USB NYET response despite of free buffer (LPC55S69 USB1 HS Controller)

pettel · ‎10-07-2025

I'm using EP2 OUT with double buffering.

Buf0 and Buf1 have 2048 bytes each
EP2 OUT is a bulk endpoint with a maximum packet size of 512 bytes

Both endpoints (EP2_OUT_BUF0, EP2_OUT_BUF1) are initialized with 2048 N-bytes and the endpoints are set to active.

Expectation: without further interaction 4096 bytes should be received, without a NYET response.

Observed behaviour: after Buf0 is full a NYET response is transmitted instead of an ACK response, despite Buf1 beeing completely empty and active.

This behaviour is especially detrimental to performance if you work with 512 byte buffers for Buf0 and Buf1, because in this case a NYET response is transmitted after every OUT transmission from the host, which requires an unnecessary USB-Ping and reduces throughput.

Is there a workaround or is this a hardware bug?

EdwinHz · ‎10-14-2025

Hi @pettel,

This is by design of the USB stack implementation of the SDK. As you can see on a comment on "PERI_USBHSD.h":

DATA_PENDING - As long as this bit is set to one and LPM supported bit is set to one, HW will
* return a NYET handshake on every LPM token it receives. If LPM supported bit is set to one and
* this bit is zero, HW will return an ACK handshake on every LPM token it receives. If SW has
* still data pending and LPM is supported, it must set this bit to 1.

pettel · ‎10-16-2025

Hi @EdwinHz ,

this topic is not about LPM tokens. It's about sending user data from the USB host to EP2 of the LPC55S69. Link Power Management and the associated transactions are a completely separate topic.