TM4C129ENCPDT: Processor appears to be driving RX (MISO) line high on SSI port

Tom Oldbury

I am using a TM4C129ENCPDT in legacy SSI mode.

It seems like the processor is keeping MISO high. My logic level on my slave varies between 3.3V and 2.5V. Disconnecting the master allows the slave voltage to swing correctly.

I am using SSI port 0 in legacy mode. Here is the configuration:

	// ** Initialise SSI0 module **
	SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI0);
	SSIDisable(SSI0_BASE);
	SSIConfigSetExpClk(SSI0_BASE, CPU_CLOCK, SSI_FRF_MOTO_MODE_0, SSI_MODE_MASTER, INTERFACE_CLOCK, 8);
	SSIEnable(SSI0_BASE);
	
	// ** Initialise GPIO **
	// Setup GPIO for SSI2
	GPIOPinConfigure(GPIO_PA2_SSI0CLK);
	GPIOPinConfigure(GPIO_PA3_SSI0FSS);
	GPIOPinConfigure(GPIO_PA4_SSI0XDAT0);
	GPIOPinConfigure(GPIO_PA5_SSI0XDAT1);
	GPIOPinTypeSSI(GPIO_PORTA_BASE, GPIO_PIN_2 | GPIO_PIN_3 | GPIO_PIN_4 | GPIO_PIN_5);

The problem goes away if I disable the initialisation for PA5, but then I won't be able to read data.

Is there a special configuration needed to get the device into legacy SSI mode? Why might it be driving the line high?

Kind regards

over 9 years ago

0 Charles Tsai over 9 years ago

TI__Guru**** 191886 points

Hi Tom,
Legacy mode is the default so if you did not otherwise change the mode then you are in legacy mode. I think there is an internal weak pullup enabled on the RX pin. Do you have any problem reading via the MISO from the external device?

0 Tom Oldbury over 9 years ago in reply to Charles Tsai

Intellectual 540 points

Hello Charles,

Thank you for your reply. The issue was resolved, it turned out to be a solder bridge from the SSI TX to RX line, so the TX line's high state was pulling up the RX line and preventing the slave from driving it properly.

Kind regards

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TM4C129ENCPDT: Processor appears to be driving RX (MISO) line high on SSI port