Re: JTAG RTCK / Jeeni question

Started by Arie de Muynck October 1, 2005
From: "Michael Johnson"
> The RTCK isn't strictly needed. Without the RTCK if the JTAG port works
> slowly, as is the case of the LPC2xxx, then the JTAG cable/debugger has
> to put in a delay before it samples the TDO. I'm surprised that
> companies are selling JTAG cables that require RTCK - the cynic in me
> says it's a way of getting repeat business.


(1) Use of RTCK for PLL'ed MCUs.

It is extremely useful when you have an MCU that starts at a slow speed,
then turns on a PLL and speeds up. The RTCK then continuously indicates the
speed at which you can clock instructions into the core and the JTAG can
track that speed.

Normally in setups like this the PLL registers are programmed first, then
the RAM is uploaded. The increase in upload speed is equal to the PLL

(2) Use of RTCK to synchronize JTAG to the core execution.

When you don't know the core clock you may have to wait a few CLK states
after loading each instruction into the pipeline so you are sure it is
executed before the JTAG sends in the next instruction.
By using RTCK you can syncronize this process and skip a few waitstates. Regards,
Arie de Muynck

An Engineer's Guide to the LPC2100 Series