DRV8873S-Q1EVM: SPI transactions says 16-bits in user manual, but are two 8-bit in evaluation code

Hi Ryan,

Thank you for reply and explanation.

The DRV8873 user manual makes it seem like SPI transitions are done as a 16-bit word, and not as two transitions of 8-bit bytes.  It is not very clear to me, nor any of my colleagues, that SPI transitions are done as two transitions of 8-bit bytes from reading the user manual.

Section 7.5.1.1 SPI Format

• "SDI input data word is 16 bits long"
• "SDO output-data word is 16 bits long"

Section 7.5.1.2 SPI for a Single Slave Device

• "The SPI input-data (SDI) word consists of a 16-bit word"
• "If the data word sent to SDI pin is less than 16 bits or more than 16 bits, a frame error occurs and the data
  word is ignored."

Why is the user manual describing a 16-bit word for the SDI and SDO?

Best regards

Erek

Hi Ryan,

I am referring to document "drv8873-q1.pdf" "DRV8873-Q1 Automotive H-Bridge Motor Driver".

I am getting a total of 16-bits from DRV8873 device, but am getting it by doing two 8-bits transfers.  

In the "drv8873x-1q.c" source code provided (below) for the EVM for the MSP430 to communicate with the DRV8873 it actually does two 8-bit data transfers. Transmits 8-bits, then waits for transmit complete, then  receives 8-bits, then transmits another 8-bits, then waits for another transmit complete, then receives another 8-bits.  This is not a 16-bit transactions.  This is two 8-bit transactions.  A 16-bit transaction would have been to transmit 16-bits, wait for transmit complete, receive 16-bits.  This is what was expected from reading the pdf on DRV8873.  The DRV8873 does not do within a single SPI frame, 16-bits of data are sent in order for it to be valid. You can actually receive a valid upper 8-bits of data by only transmitting 8-bits of data.

The data sheet really does not seem to be correct, respectfully.

uint16_t drv8873xq1_readRegister(uint8_t address)
{
volatile uint16_t reg_value = 0;
volatile uint8_t dataMSB = 0;
volatile uint8_t dataLSB = 0;

reg_value |= SPI_RW_BIT_MASK; /*Set R/W bit*/
reg_value |= ((address << SPI_ADDRESS_POS) & SPI_ADDRESS_MASK); /* Configure register address value*/
//reg_value |= ((data << SPI_DATA_POS) & SPI_DATA_MASK); /*Adding data value */

P2OUT &= ~nSCS;//SPI set

while (!(IFG2 & UCB0TXIFG)); /* USCI_B0 TX buffer ready*/
UCB0TXBUF = (uint8_t)((reg_value >>8) & 0xFF); /* Transmit the Address(MSB Byte)*/

while(UCB0STAT & SPI_BUSY_FLAG)
{
; /* Wait till Transmission is complete*/
}
dataMSB = UCB0RXBUF; /* Recieve the First byte of data, MSB byte*/

UCB0TXBUF = 0x00; /* Transmit Second byte*/
while(UCB0STAT & SPI_BUSY_FLAG)
{
; /* Wait till Transmission is complete*/
}
dataLSB = UCB0RXBUF & 0xFF; /* Recieve the Second byte,LSB byte*/

P2OUT |= nSCS; //SPI Reset
_delay_cycles(10);

reg_value = ((((dataMSB<<8) | dataLSB) & SPI_DATA_MASK)>>SPI_DATA_POS); /* complete data */

return (reg_value);
}

Erek,

Please give me 24 hours to investigate this with some team members and I will post again.