Pico machine.Serial.Write drops leading data and output unordered if data is larger than ~500 bytes.

[soypat]

Likely DMA race condition. Chunking the data into 500 sized chunks and sleeping between the writes fixes the issue.

package main

import (
	"machine"
	"strconv"
	"time"
)

func main() {
	time.Sleep(2 * time.Second) // connect via serial
	buf1 := makeBuffer('|', 600)
	buf2 := makeBuffer('/', 600)
	println("start")
	serialWrite(buf1)
	serialWrite(buf2)
}

func makeBuffer(sep byte, size int) []byte {
	buf := make([]byte, size)
	for i := 0; i < size-5; i += 5 {
		buf[i] = sep
		strconv.AppendInt(buf[i+1:i+1:i+5], int64(i), 10)
	}
	return buf
}

func serialWrite(b []byte) {
	machine.Serial.Write(b)
}

Got

tinygo flash -target=pico -scheduler=tasks -monitor ./examples/pico-bug
Connected to /dev/ttyACM0. Press Ctrl-C to exit.
s505|510|515|520|525|530|535|540|545|550|555|560|56580/485/490/495/500/505/510/515/520/525/530/535/540/545/550/555/560/565/570/575/580/585/590/100/105/110/115/120/125/130/135/140/145/150/155/160/165/170/175/180/185/190/195/200/205/210/215/220/225/230/235/240/245/250/255/260/265/270/275/280/285/290/295/300/305/310/315/320/325/330/335/340/345/350/355/360/365/370/375/380/385/390/395/400/405/410/415/420/425/430/435/440/445/450/455/460/465/470/475/480/485/490/495/500/505/510/515/520/525/530/535/540/545/550/555/560/565/570/575/580/585/590/100/105/110/115/120/125/130/135/140/145/150/155/160/165/170/175/180/185/190/195/200/205/210/215/220/225/230/235/240/245/250/255/260/265/270/275/280/285/290/295/300/305/310/315/320/325/330/335/340/345/350/355/360/365/370/375/380/385/390/395/400/405/410/415/420/425/430/435/440/445/450/455/460/465/470/475/480/485/490/495/500/505/510/515/520/525/530/535/540/545/550/555/560/565/570/575/580/585/590

Expected

start
|0|5|10|15|20|25|30|35|40|45|50|55|60|65|70|75|80|85|90|95|100|105|110|115|120|125|130|135|140|145|150|155|160|165|170|175|180|185|190|195|200|205|210|215|220|225|230|235|240|245|250|255|260|265|270|275|280|285|290|295|300|305|310|315|320|325|330|335|340|345|350|355|360|365|370|375|380|385|390|395|400|405|410|415|420|425|430|435|440|445|450|455|460|465|470|475|480|485|490|495|500|505|510|515|520|525|530|535|540|545|550|555|560|565|570|575|580|585|590
/0/5/10/15/20/25/30/35/40/45/50/55/60/65/70/75/80/85/90/95/100/105/110/115/120/125/130/135/140/145/150/155/160/165/170/175/180/185/190/195/200/205/210/215/220/225/230/235/240/245/250/255/260/265/270/275/280/285/290/295/300/305/310/315/320/325/330/335/340/345/350/355/360/365/370/375/380/385/390/395/400/405/410/415/420/425/430/435/440/445/450/455/460/465/470/475/480/485/490/495/500/505/510/515/520/525/530/535/540/545/550/555/560/565/570/575/580/585/590

[sago35]

The txringbuffer consists of eight 64-byte buffers. If you try to write more data than this simultaneously, the newer data will overwrite the earlier data. Theoretically, up to 64 * 8 = 512 bytes can be buffered.

In this case, since you're trying to write 600 x 2 bytes, some data is being overwritten before it's transmitted. To fix this, I believe it's necessary to make the txringbuffer of USBCDC behave more like io.Writer. Also, since machine.Serial.Write() behaves differently from io.Writer, I think it should be corrected.

https://github.com/tinygo-org/tinygo/blob/v0.40.1/src/machine/usb/cdc/buffer.go#L77-L102