39.4 JTAG Instructions

PIC18-Q84 family devices support the mandatory instruction set specified by IEEE 1149.1, as well as several optional public instructions defined in the specification.

The mandatory JTAG instructions are:
  • BYPASS (Fh): Used for bypassing a device in a test chain; this allows the testing of off-chip circuitry and board-level interconnections.
  • SAMPLE/PRELOAD (2h): Captures the I/O states of the component, providing a snapshot of its operation.
  • EXTEST (6h): Allows the external circuitry and interconnections to be tested by either forcing various test patterns on the output pins or capturing test results from the input pins.
The optional JTAG instructions implemented in PIC18-Q84 devices are:
  • IDCODE (1h): Causes the 32-bit device identification word to be shifted out on the TDO pin.
  • HIGHZ (0h): Places the device into a state in which all I/O pins are in a High-Impedance state (driven inactive).