Serial not reading correctly

[acrankyturtle]

Firmware

Adafruit CircuitPython 7.0.0-alpha.3 on 2021-06-03; Raspberry Pi Pico with rp2040

Code/REPL

import usb_cdc
import binascii

while True:
    # get serial data
    while usb_cdc.data.in_waiting > 0:
        print(binascii.hexlify(usb_cdc.data.read(256)))

Behavior

The data printed by the program does not match what I send.

Description

Any data I try to receive appears damaged or partially overwritten.

Here is a before and after of 128 bytes sent to my Pi:

Here is a before and after of 560 bytes sent:

[acrankyturtle]

This has to be some kind of timing related problem. Here is my current code:

import usb_cdc
import binascii

expectedData = 560
bufferSize = 512

print("Buffer size: " + str(bufferSize))

while True:
    # get serial data
    while usb_cdc.data.in_waiting > 0:
        data = bytearray(expectedData)
        pos = 0

        while pos < expectedData:
            readNum = min(bufferSize, expectedData - pos)
            time.sleep(0.01)
            buffer = usb_cdc.data.read(readNum)
            data[pos:pos+readNum] = buffer
            pos += bufferSize

        print(binascii.hexlify(data))

This reads 512 bytes at a time and it works. If I comment out the time.sleep call, it no longer works.

If anyone else reading this is having a problem, the time.sleep call is not necessary if you set bufferSize to 64. Or at least in my case that's true!