Electronic – How to wait for a response from an SPI slave with an ATmega16

avrspi

I'm trying to connect to a slave in SPI net, I need to send a command and receive a response. I've used the below code for connection:

  char SPI_sendchar(char chr) {
      char receivedchar = 0;
      SPDR = chr;
      while(!(SPSR & (1<<SPIF)));
      receivedchar = SPDR;
      return (receivedchar);
  }

But I always "receivedchar" variable equal to variable "chr", I'm try to use this code without any slave in net but the result is the same.

Best Answer

What PeterJ said in his comment is correct. For any device that works based on commands, you must send a command to it like you are, then send a "dummy command" or just a character of data, say 0xFF.

Here is what happens with most SPI communication: the master sends a command, after the command is sent, the slave reads it and prepares its response. Then the slave sends its response at the same time the master sends the next byte.

What this means for you is that you need to add a dummy command:

char SPI_sendchar(char chr) {
  char receivedchar = 0;
  SPDR = chr;
  while(!(SPSR & (1<<SPIF)));

  SPDR = 0xFF;
  while(!(SPSR & (1<<SPIF)));

  receivedchar = SPDR;
  return (receivedchar);
}

Hope this helps.

Related Solutions

Electronic – Serial Peripheral Interface

A few points. That Maxim article targets a specific type of spi device. Mainly ones that are write only. Notice only MOSI is shown, not MISO. Additionally, these devices are chained, so data is shifted from one device to the other, like a shift register. (One common use of SPI peripherals on a microcontroller is to use them to drive common shift registers.)

SPI does not require the master to receive data. A slave is not required to respond. You can even have read only SPI devices, where the Master just clocks in data from the Slave (Only uses MISO). You can even have a 3 wire variation where MOSI and MISO are tied together (half duplex only). It all depends on the type of spi device you are using. Additionally, SPI can do Full Duplex transfers, but not all devices support this. As Wikipedia states, most devices do half duplex, even when wired in 4-wire mode.

In that note, the SPI devices are write only.

As for the clock frequency question, there is no auto negotiation of clock speeds. This is decided while the master is being programmed. You have to account for it.

Electronic – holding the SS pin high in SPI master mode on AVRs

The SS line ( of this devices or others ) is completely ignored in the example since it is supposed to controlled by the software ( you have to do it ) as it states that in page 163 of that datasheet:

When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled by user software before communication can start. W hen this is do ne, writing a byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the 8 bits into the Slave.

I now this is about the other device SS but it implies that you should take care of this line on both sides ,So even in the sending part it is supposed to drive the SS of the slave low but it is not included

    void SPI_MasterTransmit(char cData)
{
    /* Start transmission */
    // set slave SS line to low is missing here
    SPDR = cData;

    /* Wait for transmission complete */
    while (!(SPSR & (1<<SPIF)));
}

Best Answer

Related Solutions

Electronic – Serial Peripheral Interface

Electronic – holding the SS pin high in SPI master mode on AVRs

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