USBPD Library Never sends PS_RDY

[JoeMck]

Hello,

I've been adapting the DRP1M1 USB-PD DRP reference design for use on a custom STM32U5A9 Board, and have run into an issue where it seems like ST's compiled USBPD library just stalls during negotiation as a SRC device.

Hardware: STM32U5A9, custom board with TCPP03-M20
Library: USBPDCORE_PD3_FULL_CM33_wc32.a
Configuration: DRP, PD3.0

## Problem

Operating as SRC, the PE state machine sends ACCEPT in response to a REQUEST but never advances to PE_SRC_TRANSITION_SUPPLY. PS_RDY is never sent and the port partner eventually triggers a hard reset.

## What We Have Verified

**Protocol layer is working correctly:**
- SOURCE_CAPABILITIES sent and GOODCRC received
- REQUEST received and GOODCRC sent
- ACCEPT sent and GOODCRC received
- All confirmed on protocol analyzer

**All DPM callbacks returning correct values:**
- USBPD_DPM_EvaluateRequest() returns USBPD_ACCEPT (verified via debugger)
- USBPD_PWR_IF_GetVBUSStatus(USBPD_PWR_VSAFE5V) returns USBPD_TRUE (VBUS at ~4.8V, threshold 4750mV)
- USBPD_DPM_SetDataInfo(USBPD_CORE_DATATYPE_RCV_REQ_PDO) storing valid RDO (0x1304b12c)
- DPM_Params[0].PE_IsConnected = 1
- DPM_Params[0].PE_Power = USBPD_POWER_DEFAULT5V (1)
- DPM_Params[0].PE_PowerRole = USBPD_PORTPOWERROLE_SRC (1)

**PE task is alive and running:**
- USBPD_PE_StateMachine_DRP() called ~13000+ times returning _timing=2ms
- PE task is not suspended, not blocked on queue
- TIM1 running correctly at 1MHz tick (PSC=159, SystemCoreClock=160MHz, APB2=DIV1)

**Callbacks never called after ACCEPT/GOODCRC:**
- USBPD_DPM_SetupNewPower() call count = 0
- USBPD_DPM_IsPowerReady() call count = 0
- USBPD_PE_TaskWakeUp() fires twice on attachment, never again after ACCEPT/GOODCRC
- TX completions (TXMSGSENT): count=3 (SOURCE_CAPABILITIES, GOODCRC for REQUEST, ACCEPT)
- RX completions (RXMSGEND): count=3 (GOODCRC for SOURCE_CAPABILITIES, REQUEST, GOODCRC for ACCEPT)

**Notification log per attach cycle (complete, never progresses beyond these 3):**
1. USBPD_NOTIFY_USBSTACK_START (104)
2. USBPD_NOTIFY_POWER_STATE_CHANGE (90)
3. USBPD_NOTIFY_SRCCAP_SENT (15)

Notifications never fired: USBPD_NOTIFY_REQUEST_ACCEPTED, USBPD_NOTIFY_CTRL_MSG_SENT, USBPD_NOTIFY_POWER_EXPLICIT_CONTRACT, USBPD_NOTIFY_AMS_INTERRUPTED

## Conclusion

The PE library sends ACCEPT at the protocol layer and increments TX completions correctly, but its internal state machine does not advance past PE_SRC_SEND_CAPABILITIES. USBPD_NOTIFY_REQUEST_ACCEPTED is never fired despite EvaluateRequest returning USBPD_ACCEPT and all observable callbacks returning correct values. The PE appears to be processing the negotiation at the protocol layer independently of its internal state machine progression.

Is there an additional callback, configuration flag, or initialization step required for the PE to transition from PE_SRC_SEND_CAPABILITIES to PE_SRC_TRANSITION_SUPPLY after receiving ACCEPT/GOODCRC?