STM32U5 IO-Link firmware prerequisites

The customer nucleo_u575zi_q Zephyr reference ELF runs its own startup, initializes TIM2 with prescaler 159, and measures time at one microsecond per 160 simulated CPU cycles. PB1 falling-edge WAKE reaches the actual Zephyr EXTI1 handler, clears FPR1, and causes the firmware to configure USART2 at COM2 (BRR 4167, 8 data bits, even parity, one stop bit). PD5 changes from SIO GPIO output to USART2 AF7; PD6 also selects USART2 AF7. The adapter error diagnostic remains zero. These assertions run in both legacy and event-scheduler modes.

The EXTI model follows the U575 register map: EXTI owns the port mux at 0x60, RPR1/FPR1 are independent W1C pending registers, and GPIO lines 0..15 route to individual NVIC IRQs 11..26. Four-bit mux slots include GPIOI. Focused tests cover wrong-port rejection, masked-edge latching and selective byte W1C. Sources are ST RM0456 and stm32u575xx.h from Zephyr's pinned hal_stm32 revision 0657d9f97d973542e438f628a704f5d8ea0cdef5.

This validates a firmware prerequisite, not an electrical IO-Link Twin. C/Q voltage/current, transceiver propagation, cable loading, supply faults, master exchange and a physical board remain unverified. The GPIO EXTI model does not implement software/internal triggers, EMR event delivery, or security, privilege and locking enforcement. Existing timer limits still apply; this firmware uses a fixed 160 MHz clock and APB1 prescaler one.

Build the customer source example with Zephyr c66235fb7346bbe3dbedd1dd76ec5a37a8e8262b and that revision's CMSIS/STM32 modules. west init --mr accepts a branch or tag, not this commit, so fetch the commit and run west init -l on that checkout, then west update cmsis hal_stm32. Then run:

export IOLINKI_U5_ELF=/absolute/path/to/zephyr.elf
cargo test -p labwired-core --test u5_exti --test u5_reference_firmware -- --include-ignored
cargo test -p labwired-core --features event-scheduler --test u5_exti --test u5_reference_firmware -- --include-ignored

The actual-ELF test is ignored in ordinary host runs because it requires an external board build; explicitly running it without the environment variable fails. The IO-Link prerequisite CI workflow builds these pinned sources, requires both execution modes and archives the ELF/map/HEX with an ELF SHA-256. Local validation used ELF SHA-256 15250b572d9139656ab8772883a78a7b0fa2b1265635212a8605ed6f667158e7.