About UM2552 sample code

[Sodag.1]

I am building a USBPD sink with reference to the UM2552, but in the sample code on page 16, it seems that the ")" corresponding to the IF on the 9th line has disappeared, is this a typo? Also, is there a correct example somewhere?

[Yohann M.]

We could find examples in STM32cubeG0 package:

https://github.com/STMicroelectronics/STM32CubeG0/tree/master/Projects/STM32G081B-EVAL/Applications/USB-PD/USB-PD_Consumer_1port

[Sodag.1]

I have checked the package you gave me. It seems that the content is slightly different from UM2552, but is it this one?

[Dominique]

Hi @Sodag.1​ ,

The USBPD_DPM_Run function differs depending on whether you are in RTOS or bare metal system.

In case of RTOS, the USBPD_DPM_Run function is used to start the OS and USBPD_DPM_UserExecute is the user application function called inside a dedicated task.

void USBPD_DPM_Run(void)
{
  OS_KERNEL_START();
}

In the case of bare metal, the USBPD_DPM_Run function is an infinite loop that calls the various entry points of the USBPD stack + USBPD_DPM_Users (for the user application)

void USBPD_DPM_Run(void)
{
 uint32_t port = 0;
 
  do
  {
      USBPD_CAD_Process();
 
    for (port = 0; port < USBPD_PORT_COUNT; port++)
    {
      if ((HAL_GetTick() - DPM_Sleep_start[port]) >= DPM_Sleep_time[port])
      {
        DPM_Sleep_time[port] =
#ifdef _DRP
          USBPD_PE_StateMachine_DRP(port);
#elif _SRC
          USBPD_PE_StateMachine_SRC(port);
#elif _SNK
          USBPD_PE_StateMachine_SNK(port);
#endif /* _DRP */
        DPM_Sleep_start[port] = HAL_GetTick();
      }
    }
 
    USBPD_DPM_UserExecute(NULL);
  } while (1u == 1u);
}

A standard USBPD sytem must handle different activities

• state machine detection in charge to detect cable event (function USBPD_CAD_Process)
• policy engine in charge manage PD exchange ( USBPD_PE_StateMachine_*** depending the port capabilities)

Please could you provide us more detail about your issue ? and the type of application you want to generate ?

BR

[Sodag.1]

thank you for your answer. I'm trying to build a USBPD on bare metal without RTOS. So I was trying to refer to the "do～while" of the USBPD_DPM_Run function

I'm trying to build a USBPD_SINK to pick up an arbitrary voltage. I think that the necessary description should be written in the USBPD_PE_StateMachine_SNK function in the USBPD_DPM_Run function function, but is it wrong?

Best regards,

[Dominique]

Hi Sodag,

Bare metal brings some constraints unlike RTOS we don't have preemption management. So you must avoid blocking the system inside a task. The main constraint is on the management of USB-PD message exchanges, the standard requires responding to a message within a maximum of 30ms. So user tasks must not block the scheduling for more than 28ms to have a margin.

in your case the function void USBPD_DPM_Run(void) manages the task schedule and USBPD_DPM_UserExecute can be used to call application code

yes a sink port must use USBPD_PE_StateMachine_SNK(port);

BR

Dominique

[Sodag.1]

thank you for your answer. I have one more question. I am thinking about USBPD_SINK referring to the following Wiki. In this sample,

https://wiki.st.com/stm32mcu/wiki/STM32StepByStep:Getting_started_with_USB-Power_Delivery_Sink

I think that PD is managed by two tasks, "USBPD_CAD_Task" and "USBPD_PE_task", but where is the function actually executed in the task defined?

Best regards,​

[Dominique]

Hi @Sodag.1​ 

In your case, the scheduler is the while(1) instruction which will sequentially call the different functions: USBPD_CAD_Process, USBPD_PE_StateMachine_SNK and USBPD_DPM_UserExecute.

Present inside the delivered stack, there is a file which could help you better understand the difference between OS and bare metal system : Middlewares\ST\STM32_USBPD_Library\Core\src\usbpd_dpm_core.c

What is important for you is to see the correspondence

the call to USBDP_CAD_Process is made inside USBPD_CAD_Task

the call to USBPD_PE_StateMachine_SNK is made inside USBPD_PE_task

BR

Dominique

[Sodag.1]

thank you for your answer.

As a first step I'm trying to recreate the selected PDO retrieval system running on the Wiki without an RTOS.

In the sample, I think that the following two tasks are executed within the RTOS

・USBPD_CAD_Task (cable detection: USBPD_CAD_Process)

・USBPD_PE_Task (voltage adjustment, etc.: USBPD_PE_StateMachine_SNK)

So, if I don't use RTOS, I think that the basic operation will work if the above function in () is executed in While(1), but is that correct?

Also, looking at the explanations in the USBPD_CORE_RELEASE_User_Manual and UM2552, I didn't really understand the actual contents of the functions such as USBPD_CAD_Process.Is there a detailed explanation manual?

Best regards,

[Dominique]

Hello Sodag,

The USB-PD standard defines some rules for the cable detection (information available there https://www.usb.org/document-library/usb-type-cr-cable-and-connector-specification-release-22 ), the CAD process manages these rules through a state machine detection.

BR

Dominique