RTOS/SW-EK-TM4C1294XL: SPI without CS results in bitshift

Volker Weber

Part Number: SW-EK-TM4C1294XL

Tool/software: TI-RTOS

Hi,

I continued playing around with your launchpad and your SPI loopback example. I modified it a bit by disconecting the CS line. The reason for this is a sensor that does acts a master and does not provide a CS line. Furthermore I pulled the PIN up and removed the handling out of the SPI configuration:

    GPIOTiva_PQ_1 | GPIO_CFG_IN_PU | GPIO_CFG_IN_INT_NONE

    SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI3);

    GPIOPinConfigure(GPIO_PQ0_SSI3CLK);
//    GPIOPinConfigure(GPIO_PQ1_SSI3FSS);
    GPIOPinConfigure(GPIO_PQ2_SSI3XDAT0);
    GPIOPinConfigure(GPIO_PQ3_SSI3XDAT1);

    GPIOPinTypeSSI(GPIO_PORTQ_BASE,
                   GPIO_PIN_0
//                   | GPIO_PIN_1
                   | GPIO_PIN_2
                   | GPIO_PIN_3);

As a result it works - at first glance. Sending the bytes 0xFF 0x00 results in receiving "bitshifted" messages over time. First I got a 0x7F 0x80 and finally a 0x00 0xFF. It seems that the application cannot detect the start of the frame properly. Is this an issue of setting up the SPI without CS?

over 8 years ago

0 Darius Williams over 8 years ago

TI__Expert 4440 points

Volker,

What type of behavior do you observe if you physically connect the CS pin to ground?

Derrick

0 Volker Weber over 8 years ago in reply to Darius Williams

Intellectual 570 points

Hi Derrick,

thanks for the input. Unfortunately the behavior stays the same. With a CS Pin the loopback example works totally well, without a still see this "bitshifting" behavior. In addition I saw that the effect increases when I choose a bigger dataSize (masterSpiParams.dataSize = 8, slave same). Are there any settings to tell the RTOS driver that I do not want to use the CS line or are there any operations that are triggered using this input?

0 Volker Weber over 8 years ago in reply to Volker Weber

Intellectual 570 points

/Update: I noticed that I used a slaveSpiParams.dataSize that is out of range for the normal implementation (>16). As a result: if I want to use such a large frame (>200), I cannot fall back to the RTOS solution and have to do it on my own, right? The device is just acting as a slave.

0 Darius Williams over 8 years ago in reply to Volker Weber

TI__Expert 4440 points

Volker,

You are correct; the SPI data frame size can be between 4-bits to 16-bits. This particular SPI driver uses the DMA controller. The DMA controller only supports data transfers of up to 1024 data frames.

You can therefore achieve (1024 data frames * 2 bytes) = 2048 byte transfers.

Refer here for more on the SPITivaDMA driver.

Derrick

0 Volker Weber over 8 years ago in reply to Darius Williams

Intellectual 570 points

Thanks for clarifying this issue Derrick, I appreciate this.

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RTOS/SW-EK-TM4C1294XL: SPI without CS results in bitshift