RTOS/TM4C123GH6PGE: UART baud rate wrong

Richard Bland

Expert 2050 points

Part Number: TM4C123GH6PGE

Tool/software: TI-RTOS

I have a project using RTOS v 2_16_01_14, I am setting the Tiva clock source like this...

ROM_SysCtlClockSet(SYSCTL_SYSDIV_4 | SYSCTL_USE_PLL | SYSCTL_OSC_MAIN | SYSCTL_XTAL_25MHZ); //clocking from PLL at 50MHz
SystemClockFreq = ROM_SysCtlClockGet(); //verify 50MHz

I am configuring a UART for 38400 8N1 like this...

void initUART(void)
{
   UART_Params uartParams;

   SysCtlPeripheralEnable(SYSCTL_PERIPH_UART5);
   while(!SysCtlPeripheralReady(SYSCTL_PERIPH_UART5)) ;

   MAP_GPIOPinConfigure(GPIO_PE4_U5RX);
   MAP_GPIOPinTypeUART(GPIO_PORTE_BASE, GPIO_PIN_4);   //UART RX
   MAP_GPIOPinConfigure(GPIO_PE5_U5TX);
   MAP_GPIOPinTypeUART(GPIO_PORTE_BASE, GPIO_PIN_5);   //UART TX

   UART_init();

   UART_Params_init(&uartParams);
   uartParams.readDataMode = UART_DATA_BINARY;
   uartParams.writeDataMode = UART_DATA_BINARY;
   uartParams.baudRate = 38400;

   uart5 = UART_open(UART5, &uartParams);
   if (uart5 == NULL) {
       System_printf("Error opening UART5 midi");
       System_flush();
    }
}

But the baud rate is wrong when I use the UART. I observe UART_CC is 0x00 (System clock), but UART_IBRD has been set to 0x82(130) and UART_FBRD set to 0x0D(13). According to data sheet these should be IBRD: 50000000/(16x38400)=81.3802 and FBRD: 0.3802x64+0.5=24.

Can someone advise what I have got wrong please.

Thanks in advance, Richard

over 6 years ago

0 ToddMullanix over 6 years ago

TI__Guru* 96960 points

Hi Richard,

Can you make a simple CCS project that shows the issue and export it. It would be nice to have all the pieces (e.g. driver configuration, .cfg, etc.). The easiest way is to right-click the project, select Export..., General->Archive File, Next, verify the correct project was selected and give it a name (and Finish).

Todd

Edit: and have it be able to run on a TI LaunchPad.

0 Chester Gillon over 6 years ago

Guru 92251 points

Richard Bland said:
I have a project using RTOS v 2_16_01_14, I am setting the Tiva clock source like this...
ROM_SysCtlClockSet(SYSCTL_SYSDIV_4 | SYSCTL_USE_PLL | SYSCTL_OSC_MAIN | SYSCTL_XTAL_25MHZ); //clocking from PLL at 50MHz
SystemClockFreq = ROM_SysCtlClockGet(); //verify 50MHz

I think the clock configuration in the TI-RTOS setup and the ROM_SysCtlClockSet may be conflicting.

By default for a TM4C123 device during start-up, before the application main is called, TI-RTOS will configure the clock for 80MHz as per the following:

And TI-RTOS "computed" CPU frequency calculated by TI-RTOS is used as the expected system frequency which is passed to UARTConfigSetExpClk by the UARTTiva_open function in tirtos_tivac_2_16_00_08\products\tidrivers_tivac_2_16_00_08\packages\ti\drivers\uart\UARTTiva.c.

What I think may be causing your failure is that:

a. The expected CPU frequency used by TI-RTOS is 80MHz (the default when a TM4C123 project is created).

b. While your code calls ROM_SysCtlClockSet to set the system frequency to 50 MHz, the TI-RTOS UART driver is assuming the system frequency is 80 MHz leading to the incorrect baud rate.

Also, since the default TI-RTOS Clock Configuration assumes a 16.0 MHz crystal, but your board has a 25MHz crystal the system frequency set by the TI-RTOS startup code (before main is reached) may be overclocking the device during startup.

Therefore, suggest:

a. Remove the ROM_SysCtlClockSet call from the application.

b. In the TI-RTOS Clock Configuration set the desired 50 MHz system frequency based upon the 25 MHz crystal in use by setting the Boot - Clocking Options to:

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RTOS/TM4C123GH6PGE: UART baud rate wrong