Received Data Zeroed Out After Encountering First Byte Equal to Zero - raspberry-pi

Question

Summarize the Problem:
I wrote a userspace SPI driver in linux for the NRF24L01+ transceiver. My goal is to send files to a server. A jetson nano is the sender, and a raspberry pi 3b+ the receiver. Both the spi and nano are running Linux.
I can consistently send packets and receive acknowledgements.
However, the issue is whenever I send a packet such as 0x ff ee dd 00 cc bb aa the receiver only receives the packet 0x ff ee dd 00 00 00 00. So what is happening is that whenever the first byte encountered is zero, the rest of the packet becomes zero. This causes the files I send to become corrupted.
I was able to reproduce this bug with a char array having a similar pattern. I noticed this trend when I printed out the file contents I was sending on the transmitter and receiver.
What I've tried:
I've tried altering my SPI read function. What I thought was happening was the chip select line was being flipped high early. This did not work, I got the same results.
I've printed the packets before calling the ioctl() function from the transmitter and the packet remains intact.
I've printed the return value of the ioctl() function to see how many bytes I was receiving and sending. I was sending 31 bytes from the transmitter, and receiving 32 bytes from the receiver. So it doesn't look like my reads and sends are failing.
If I had a logic analyzer my next step would be to check the SPI pins on the transmitter, but unfortunately I don't have one.
I've added a 10uF decoupling capacitor on the transceivers and that sped up communication.
Show Some Code:
Receiver side:
/**
* Reads the payload when data pipe
* is available.
*
* spi_dev_fd: file descriptor for spi device.
* */
int nrf_rx_read(int spi_dev_fd, char * payload, int * pipe, int * bytes)
{
int pipe_temp, rtn;
// TODO: Add timeout.
do
{
rtn = nrf_rx_pipe_available(spi_dev_fd, &pipe_temp);
}while(rtn != 0);
if(rtn == 0)
{
char status;
if(bytes != NULL)
{
char size;
spi_read_msg(spi_dev_fd, R_RX_PL_WID, &status, &size, 1);
*bytes = (int) size;
}
spi_read_msg(spi_dev_fd, R_RX_PAYLOAD , &status, payload, (int) NUM_PAYLOAD_BYTES);
*pipe = pipe_temp;
char msg;
msg = RX_DR;
spi_send_msg(spi_dev_fd, W_REGISTER | STATUS, &msg, 1);
return 0;
}
return 1;
}
bool nrf_rx_pipe_available(int spi_dev_fd, int * pipe)
{
char addr = NOP;
char status;
spi_read_msg(spi_dev_fd, addr, &status, NULL, 0);
if((status & RX_DR) > 0)
{
*pipe = (status >> RX_P_NO) & 0x07;
if(*pipe > 5)
{
return 1;
}
return 0;
}
return 1;
}
int spi_read_msg(int spi_dev_fd, char addr, char * status, char * copy_to, int len)
{
char data_buffer;
char recv_buffer[len + 1];
struct spi_ioc_transfer xfer;
memset(&xfer, 0, sizeof(xfer));
memset(&recv_buffer, 0, sizeof(recv_buffer));
data_buffer = addr;
xfer.tx_buf = (unsigned long) &data_buffer;
xfer.rx_buf = (unsigned long) recv_buffer;
xfer.len = len + 2;
xfer.bits_per_word = 8;
xfer.speed_hz = 1000000;
xfer.cs_change = 0;
xfer.rx_nbits = len * 8;
xfer.tx_nbits = 8;
int res = ioctl(spi_dev_fd, SPI_IOC_MESSAGE(1), xfer);
if(res > 0)
{
status[0] = recv_buffer[0];
if(copy_to != NULL)
