How to correctly receive data through SPI enhanced buffer mode of Microchip PIC24F microcontrollers?

Question

I'm programming a SPI communication with an external RF chip. The microcontroller is the model PIC24FJ64GA102 from Microchip.

I want to use the enhanced buffer mode of the SPI.

Problem: Get the received bytes out of the receive buffer.

Used SPI function:

void SPI1_get(uint8_t* data, uint16_t length) {
    uint16_t i = 0, l = length;
    uint8_t dummy;
    while (length > 0) {
        while (SPI1STATbits.SPITBF || SPI1STATbits.SPIRBF) {
            dummy = SPI1STAT;
        }
        do {
            SPI1BUF = 0xff;
        } while (SPI1STATbits.SPIRBF == 0 && --length > 0);
        do {
            while (SPI1STATbits.SRMPT == 0) {
            }
            data[i] = SPI1BUF;
            ++i;
        } while (i < l && SPI1STATbits.SRXMPT != 1);
    }
}

Here the calls:

uint8_t cmd[2]; cmd[0] = length; cmd[1] = address;
SPI1_put(cmd,2); // e.g: 0x02, 0x01
SPI1_get(buf,2); // e.g: 0x05, 0x01 (received data)

The communication is just fine, checked with an oscilloscope and SPI decoding module. The data on the SPI bus are like in the comment above: sent 0x02 0x01 0xff 0xff, received 0x00 0x00 0x05 0x01, but the function above does not correctly retrieve the data out of the receive buffer. I've already tested a lot of constellations of checking flags and interrupts but in the end the best result I can get is: 0x00 0x01 (only the last byte is correct).

Also already checked the errata sheet, where two SPI problems are mentioned which do not (should not) affect my code.

What the hell am I doing wrong?!

As requested here the SPI1_put() function:

void SPI1_put(uint8_t* data, uint16_t length) {
    uint16_t i = 0;
    uint8_t dummy;
    for (; i < length; ++i) {
        while (SPI1STATbits.SPITBF)
            ; // maybe change to (_SPI1BEC == 7) ?
        SPI1BUF = data[i];
        dummy = SPI1BUF; //dummy read
    }
}

[latest edit: 2015-02-05]

So today I was able to spend some more time on this particular issue, and came up with a port of ElderBug's suggestion, also taking care of the bugs mentioned in the errata sheet:

uint8_t out_buf[128];
uint8_t in_buf[128];

void SPI1_com(uint8_t* out, uint8_t* in, uint16_t out_len, uint16_t in_len) {
    uint16_t len = out_len + in_len;
    uint16_t sent = 0, recv = 0, i = 0;
//  while (!SPI1STATbits.SRXMPT)
    sent = SPI1BUF; // empty buffer
    sent = SPI1BUF; // empty buffer
    sent = 0;
    if (out != out_buf && out != 0)
        memcpy(out_buf, out, out_len);
    while (sent < len && recv < len) {
        if (SPI1STATbits.SPIBEC != 7 && sent < len) {
            SPI1BUF = out_buf[sent++];
        }
        if (!SPI1STATbits.SRXMPT && recv < len) {
            in_buf[recv] = SPI1BUF, recv++;
        }
    }
    if (in != 0) {
        for (i = 0; i < in_len; ++i) {
            in[i] = in_buf[out_len + i];
        }
//      memcpy(in, in_buf + out_len, in_len);
    }
    for (i = 0; i < len; ++i) {
        out_buf[i] = 0xff;
        in_buf[i] = 0xff;
    }
}

This code basically works. With an exception which I was not able to work around:

The communication works as follows:

1byte: r/w-bit+length
1byte: address
1-127byte: data

So, when I read 1 byte from the slave chip, meaning sending 3 bytes (rw+len, address, dummybyte), the data I have in my in_buf buffer is 0xFF. Now a weird thing: When I just read one more byte, without changing anything (just one more dummy byte on the bus), the 1st byte of my in_buf has got the correct data. But this seems not to work always, because my program still stucks at some points.

I'm left sitting here with a lot of question marks.

Weird thing the 2nd: Reading 8 bytes, data correct in my buffer until the last byte, last byte is 0xFF, should be 0x00. Wtf?

