On Fri, Jul 8, 2011 at 1:19 AM, Matthew Lai <[email protected]> wrote:
> Hello! > > I'm trying to embed a FT2232D based programmer into my board with a STM32 > (Cortex-M3 MCU). > > I want the programmer to be compatible with jtagkey, so I looked at > schematics of compaible designs. > > I noticed that while the JTAG signals (TCK, TDI, TDO, TMS) are only > buffered when translation is necessary, the SRST and TRST signals are always > tri-state buffered, with OE going into the FTDI chip. > > Is there a reason for that? Can I omit the buffers? > > Maybe you've already got the answers you needed, but a clarification may be in order: It's important that SRST is open-drain because it is a bidirectional signal. The STM32 drives it low when it is reset internally. If it's driven high, reset may not work and the chip may possibly even take damage. > Also, I heard it's possible to omit the TRST signal and only keep the SRST > signal, because system reset will also reset the TAP controller. Is that > true? Are there problems with that? > TAP or debug logic is not reset by SRST, but you can skip TRST anyway because TAP reset can be achieved by clocking in a pattern on TMS, which OpenOCD does if no TRST is available. I'm not aware of any problems, we have made several STM32 designs without TRST. Even the SRST is optional, it can be asserted via debug logic, but it's good to have because it may be the only way to reset the chip (other than POR) if you're stuck in low power mode (JTAG unresponsive). /Andreas
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