{
string temp = string(recv_buffer);
temp = temp.substr(1);
strncpy(copy_to, temp.c_str(), len);
}
// debug code
for(int i = 0; i < len; ++i)
{
printf("copy_to: %x \n ", copy_to[i]);
}
// end debug code.
}
return res;
}
Transmitter side:
/**
* Function to load a payload and send a packet.
*
*
* spi_dev_fd: file descriptor for spi device.
* */
int nrf_tx_send_packet(int spi_dev_fd, char * payload, int len)
{
int rtn;
// Put low so we can add the payload.
gpio_set_value((unsigned int) GPIO_CE, (unsigned int) GPIO_LVL_LOW);
// Set a new payload.
nrf_tx_new_payload(spi_dev_fd, payload, len);
// Start tx transmission.
gpio_set_value((unsigned int) GPIO_CE, (unsigned int) GPIO_LVL_HIGH);
do
{
rtn = nrf_tx_pending_send(spi_dev_fd);
if(rtn == 2)
{
char clr = MAX_RT;
spi_send_msg(spi_dev_fd, W_REGISTER | STATUS, &clr, 1);
}
}while(rtn != 1);
// Go back to standby mode
gpio_set_value((unsigned int) GPIO_CE, (unsigned int) GPIO_LVL_LOW); // Setting chip enable to 0.
char reg = W_REGISTER | STATUS;
char val = RX_DR | TX_DS | MAX_RT;
spi_send_msg(spi_dev_fd, reg, &val, 1);
return 0;
}
int spi_send_msg(int spi_dev_fd, char addr, char * data, int len)
{
char data_buffer[len + 1];
char recv_buffer;
struct spi_ioc_transfer xfer;
memset(&xfer, 0, sizeof(xfer));
memset(&recv_buffer, 0, sizeof(recv_buffer));
data_buffer[0] = addr;
for(int i = 1; i < len + 1; ++i)
{
data_buffer[i] = data[i-1];
printf("databuffer[i]: %x \n", data_buffer[i]);
}
xfer.tx_buf = (unsigned long) data_buffer;
xfer.rx_buf = (unsigned long) NULL;
xfer.len = len + 1;
xfer.bits_per_word = 8;
xfer.speed_hz = 1000000;
xfer.cs_change = 0;
//xfer.rx_nbits = 8;
xfer.rx_nbits = 0;
xfer.tx_nbits = (8 * len) + 8;
int res = ioctl(spi_dev_fd, SPI_IOC_MESSAGE(1), xfer);
printf("res: %i \n", res);
return res;
}
I tried to add all the relevant code, sorry if it is a bit much. Main thing to look at is the send and receive functions. They all work as expected until I encounter the zeroed out byte.
If I am missing any information that can help someone out please let me know and I can add it. I think the send and receive functions are the most important however. I'm able to set and read the registers of the transceiver.

Answer 1

I can send files now!
The fix was done in spi_read_msg() function.
The problem was I was converting the buffer received to a string, which caused the data to be trimmed when the byte 0x00 was encountered. This is also equivalent to the null terminating character.
Receiver code:
int spi_read_msg(int spi_dev_fd, char addr, char * status, char * copy_to, int len)
{
char data_buffer;
char recv_buffer[len + 1];
struct spi_ioc_transfer xfer;
memset(&xfer, 0, sizeof(xfer));
memset(&recv_buffer, 0, sizeof(recv_buffer));
data_buffer = addr;
xfer.tx_buf = (unsigned long) &data_buffer;
xfer.rx_buf = (unsigned long) recv_buffer;
xfer.len = len + 2;
xfer.bits_per_word = 8;
xfer.speed_hz = 1000000;
xfer.cs_change = 0;
xfer.rx_nbits = len * 8;
xfer.tx_nbits = 8;
int res = ioctl(spi_dev_fd, SPI_IOC_MESSAGE(1), xfer);
if(res > 0)
{
status[0] = recv_buffer[0];
if(copy_to != NULL)
{
for(int i = 0; i < len; ++i)
{
copy_to[i] = recv_buffer[i + 1];
}
}
}
return res;
}