voltage divider to turn 5v into 3.3v and, if your hardware
is properly TTL compatible (2.5v threshold), you can simply wire the
outputs through.

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.

FWIW, I would grab yourself an ARM-USB-TINY from Olimex for doing any
work - using a 'Wobbler' from the parallel port will take AGES to
re-write the whole firmware. They're cheap enough and are useful for
having around.

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.

 

[Nico Coesel]

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?

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