lpc1788: Weird behaviour of nxpusblib

Discussion created by lpcware Employee on Jun 15, 2016
Content originally posted in LPCWare by TKoe on Wed Jul 18 07:32:41 MST 2012

The nxpusblib seemed to have trouble in its current version with the length of the received bytes in the VirtualSerial mode. A patch by Vu Nguyen (http://lpcware.com/content/forum/nxpusblib-dualvirtualserial-example#comment-1419) fixed that. However, now I have the problem that it doesn't really receive any data anymore after I send data that's bigger than the maximum packet length minus one to the device. I set that to 64, so sending 63 bytes works fine, but one more and the data doesn't arrive in the end program.
For testing I made it so that my program will always send 63 Bytes, but any more and it stops working...

Here's the code that reads the received data. The FIFO-functions that I have were tested on other projects and work flawlessly. the FIFO is initialized to a size of 1000 bytes.
So whenever I get any data I push it into that FIFO which is read by a very simple state machine which basically just waits until enough data is present in the FIFO to continue operation.
void Get_USB_Data(void)
  uint16_t bytes_rec = CDC_Device_BytesReceived(&VirtualSerial_CDC_Interface);
  if (bytes_rec > 0)
    FIFO_push_byte(&RxFIFO, CDC_Device_ReceiveByte(&VirtualSerial_CDC_Interface));

// Pseudocode!
void simple_state_machine(void)
  uint8_t data[10];

  if (state == 1)
    if (FIFO_pop(&RxFIFO, data, 1))
      /* See what data[0] contains and switch state */
/* ..... */
  else if (state == 5)
    if (FIFO_pop(&RxFIFO, data, expected_length))
      /* got all data, return to wait */
      state = 1;

int main()

  for (;;)

And here's the C# code of the program that sends the data:
SerialPort p = new SerialPort("COM14", 115200, Parity.None, 8, StopBits.One);
int DATA_LEN = 63;
byte[] data = new byte[DATA_LEN];
byte[] get_data = new byte[DATA_LEN];

byte[] data_len = new byte[4];
data_len[0] = (byte)((DATA_LEN >> 24) & 0xFF);
data_len[1] = (byte)((DATA_LEN >> 16) & 0xFF);
data_len[2] = (byte)((DATA_LEN >>  8) & 0xFF);
data_len[3] = (byte)(DATA_LEN & 0xFF);
Random rand = new Random();
// Fill Tx-array with random data
for (int i = 0; i < DATA_LEN; i++)
  data = (byte)rand.Next(0xFF);

// Get ready to receive data
p.Write(new byte[] {0xAA}, 0, 1);

// Transmit data length that the device will receive
p.Write(data_len, 0, 4);

// Transmit data
p.Write(data, 0, DATA_LEN);

// Tell the device to transmit saved data back to the PC
p.Write(new byte[] {0xBB}, 0, 1);

// Receive data
for (int g = 0; g < DATA_LEN; g++)
  get_data[g] = (byte)(p.ReadByte() & 0xFF);

/* Checking if data and get_data are equal here */
Now if DATA_LEN is equal to or smaller than 63 everything works as expected. But as soon as I set it to 64 or higher the device doesn't seem to get the data as it is stuck in the "receive data" state.

How would I fix this?