Month: May 2018

Using Flash memory on the STM32F103

Frank6 Comments

Some microcontrollers have a dedicated Non-Volatile-Memory (NVM) bank for storing calibration data, program settings and so on. The STM32F103C8T6 does not have NVM like this but it’s Flash program memory can be used with care for the same purpose. The Flash memory in this chip is divided into 1kiB sectors and there are 64 if them (0 to 63). The code to erase, write and read a sector is shown below:

int  writeSector(uint32_t Address,void * values, uint16_t size)
{              
    uint16_t *AddressPtr;
    uint16_t *valuePtr;
    AddressPtr = (uint16_t *)Address;
    valuePtr=(uint16_t *)values;
    size = size / 2;  // incoming value is expressed in bytes, not 16 bit words
    while(size) {        
        // unlock the flash 
        // Key 1 : 0x45670123
        // Key 2 : 0xCDEF89AB
        FLASH->KEYR = 0x45670123;
        FLASH->KEYR = 0xCDEF89AB;
        FLASH->CR &= ~BIT1; // ensure PER is low
        FLASH->CR |= BIT0;  // set the PG bit        
        *(AddressPtr) = *(valuePtr);
        while(FLASH->SR & BIT0); // wait while busy
        if (FLASH->SR & BIT2)
            return -1; // flash not erased to begin with
        if (FLASH->SR & BIT4)
            return -2; // write protect error
        AddressPtr++;
        valuePtr++;
        size--;
    }    
    return 0;    
}
void eraseSector(uint32_t SectorStartAddress)
{
    FLASH->KEYR = 0x45670123;
    FLASH->KEYR = 0xCDEF89AB;
    FLASH->CR &= ~BIT0;  // Ensure PG bit is low
    FLASH->CR |= BIT1; // set the PER bit
    FLASH->AR = SectorStartAddress;
    FLASH->CR |= BIT6; // set the start bit 
    while(FLASH->SR & BIT0); // wait while busy
}
void readSector(uint32_t SectorStartAddress, void * values, uint16_t size)
{
    uint16_t *AddressPtr;
    uint16_t *valuePtr;
    AddressPtr = (uint16_t *)SectorStartAddress;
    valuePtr=(uint16_t *)values;
    size = size/2; // incoming value is expressed in bytes, not 16 bit words
    while(size)
    {
        *((uint16_t *)valuePtr)=*((uint16_t *)AddressPtr);
        valuePtr++;
        AddressPtr++;
        size--;
    }
}

Writes and reads must be performed in 16 bit (half word) chunks so a little bit of type casting is necessary. The data type for the values to be read or written is void *. This allows a pointer to any type be passed without generating warnings. The sector start address must be on a 1k boundary. In this chip, Flash memory starts at 0x8000000 so sector start addresses can be 0x8000000, 0x8000400, 0x8000800, 0x8000c00, etc.
The data sheet states that the flash can withstand at least 10000 write cycles (does this include erases?). This seems like a pretty large amount but you would be surprised how quickly it can get used up. If your program requires to write to Flash memory regularly consider some wear leveling scheme.
Full code is available over here on GitHub

Serial transmission on the SAMD20

FrankLeave a comment

This is a follow up to a previous post about bare-metal programming of the SAMD20 MCU. It has been tricky. There is a very complex clock system in the SAMD20 which is difficult to set up. The UART itself is pretty straight forward however and I’ve added a simple UART transmit example to the github repository.
Shown below is a listing for the UART related and main functions. System clock setup is done in another file (init.c).
Next step: Interrupts.

/* User LED is on PA0 */
/* This example uses SERCOM0 (UART mode) to send data to a host PC
 * 
 */
#include 
#include "../samd20e16.h"
/*
 How to program: run openocd first
 Next, run arm-none-eabi-gdb in another window and execute these commands (adjust directory to suit)
 target remote :3333
 monitor at91samd chip-erase
 monitor reset halt
 monitor load_image /home/frank/Documents/programming/samd/Baremetal/samd20_examples/serial/main.elf 0
 monitor reset 
*/
void delay(uint32_t dly)
{
    while(dly--);
}

void initUART()
{    
    // UART is on:    
    PM_APBCMASK |= BIT2; // turn on SERCOM0
    // Reset SERCOM 0
    SERCOM0_CTRLA = 1; // reset the UART
    while(SERCOM0_STATUS & BIT15); // wait for sync to complete   
    while ( SERCOM0_CTRLA & BIT0 ); // wait for reset to complete
    // Will route the main 48MHz clock to SERCOM0 as the baud rate base clock.
    GCLK_CLKCTRL = 0x0d + BIT14;        
    while(  (GCLK_STATUS & BIT7)); // wait for sync to complete
    SERCOM0_BAUD = 65325; // configure for 9600 bps
    while(SERCOM0_STATUS & BIT15); // wait for sync to complete   
    SERCOM0_CTRLB = BIT17 + BIT16 + BIT9 ; // enable TX, RX, Start-of-frame wakeup, 1 stop bit, 8 bits;
    // 0x00030200 RXEN,TXEN,SFDE,
    while(SERCOM0_STATUS & BIT15); // wait for sync to complete
    SERCOM0_CTRLA = BIT30 + BIT20 + BIT2 + BIT1; // LSB first, UART mode , internal clock, PAD1 = RX, PAD0 = TX, ENABLE    
    // 0x40000006 DORD,ENABLE,MODE001 (USART INTERNAL CLOCK), ENABLE
    while(SERCOM0_STATUS & BIT15); // wait for synchronization between clock domains to complete    
    // The actual special function is selected by writing to the upper 4 bits of PAPMUX5 
    // To select function C, write  '2' to these bits
    PAPMUX4 = 0x22;    
    PAPPINCFG8 |= BIT0; // enable special function multplexing on Pin 11                
    PAPPINCFG9 |= BIT0; // enable special function multplexing on Pin 12
}
void eputc(char c)
{
    SERCOM0_DATA = c;
    while((SERCOM0_INTFLAG & BIT1)==0);// wait for TXC flag
    
}
void printString(char *String)
{
    while (*String)
    {
        eputc(*String++);
    }
}
int main()
{
    int i=0;
    PADIRSET = BIT0; // make Port A bit 0 an output
    // Startup delay to allow escape from a potentially bricked chip: ONLY DELETE WHEN SURE OF PROPER CLOCK OPERATION
    for (i=0;i<10;i++)
    {
        PAOUTTGL = BIT0; // Toggle Port A bit 0
        delay(1000000);   // Wait  
    }    
    initUART(); // get SERCOM0 going
    while(1)
    {
        printString("Hello SamD!\r\n");
        i = SERCOM0_INTFLAG;
        PAOUTTGL = BIT0; // Toggle Port A bit 0
        delay(10000);   // Wait  
        
    }
   
}