10 Reset Configuration

Every system configuration may require a different reset configuration. This can also be quite confusing. Please see the various board files for example.

10.1 jtag_nsrst_delay <ms>

How long (in milliseconds) OpenOCD should wait after deasserting nSRST before starting new JTAG operations.

10.2 jtag_ntrst_delay <ms>

Same jtag_nsrst_delay, but for nTRST

The jtag_n[st]rst_delay options are useful if reset circuitry (like a big resistor/capacitor, reset supervisor, or on-chip features). This keeps the signal asserted for some time after the external reset got deasserted.

10.3 reset_config

Note: To maintainer types and integrators. Where exactly the “reset configuration” goes is a good question. It touches several things at once. In the end, if you have a board file - the board file should define it and assume 100% that the DONGLE supports anything. However, that does not mean the target should not also make not of something the silicon vendor has done inside the chip. Grr.... nothing is every pretty.

Problems:

1. Every JTAG Dongle is slightly different, some dongles impliment reset differently.
2. Every board is also slightly different; some boards tie TRST and SRST together.
3. Every chip is slightly different; some chips internally tie the two signals together.
4. Some may not impliment all of the signals the same way.
5. Some signals might be push-pull, others open-drain/collector.

OpenOCD defines these signals in these terms:

• TRST - is Tap Reset - and should reset only the TAP.
• SRST - is System Reset - typically equal to a reset push button.

The Command:

• reset_config <signals> [combination] [trst_type] [srst_type]
  The reset_config command tells OpenOCD the reset configuration of your combination of Dongle, Board, and Chips. If the JTAG interface provides SRST, but the target doesn't connect that signal properly, then OpenOCD can't use it. <signals> can be none, trst_only, srst_only or trst_and_srst.

  [combination] is an optional value specifying broken reset signal implementations. srst_pulls_trst states that the testlogic is reset together with the reset of the system (e.g. Philips LPC2000, "broken" board layout), trst_pulls_srst says that the system is reset together with the test logic (only hypothetical, I haven't seen hardware with such a bug, and can be worked around). combined imples both srst_pulls_trst and trst_pulls_srst. The default behaviour if no option given is separate.

  The [trst_type] and [srst_type] parameters allow the driver type of the reset lines to be specified. Possible values are trst_push_pull (default) and trst_open_drain for the test reset signal, and srst_open_drain (default) and srst_push_pull for the system reset. These values only affect JTAG interfaces with support for different drivers, like the Amontec JTAGkey and JTAGAccelerator.