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TMS320F28379D: SPI communication with 8 bits length

Henry Liu
Prodigy 120 points
Part Number: TMS320F28379D

Hello,

I have TMS320F28379D which connects to a RTC module via SPI interface (4-wire).
The RTC module provides 8 bits length address and data.

When I configured the SPI with 16 bits (SPICHAR = 15) and used 16 bits read/write, the results are correct:

 spi_writeByte(0x0100); 
 second = spi_readByte(); //read register 0x01
However if the length of data is set to 8 (SPICHAR = 7), all the received data is wrong in the following two cases:

1.
spi_writeByte(0x01);            // seconds register read address
spi_readByte();                 // dummy read
spi_writeByte(0x01); 
second = spi_readByte(); //read register 0x01

2.
spi_writeByte(0x0100); 
second = spi_readByte(); //read register 0x01

Am I missing something?

Thanks,
HL

  • 0
    Expert 1500 points
    Hi Henry,

    For my application:
    SpiaRegs.SPICCR.bit.SPICHAR = 0xB; // 3:0 Character Length Control 12 bits

    This is how I transmitted 12 bits
    SpiaRegs.SPITXBUF = Vout1R << 4;

    This is how I got the slave data back.
    rdata_b = SpibRegs.SPIRXBUF & 0x0FFF;
  • 0 in reply to programmable solution
    Prodigy 120 points
    Thanks for your reply.

    When the length is set to 8, The scope shows 8 clock pulses when a write is in progress, but there is no clocks for the following read.
    Do I have to do a dummy or next write in order to read? Or am I missing something in SPI master configuration?

    HL
  • 0 in reply to Henry Liu
    Expert 1500 points

    The read serial bit stream is simultaneously clocked into the register with the transmit.clock

    After the data is transmitted the slave data has already shifted into the register.
    The read reads the contents of the register.

  • 0 in reply to programmable solution
    Prodigy 120 points
    The master needs to send the address first and then the slave shifts out the corresponding value to the master.

    HL
  • 0
    TI__Mastermind 23610 points

    Hi Henry

    Did you get your problem resolved or do you still have some problems?

    -Marlyn

  • 0 in reply to Marlyn Rosales Castaneda20
    Prodigy 120 points
    Hi Marlyn,

    Yes, I send the address left-justified – 0x0100 for address 01.
    The problem is that the read value will come after 8 clock pulses, but SPI master provides only 8 clock pulses if the length is set to 8.

    Thanks,
    HL
  • 0 in reply to Henry Liu
    Expert 1500 points

    Henry,
    For SPI, the master is the only one that supplies the clocks and it does so only during transmit.
    If your custom design is to transmit an address, then the slave receives it and it is in the pipeline to respond during the next transmit.

    Is your slave a 8 bit or 16 bit slave? normally it is the same as the master.

    For my application example, the address is always the same so I transmit out the value that was converted by the ADC in the uC that I use to graph the response and the slave transmits a voltage representation of the model's calculated output back to the uC during the same transmit.