Overriding Constant Pins

Constant Pins are intended to protect the board during JTAG chain initialisation and connection tests as these are controlled automatically by XJTAG. User specified tests can, however, override the settings using the SET statement. If a Constant Pin is changed by a test device file, it will keep the value that has been SET until the safe bitstream is reapplied, at which point it will be returned to the safe value.

If you want to change the safe bitstream during testing, XJEase has two relevant commands:

• SET_DEFAULT_OVERRIDE
• RESET_DEFAULT_OVERRIDE

If the Constant Pin is changed by an override in the XJEase code, it will both set the pin to the new value and also update the safe bitstream, meaning that the change will be remembered when the safe bitstream is applied in the future. The value of the Constant Pin can be changed back and forth by SET_DEFAULT_OVERRIDE commands as often as required; the RESET_DEFAULT_OVERRIDE command will always set it back to its initial value.

This could be used to bring up a series of power supplies by setting the default overrides in the appropriate sequence to bring them up in a controlled manner. This means that if you subsequently do another connection test, the power supplies will stay up. Once the testing has been completed, additional SET_DEFAULT_OVERRIDE or RESET_DEFAULT_OVERRIDE commands could be used to turn the power supplies off again in the correct order.

• Whenever a test reset sequence is run, the JTAG devices are likely to briefly leave boundary scan mode, which can lead to a glitch in values set by Constant Pins. If this is found to cause problems for your circuit, please contact XJTAG support for assistance.