Wait for PRE to finish. Comment timeout

src/pio_usb.c

@@ -49,14 +49,21 @@ static void __no_inline_not_in_flash_func(send_pre)(pio_port_t *pp) {
   SM_SET_CLKDIV(pp->pio_usb_tx, pp->sm_tx, pp->clk_div_fs_tx);
 
   pio_sm_exec(pp->pio_usb_tx, pp->sm_tx, pp->tx_start_instr);
+  pp->pio_usb_tx->irq = IRQ_TX_ALL_MASK;       // clear complete flag
   dma_channel_transfer_from_buffer_now(pp->tx_ch, pre_encoded,
                                        sizeof(pre_encoded));
-  pp->pio_usb_tx->irq = IRQ_TX_ALL_MASK;       // clear complete flag
 
   while ((pp->pio_usb_tx->irq & IRQ_TX_EOP_MASK) == 0) {
     continue;
   }
-  pio_sm_clear_fifos(pp->pio_usb_tx, pp->sm_tx);
+  // Wait for complete transmission of the PRE packet. We don't want to
+  // accidentally send trailing Ks in low speed mode due to an early start
+  // instruction that re-enables the outputs.
+  uint32_t stall_mask = 1 << (PIO_FDEBUG_TXSTALL_LSB + pp->sm_tx);
+  pp->pio_usb_tx->fdebug = stall_mask; // clear sticky stall mask bit
+  while (!(pp->pio_usb_tx->fdebug & stall_mask)) {
+    continue;
+  }
 
   // change bus speed to low-speed
   pp->low_speed = true;
@@ -149,8 +156,15 @@ static uint8_t __no_inline_not_in_flash_func(pio_usb_bus_wait_packet)(pio_port_t
 
   const uint16_t rx_buf_len = sizeof(pp->usb_rx_buffer) / sizeof(pp->usb_rx_buffer[0]);
 
+  // USB 2.0 specs: 7.1.19.1: handshake timeout
+  // Full-Speed (12 Mbps): 1 bit time = 1 / 12 MHz = 83.3 ns --> 16 bit times = 1.33 µs
+  // Low-Speed (1.5 Mbps): 1 bit time = 1 / 1.5 MHz = 666.7 ns --> 16 bit times = 10.67 µs
+
+  // We're starting the timing somewhere in the current microsecond so always assume the first one
+  // is less than a full microsecond. For example, a wait of 2 could actually be 1.1 microseconds.
+  // We will use 3 us (24 bit time) for Full speed and 12us (18 bit time) for Lowspeed.
   uint32_t start = time_us_32();
-  uint32_t timeout = pp->low_speed ? 20 : 3;
+  uint32_t timeout = pp->low_speed ? 12 : 3;
   while (time_us_32() - start <= timeout) {
     if ((pp->pio_usb_rx->irq & IRQ_RX_START_MASK) != 0) {
       break;