Configuration Register Read Procedure (JTAG) - Configuration Register Read Procedure (JTAG) - UG570

The simplest read operation targets a configuration register such as the COR0 or STAT register. Any configuration register with read access can be read through the JTAG interface, although not all registers offer read access. The procedure for reading the STAT register through the JTAG interface follows:

1. Reset the TAP controller.
2. Shift the CFG_IN instruction into the JTAG Instruction register through the Shift-IR state. The LSB of the CFG_IN instruction is shifted first; the MSB is shifted while moving the TAP controller out of the SHIFT-IR state.
3. Shift packet write commands into the CFG_IN register through the Shift-DR state:
   1. Write the synchronization word to the device.
   2. Write at least one NOOP instruction to the device.
   3. Write the read STAT register packet header to the device.
   4. Write two dummy words to the device to flush the packet buffer.

   The MSB of every configuration packet sent through the CFG_IN register must be sent first. The LSB is shifted while moving the TAP controller out of the SHIFT-DR state.

4. Shift the CFG_OUT instruction into the JTAG Instruction register through the Shift-IR state. The LSB of the CFG_OUT instruction is shifted first; the MSB is shifted while moving the TAP controller out of the SHIFT-IR state.
5. Shift 32 bits out of the Status register through the Shift-DR state.
6. Reset the TAP controller.

The following table describes the readback command sequence.

The packets shifted into the JTAG CFG_IN register are identical to the packets shifted in through the SelectMAP interface when reading the STAT register through SelectMAP.