How can I determine Source PDO capabilities?

ardnew · ‎2019-12-10

I am developing an application using an STUSB4500 and an external MCU STM32G031K8 that should be able to read USB PD Source capabilities and allow the user to dynamically set custom PDOs.

I have based the majority of my code off of the example software package STSW-STUSB003.

According to that software, you determine Source capabilities by first sending a USB PD soft reset command (the USB Power Delivery command, not a STUSB4500 soft reset), int PdMessage_SoftReset() and int PdMessage_SoftReset_WithTimeout() in the firmware package, after which the Source will broadcast is capabilities:

1. The STUSB4500 raises the ALRT line as expected after this PD command is sent

2. I read the PRT_STATUS (0x16) register to verify a message is received

3. I then read the RX_HEADER (0x31) register, unfortunately there are no docs on this register content

4. So using the firmware package's definition (USBPD_MsgHeader_TypeDef), I am decoding the content of RX_HEADER

5. At this point, regardless of what PD source I use (testing with Apple 60W PD charger), the reported number of PDOs in RX_HEADER is always 0

6. The message type of these headers is either 0x3 (USBPD_CTRLMSG_Accept) or 0x6 (USBPD_CTRLMSG_PS_RDY)

I'm not sure if I am decoding the header incorrectly or if something else is wrong.

My full source code can be found here:

https://github.com/ardnew/upd-touch/tree/alert-signal/Core/STUSB4500

My subroutine void stusb4500_alert(stusb4500_device_t *) in stusb4500_api.c corresponds to void ALARM_MANAGEMENT(uint8_t) in the STSW-STUSB003 package.

Any suggestions would be appreciated, thanks!

ardnew · ‎2019-12-10

Disregard this question, the code is actually behaving as expected! My problem appears to be related to a timing issue, where halting with breakpoints in the debugger was causing Type-C or PD messages to be dropped.

Not setting breakpoints during the PD negotiations was the fix.

Gregory Go. · ‎2020-01-17

Hi,

The Source PDOs are stored in the STUSB4500 buffer only for a short time.

If the software is too slow to read these registers (for instance because of a breakpoint), then the buffer will be overwritten by upcoming messages, and you will lose the Source PDO information.