STM8 UART polling receive is mangles

Question

I'm trying to connect to an STM8 using uart. The STM seems to transmit data OK, but what it receives seems to be mostly junk, and often seems to receive 2 bytes at once. Here's the code:

#include "../stm8.h"


//
//  Setup the system clock to run at 16MHz using the internal oscillator.
//
void InitialiseSystemClock()
{
    CLK_ICKR = 0;                       //  Reset the Internal Clock Register.
    CLK_ICKR |= CLK_ICKR_HSIEN ;                 //  Enable the HSI.
    CLK_ECKR = 0;                       //  Disable the external clock.
    while ((CLK_ICKR & CLK_ICKR_HSIRDY) == 0);       //  Wait for the HSI to be ready for use.
    CLK_CKDIVR = 0;                     //  Ensure the clocks are running at full speed.
    CLK_PCKENR1 = 0xff;                 //  Enable all peripheral clocks.
    CLK_PCKENR2 = 0xff;                 //  Ditto.
    CLK_CCOR = 0;                       //  Turn off CCO.
    CLK_HSITRIMR = 0;                   //  Turn off any HSIU trimming.
    CLK_SWIMCCR = 0;                    //  Set SWIM to run at clock / 2.
    CLK_SWR = 0xe1;                     //  Use HSI as the clock source.
    CLK_SWCR = 0;                       //  Reset the clock switch control register.
    CLK_SWCR |= CLK_SWCR_SWEN;                  //  Enable switching.
    while ((CLK_SWCR & CLK_SWCR_SWBSY) != 0);        //  Pause while the clock switch is busy.
}


//
//  Setup the UART to run at 115200 baud, no parity, one stop bit, 8 data bits.
//
//  Important: This relies upon the system clock being set to run at 16 MHz.
//
void init_uart()
{
    //
    //  Clear the Idle Line Detected bit in the status register by a read
    //  to the UART1_SR register followed by a Read to the UART1_DR register.
    //
    //unsigned char tmp = UART1_SR;
    //tmp = UART1_DR;

    //UART1_SR = 0xC0; // mcarter set to default value

    //
    //  Reset the UART registers to the reset values.
    //
    UART1_CR1 = 0;
    UART1_CR2 = 0;
    UART1_CR4 = 0;
    UART1_CR3 = 0;
    UART1_CR5 = 0;
    UART1_GTR = 0;
    UART1_PSCR = 0;
    //
    //  Now setup the port to 115200,n,8,1.
    //

    // clear certain bits
    UART1_CR1 &= ~UART1_CR1_M ;        //  8 Data bits.
    UART1_CR1 &= ~UART1_CR1_PCEN;     //  Disable parity

    // stop bits
    UART1_CR3 &= 0b11001111; // unmask the stop bit to default (1 stop bit)
    //UART1_CR3 |= 0b00100000; // two stop bits
    //UART1_CR3 |= 0b00110000; // 1.5 stop bits
    //UART1_CR3 &= ~UART1_CR3_STOP;     //  1 stop bit.

#if 1 //115200 baud
    //UART1_BRR2 = 0x0a;      //  given in original example
    UART1_BRR2 = 0x0b;      //  Set the baud rate registers to 115200 baud
    UART1_BRR1 = 0x08;      //  based upon a 16 MHz system clock.
#else // 9600 baud, but seems to be worse than 115200
    UART1_BRR2 = 0x03;
    UART1_BRR1 = 0x69;
#endif

    //
    //  Disable the transmitter and receiver.
    //
    //UART1_CR2_TEN = 0;      //  Disable transmit.
    //UART1_CR2_REN = 0;      //  Disable receive.
    //
    //  Set the clock polarity, lock phase and last bit clock pulse.
    //
    UART1_CR3 |= UART1_CR3_CPOL;
    UART1_CR3 |= UART1_CR3_CPHA;
    //UART1_CR3 |= UART1_CR3_LBCL; // this seems to cause problems

    UART1_CR2 |= UART1_CR2_TEN; // enable transmit
    UART1_CR2 |= UART1_CR2_REN; // enable receive
    UART1_CR3 |= UART1_CR3_CLKEN; // unable uart clock
}


char uart_getc()
{
    while((UART1_SR & UART1_SR_RXNE)==0); //  Block until char rec'd
    //char c =  UART1_DR;
    //return c;
    return UART1_DR;
}

void uart_putc(char c)
{
    while((UART1_SR & UART1_SR_TXE)==0); //  Wait for transmission complete
    UART1_DR = c; //  transmit char
}

void UARTPrintf(char *message)
{
    char *ch = message;
    while (*ch)
        uart_putc(*ch++);
}

void main()
{
    disable_interrupts();
    InitialiseSystemClock();
    init_uart();
    enable_interrupts();
    UARTPrintf("Uart example: you type, I echo\n\r");
    while (1)
    {
        //continue;
        char c = uart_getc();
        uart_putc(c);
        //UARTPrintf("Hello from my microcontroller....\n\r");
        //for (long counter = 0; counter < 2500000; counter++);
    }
}

Relevant declaration headers are:

#define UART1_SR   *(uchar*)(0x5230)
#define UART1_DR   *(uchar*)(0x5231)
#define UART1_BRR1 *(uchar*)(0x5232)
#define UART1_BRR2 *(uchar*)(0x5233)
#define UART1_CR1  *(uchar*)(0x5234)
#define UART1_CR2  *(uchar*)(0x5235)
#define UART1_CR3  *(uchar*)(0x5236)
#define UART1_CR4  *(uchar*)(0x5237)
#define UART1_CR5  *(uchar*)(0x5238)
#define UART1_GTR  *(uchar*)(0x5239)
#define UART1_PSCR *(uchar*)(0x523A)


#define UART1_CR1_M (1<<4)
#define UART1_CR1_PCEN (1<<2)

#define UART1_CR2_TEN           (1<<3)
#define UART1_CR2_REN           (1<<2)

#define UART1_CR3_STOP 4
#define UART1_CR3_CPOL (1<<2)
#define UART1_CR3_CPHA (1<<1)
#define UART1_CR3_LBCL (1<<0)
#define UART1_CR3_CLKEN (1<<3)

#define UART1_SR_TXE            (1<<7)
#define UART1_SR_TC             (1<<6)
#define UART1_SR_RXNE           (1<<5)

I'm not really sure about stop bits, and all that. It's just "regular" serial communication.

I found that if I uncommented the line

//UART1_CR3 |= UART1_CR3_LBCL; // this seems to cause problems

then the stm8 prints out a continuous stream of junk. But with it commented out, the mcu seems to correctly know that there has been a transmission. There doesn't seem to be any pattern as to what it sees, though.

Answer 1

Hmm. The offending line seems to be

UART1_CR3 |= UART1_CR3_CLKEN;

It's purpose seem to be to "enable the SCLK pin". I don't really understand what's going on here, but according to a pinout diagram, one of the purposes of pin PD4 is UART1_CK. So you can attach a UART clock to the STM8 and this enables it?? And thereby causes problems if a clock isn't attached. It doesn't make that much sense, really; I didn't know uarts could have external clocks.

Anyway, commenting out the line seems to have fixed things.