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'[EE] JTAG TMS pin'
2009\04\29@113107 by Sean Breheny

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Hi all,

I have a design with a Lattice FPGA in it and it is set up so that
another PCB which attaches to it can reprogram the FPGA via JTAG. This
is part of a field update capability of this system.

The datasheet for the FPGA gives a diagram of how they recommend the
JTAG pins should be tied when not in use. They show TMS tied high and
TCK tied low. When they explain the operation of the JTAG module, they
say that TMS is sampled on the rising edge of TCK.

In our system, since there is a microcontroller which can re-program
the FPGA via JTAG, we have a "lockout" signal. That is, an extra
signal which ensures that the JTAG interface cannot operate unless it
is asserted. This is an extra protection against accidental
programming.

I am having a debate with a co-worker about how to use this signal. I
was suggesting that the signal should force BOTH TMS and TCK to their
idle states. He is suggesting that as long as we force TCK low, the
JTAG interface should not be able to cause improper operation of the
FPGA, since TMS is sampled when TCK changes.

This makes a difference because in order to make the lockout signal
affect both signals, we need to add another IC to the board as we are
out of extra gates.

Does anyone have any knowledge of whether changing TMS can typically
cause any improper operation if TCK is held low?

Thanks,

Sean

2009\04\29@113614 by Shawn Tan

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On Wednesday 29 April 2009 23:11:02 Sean Breheny wrote:
> Does anyone have any knowledge of whether changing TMS can typically
> cause any improper operation if TCK is held low?

I don't really know. But I was just wondering if a spurious spike in TCK is
possible? If TCK will never go high by accident, then it is probably okay.

Cheers.

with metta,
Shawn Tan.


'[EE] JTAG TMS pin'
2009\05\02@023520 by Vasile Surducan
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Your colleague seems to have right.

On 4/29/09, Sean Breheny <spam_OUTshb7TakeThisOuTspamcornell.edu> wrote:
{Quote hidden}

> -

2009\05\02@143154 by Raymond Hurst

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You can download a JTAG module from OpenCores.org and study it.
Ray

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