fix(usb): add read delay, improved CDC latency

New CDC behavior: when the code reads data from CDC and the Tx FIFO is empty, we just return an empty buffer instead of waiting until there's some data to send.

Otherwise, this causes output latency in CircuitPython. Also, tested it against the hardware and it seem to be sending empty buffers when there's nothing to send over CDC, matching the new behavior.

Author: urish

src/peripherals/usb.ts

@@ -87,6 +87,7 @@ export class RPUSBController extends BasePeripheral {
   onEndpointWrite?: (endpoint: number, buffer: Uint8Array) => void;
   onEndpointRead?: (endpoint: number, byteCount: number) => void;
 
+  readDelayMicroseconds = 1;
   writeDelayMicroseconds = 1;
 
   get intStatus() {
@@ -197,7 +198,17 @@ export class RPUSBController extends BasePeripheral {
     }
   }
 
-  endpointReadDone(endpoint: number, buffer: Uint8Array) {
+  endpointReadDone(endpoint: number, buffer: Uint8Array, delay = this.readDelayMicroseconds) {
+    if (delay) {
+      this.rp2040.clock.createTimer(delay, () => {
+        this.finishRead(endpoint, buffer);
+      });
+    } else {
+      this.finishRead(endpoint, buffer);
+    }
+  }
+
+  private finishRead(endpoint: number, buffer: Uint8Array) {
     const bufferOffset = this.getEndpointBufferOffset(endpoint, true);
     const bufControlReg = EP0_OUT_BUFFER_CONTROL + endpoint * 8;
     let bufControl = this.rp2040.usbDPRAMView.getUint32(bufControlReg, true);

src/usb/cdc.ts

@@ -67,8 +67,6 @@ export class USBCDC {
   private descriptors: number[] = [];
   private outEndpoint = -1;
   private inEndpoint = -1;
-  private deviceWaitingForData = false;
-  private outBufferSize = 0;
 
   constructor(readonly usb: RPUSBController) {
     this.usb.onUSBEnabled = () => {
@@ -118,28 +116,15 @@ export class USBCDC {
     };
     this.usb.onEndpointRead = (endpoint, size) => {
       if (endpoint === this.outEndpoint) {
-        this.deviceWaitingForData = true;
-        this.outBufferSize = size;
-        if (!this.txFIFO.empty) {
-          this.sendDataToDevice();
+        const buffer = new Uint8Array(Math.min(size, this.txFIFO.itemCount));
+        for (let i = 0; i < buffer.length; i++) {
+          buffer[i] = this.txFIFO.pull();
         }
+        this.usb.endpointReadDone(this.outEndpoint, buffer);
       }
     };
   }
 
-  protected sendDataToDevice() {
-    if (!this.deviceWaitingForData) {
-      return;
-    }
-
-    const buffer = new Uint8Array(Math.min(this.outBufferSize, this.txFIFO.itemCount));
-    for (let i = 0; i < buffer.length; i++) {
-      buffer[i] = this.txFIFO.pull();
-    }
-    this.usb.endpointReadDone(this.outEndpoint, buffer);
-    this.deviceWaitingForData = false;
-  }
-
   private cdcSetControlLineState(value = CDC_DTR | CDC_RTS, interfaceNumber = 0) {
     this.usb.sendSetupPacket(
       createSetupPacket({
@@ -157,6 +142,5 @@ export class USBCDC {
 
   sendSerialByte(data: number) {
     this.txFIFO.push(data);
-    this.sendDataToDevice();
   }
 }