Mandatory JTAG instructions#
The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and EXTEST.
Boundary Scan#
The EXTEST instruction is used for sampling external pins and loading output pins with data. The data from the output latch will be driven out on the pins as soon as the EXTEST instruction is loaded into the JTAG IR-register. Therefore, the SAMPLE/PRELOAD should also be used for setting initial values to the scan ring, to avoid damaging the board when issuing the EXTEST instruction for the first time. SAMPLE/PRELOAD can also be used for taking a snapshot of the external pins during normal operation of the part.
EXTEST#
The active states are:
- Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.
- Shift-DR: The Internal Scan Chain is shifted by the TCK input.
- Update-DR: Data from the scan chain is applied to output pins.
Ressources#
AVR JTAG System Overview : https://onlinedocs.microchip.com/oxy/GUID-74F8229E-4C43-4FA0-BE7D-1AA303C6F8A4-en-US-6/GUID-86F1AB9A-120D-42D4-8B59-7A9F04E34236.html?hl=extest