F429 USB OTG IN endpoint 0 enable bit is read-only (ref. manual mistake?)

[Albru]

EPENA bit for IN endpoint 0 (OTG_FS_DIEPCTL0 register) is marked as read-only both in the reference manual and in the SVD files for F429.

I believe that is a mistake, as the EPENA bit is used to start transmission on the endpoint and both libopencm3 and STM32 LL drivers ignore the fact that the bit is marked as read-only.

HAL_StatusTypeDef USB_EP0StartXfer(USB_OTG_GlobalTypeDef *USBx , USB_OTG_EPTypeDef *ep, uint8_t dma) {
  // other code
 
  USBx_INEP(ep->num)->DIEPCTL |= (USB_OTG_DIEPCTL_CNAK | USB_OTG_DIEPCTL_EPENA);
  
  // other code
}

It's possible that the DIEPCTL0 EPENA bit was confused with the DOEPCTL0 EPENA bit, which is marked as write-only, even though it should probably be read-only.

Would someone clarify this for me?

[waclawek.jan · Accepted solution]

> ref. manual mistake?

Yes.

These bits are not even rw, as they are set by user and cleared by hardware (DOEPCTL0 EPDIS is read-only, as EP0 Rx can't be disabled). This Synopsys OTG IP contains several such control bits which are changed by both user and hardware.

JW

[Albru]

Ah, makes sense, I completely forgot that bits the core controls have special access rights. Thanks for the answer.

Do you know where should I point this to get it fixed? I don't really mind that it's wrong in the reference manual, but my tooling is based upon SVD files ST provides, so I'd really like to have it fixed officially.

[waclawek.jan]

> my tooling is based upon SVD files ST provides, so I'd really like to have it fixed officially.

Unless some of the moderators chime in here - and I don't know if there is any path from here to SVD files - you might better contact ST directly through the web support form or through a FAE.

JW