In the AT91SAM764 datasheet there is a referrance to the ARM7TDMI (Rev 4) Technical Reference Manual. ( DDI0210B)

http://www.arm.com/pdfs/DDI0210B_7TDMI_R4.pdf

I can't cut and paste the text (darn protection) but it discusses some pullup resistors connected to the JTAG lines. Are these resistors necessary or are they implemented inside of the AT91SAM? I wasn't clear on this. I don't see anything about necessary pullups in the datasheet! Thanks for any info!

-Henk Visser

JTAG pull-up resistors are needed !!!! Take a look at the Evaluation Board schematics

I looked at the evaluation board schematics and indeed I see the pullup resistors. What is the point of pulling these lines up? Why not just keep them at the levels there are at? And once "real" data is on these lines won't forcing the voltage with the pullups censored things up!?

Thanks for the info!

-Henk

Hi henkvisser,
the JTAG pins are mostly input pins. Inputs should be a valid levels. If there are no pull up/down resistors inside, you have to make this with external resistors. It could also be possible that wrong or undefined input levels disturbs to JTAG state machine. So you have to put it into a defined state.
The TDO is a output pin. If the driving source is a open collector or open drain, you have to insert a pull up to get valid logical levels.

Hope this helps,
Edi

Edi,

Thank you very much.

-Henk