System debug and cross-trigger

The debugger shows software state; ILA captures PL signals. For a PS/PL failure, find one event visible from both sides.

Prepare observation

Connect ILA probes or mark signals before implementation. Width and depth consume resources, including BRAM. Its clock must suit captured signals; an asynchronous bus can produce an inconsistent snapshot.

VIO reads or drives small internal signals such as test modes. Check that a debug override does not conflict with another control source.

Link a C value to a transaction

This fragment writes a counter into AXI GPIO:

for (count = 0U; count < 256U; count++) {
    XGpio_DiscreteWrite(&gpio, 1, count);
    if (count == 0x85U) {
        xil_printf("Special count reached\r\n");
    }
}

0x85 is a marker in the code and captured AXI transaction. A nearby breakpoint shows variables subject to binary optimization. A variable may be removed or kept only in a register.

Trigger between debug resources

Cross-trigger transfers events between the processor and PL debug resources, or between ILAs. Configure trigger and acknowledgement signals in the design and tools.

It correlates observations rather than making events exactly simultaneous. Distribution, capture, and stopping introduce latency; the processor may continue before the halt takes effect.

Build a useful condition

Start with a hypothesis: a write of 0x85 to a register, an AXI error response, or an unacknowledged interrupt. Write address and data handshakes are independent and need not occur in the same cycle.

Broad conditions fill capture too early; overly strict conditions never trigger. Add criteria progressively.

Capture before and after the event to see cause and consequence. Long sequences may need fewer probes, greater depth, or several captures.

Compare against the intended behavior

State the invariant, identify software and hardware representations, then capture a correct case before the failure. Compare addresses, data, AXI responses, and driver state. Both views then support the same diagnosis.

UG908 documents ILA cross-trigger and debug probes.

Test your knowledge - Chapter quiz