Flow control of BULK OUT endpoint

> When there is data available on the endpoint, I USB_ReadEP() it out into a circular buffer IF I've got room in that buffer. If there's no room (ie the device is still processing old data), I don't do the USB_ReadEP() and hence the CMD_CLR_BUF bit does not get set in the peripheral.

It's right on the rail, so far. But,

> I would have expected the Host to retry sending that packet some time later, at which point I would have room and read the endpoint and everyone would be happy. ... I can see at this time there are no interrupts being generated from the USB peripheral

This is a wrong assumption.
On the device side, the endpoint interrupt occurs just when a transaction completes successfully. But as the endpoint buffer is still occupied by the last transaction, the endpoint keeps NAKing to the OUT transactions. You have to call USB_ReadEP() to unload the data from the endpoint buffer, to receive the next transaction. The bulk endpoint has double buffer. It may keep two transactions, at most, pending on the buffer.

The outline of the code flow is as follows, based on USBCDC example of this samples.

Sample Code Bundle for LPC13xx Peripherals using Keil's MDK-ARM V1.06 (Jun 8, 2011)
ics.nxp.com/.../code.bundle.lpc13xx.keil.zip

1) A global variable, say U8 bulk_out_count, is declared and initialized to 0.
2) bulk_out_count is cleared in Set_Configuration handler

usbuser.c

#if USB_CONFIGURE_EVENT
void USB_Configure_Event (void) {

  if (USB_Configuration) {                  /* Check if USB is configured */
    /* add your code here */
    bulk_out_count = 0;      // <---------
  }
}
#endif

3) Read out bulk transaction in CDC_BulkOut(), when there is enough room for the packet. Else, count up bulk_out_count.

cdcuser.c

void CDC_BulkOut(void) {
  int numBytesRead;

  if ( _there_is_room_on_the_buffer_ ) {
    // read out the data to the buffer using USB_ReadEP()
  } else {
    ++bulk_out_count;
  }
}

4) The buffer is periodically checked in SOF handler.

usbcfg.h

#define USB_SOF_EVENT       1

usbuser.c

#if USB_SOF_EVENT
void USB_SOF_Event (void) {
  if ( bulk_out_count ) {
    // check the room on the buffer
    // if there is enough room, read a packet using USB_ReadEP()
    // and decrement bulk_out_count by one
  }
}
#endif

Tsuneo

Hi Tsuneo,

Thanks very much for the reply, nice idea putting the check in the SOF event handler.

By doing bulk_out_count++ it seems I get 1 more bulk_out_count than necessary, and get stuck in USB_ReadEP() in this loop:

    do
    {
        cnt = LPC_USB->RxPLen;
    }
    while ((cnt &PKT_DV) == 0);

By changing bulk_out_count++ to bulk_out_count=1 in CDC_BulkOut() things are working really well.

Thanks again,
Peter