Compiler: SPI MISO idle state

user5724612

Tool/software: TI C/C++ Compiler

Good morning,

I am workying on a device that uses SPI communications. I have notices that the MISO cable is always high when a trasnference is not in process. Is there any way to make it stay low?

Thank you.

over 6 years ago

0 George Mock over 6 years ago

TI__Guru**** 251180 points

Unfortunately, I lack the expertise needed to help you. Which TI device do you use? Based on that, I can probably move this thread to the right forum.

Thanks and regards,

-George

0 user5724612 over 6 years ago in reply to George Mock

Intellectual 340 points

I'm using a MSP432P4111.

What I am trying to do is communicate one master with multiple slaves. My problema was that the MISO stayed high corrupting the communications. I was able to make the System workying by doing this:

sem_wait(&slaveSem);

        slaveSpi = SPI_open(Board_SPI1, &slaveSpiParams);
        if (slaveSpi == NULL) {
            printf("Error initializing slave SPI\n");
            while (1);
        }
        else {
            printf("\nSlave SPI initialized");
        }

        transferOK = SPI_transfer(slaveSpi, &slaveTransaction);
        SPI_close(slaveSpi);
        GPIO_setAsInputPin(GPIO_PORT_P3, GPIO_PIN7);

Port 3_7 is the SS. As I set it to an input it no longer stays in ghigh but I am sure this is no the proper way of solving this.

Thank you very much for your answer.

0 Chris Sterzik over 6 years ago in reply to user5724612

TI__Guru 68021 points

I do not see an issue with doing it that way. You can also try to use the 4-pin SPI slave mode as a type of signal to put SOMI in the input direction.

const SPIMSP432DMA_HWAttrsV1 spiMSP432DMAHWAttrs[MSP_EXP432P4111_SPICOUNT] = {
    {
        .baseAddr = EUSCI_B0_BASE,
        .bitOrder = EUSCI_B_SPI_MSB_FIRST,
        .clockSource = EUSCI_B_SPI_CLOCKSOURCE_SMCLK,
        .defaultTxBufValue = 0xFF,
        .dmaIntNum = INT_DMA_INT1,
        .intPriority = (~0),
        .rxDMAChannelIndex = DMA_CH1_EUSCIB0RX0,
        .txDMAChannelIndex = DMA_CH0_EUSCIB0TX0,
        .clkPin  = SPIMSP432DMA_P1_5_UCB0CLK,
        .simoPin = SPIMSP432DMA_P1_6_UCB0SIMO,
        .somiPin = SPIMSP432DMA_P1_7_UCB0SOMI,
        .stePin  = SPIMSP432DMA_P1_4_UCB0STE,
        .pinMode  = EUSCI_SPI_3PIN,
        .minDmaTransferSize = 10
    },

#define EUSCI_SPI_3PIN                      EUSCI_B_CTLW0_MODE_0
#define EUSCI_SPI_4PIN_UCxSTE_ACTIVE_HIGH   EUSCI_B_CTLW0_MODE_1
#define EUSCI_SPI_4PIN_UCxSTE_ACTIVE_LOW    EUSCI_B_CTLW0_MODE_2

See the trm for a more details, ti.com/lit/pdf/slau356 . Section 25.3.4.1 4-Pin Slave Mode

25.3.4.1 4-Pin SPI Slave Mode
In 4-pin slave mode, UCxSTE is a digital input used by the slave to enable the transmit and receive
operations and is driven by the SPI master. When UCxSTE is in the slave-active state, the slave operates
normally. When UCxSTE is in the slave- inactive state:
• Any receive operation in progress on UCxSIMO is halted.
• UCxSOMI is set to the input direction.
• The shift operation is halted until the UCxSTE line transitions into the slave transmit active state.
The UCxSTE input signal is not used in 3-pin slave mode.

Regards,

Chris

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Compiler: SPI MISO idle state