What is EXTEST mode?

EXTEST is mandatory IEEE Std. 1149.1 Joint Test Action Group (JTAG) instruction. It tests the "external" traces of a device.

The following steps provide a brief explanation of how the EXTEST testing works:

1. The boundary scan register is connected between TDI and TDO and the device is placed in an “external” test mode
2. Boundary-scan output cells (the source) drive a known logic high and low value onto the trace being tested.
3. Boundary-scan input cells (the destinations) connected to this trace capture the value on this trace.
4. If the expected values match, the trace is fine. If not, the trace is either open or shorted.

The high-impedance state of EXTEST is overridden by bus hold and weak pull-up resistor features.