PS: Already filed a ticket at Microchip for support in this issue.

Could you post the code for SPI1_put ? The problem can also come from there. — ElderBug
Just a wild guess: in SPI1_put function, shouldn't the for loop's step section have i++ instead of ++i? Because now you never process the 0th byte of the buffer. — Eimantas
That's just not correct. In a for loop it doesn't matter if your counter is incremented with c++ or ++c. And as I wrote in my question, the data on the bus itself (measured with a scope) is completely correct. The problem here is to get the received data out of the SPI enhanced buffer, not the communication itself. — qwc

ElderBug ElderBug · Accepted Answer · 2015-01-22T11:17:35

There is a problem in your SPI1_put.

In your SPI1_put function, you read SPI1BUF right after starting the transfer, and thus you read when the transfer is not complete (correct me if I'm wrong, but reading when the FIFO is empty doesn't block and just return the previous byte). The function should be something like :

void SPI1_put(uint8_t* data, uint16_t length) {
    uint16_t sent = 0;
    uint16_t rec = 0;
    uint8_t dummy;
    while(1)
    {
        if( !SPI1STATbits.SPITBF && sent < length )
            SPI1BUF = data[sent++];
        if( !SPI1STATbits.SRXMPT && rec < length )
            dummy = SPI1BUF, rec++;
        if( sent == length && rec == length )
            break;
    }
}

About the SPI1_get, I think it's fine, but I'm not sure. The thing is, it should look very similar to SPI1_put, because SPI is really symmetric in how it works :

void SPI1_get(uint8_t* data, uint16_t length) {
    uint16_t sent = 0;
    uint16_t rec = 0;
    while(1)
    {
        if( !SPI1STATbits.SPITBF && sent < length )
            SPI1BUF = 0xff, sent++;
        if( !SPI1STATbits.SRXMPT && rec < length )
            data[rec++] = SPI1BUF;
        if( sent == length && rec == length )
            break;
    }
}

Overall, since you only do synchronous calls, the enhanced buffer is not really useful. You could have simpler functions with the same results just by doing 'send, wait, receive' for each byte.

EDIT after your update :

Your new code looks correct, except for the condition in the while that you did wrong. Here is the corrected code with some modifications with comments that may improve it (removed the buffer copies) :

// No additional buffers

void SPI1_com(uint8_t* out, uint8_t* in, uint16_t out_len, uint16_t in_len) {
    uint16_t len = out_len + in_len;
    uint16_t sent = 0, recv = 0, i;
    uint16_t dummy;


    // After the call, SPI comms should have ended and RX FIFO should be empty.
    // If SPI1STATbits.SRXMPT is not 1 here,
    // it means there is a problem in the code. Report it somehow for debug ?
    // if( !SPI1STATbits.SRXMPT )
    //     error!

    // This loop is harmless but shouldnt be needed
    //while (!SPI1STATbits.SRXMPT)
        //dummy = SPI1BUF; // empty buffer

    i = 0;
    while (sent < len || recv < len) {
        if (SPI1STATbits.SPIBEC != 7 && sent < len) {
            // Here we are out of the buffer when sent>=out_len,
            // but it's ok because random bytes are fine here
            SPI1BUF = out[sent++];
        }
        if (!SPI1STATbits.SRXMPT && recv < len) {
            if( recv < out_len )
                // Before out_len, discard the data
                dummy = SPI1BUF;
            else
                // After out_len, store the data in in[]
                in[i++] = SPI1BUF;
            recv++;
        }
    }

}

Does this correct any bug ? Also, this may sound stupid, but are you calling that correctly ? I don't know what you did, but for your 1 byte read, it should be SPI1_com(two_byte_buffer,one_byte_buffer,2,1);.

One more point : it seems that you always try to empty the RX FIFO at the beginning. Is there actually something in the FIFO ? SPI1STATbits.SRXMPT MUST be 1 without even trying to empty. If it isn't, there IS a bug somewhere in software. Do you have any other code using the SPI ?

How to correctly receive data through SPI enhanced buffer mode of Microchip PIC24F microcontrollers?

1 Answers