How to Use SPM for Flash to Flash Programming- (Part 33/46)

Self-Programming Mode (SPM) is a feature of the AVR microcontroller which enables the microcontroller to program its own flash memory. Only the code running on the BLS can make use of this SPM feature. The microcontroller can be made to start executing from the BLS from there the code can access the application flash area. The BLS can read or write the content of the entire flash including the BLS itself.

Fig. 2: Block Diagram of SPM with BLS in AVR

The task of writing the BLS code with SPM has been made simple by the APIs available in the header file <avr/boot.h>. The following are the important APIs available in the header file which helps in the SPM.

Fig. 3: Important APIs in AVR’s header file for SPM 

Using the above APIs one can write a code for SPM in an AVR microcontroller provided that the code should follow certain steps in the order. In this project code which has been programmed from the beginning of the flash memory is re-programmed into another region of the flash memory as such. The task of programming one region of flash memory with the binary taken from the other region can be done in the following three major steps.

Step: 1 Erase the flash page which is about to write into

The first step is to erase the flash page which is about to be written with the new values. The API which helps in executing this step is;

Boot_page_erase (address)

This API can erase an entire page in the flash which the parameter addresses. In the code the address of the page erased is 256. The following image shows the status of the temporary page buffer and the flash memory at the step 1. The temporary page buffer is a buffer in which an entire page can be stored before it is flashed into a page in the flash memory.

Fig. 4:  Figure represents Status of temporary page buffer and flash memory in SPM of AVR

Step: 2 Store the values in a temporary buffer before write into a flash page

This is the second step in which one should store the required binary in a temporary buffer, before writing to any of the flash memory page. The API that can be used for this purpose is;

boot_page_fill             (address, data)

This API fills the Boot-Loader temporary page buffer byte by byte before flashing the data in the temporary page buffer into a page as such. The parameter data represents each byte in the buffer and the parameter address represents the page address + offset of the buffer location where the data byte need to be stored.

The following figure represents the operation in which the temporary page buffer is filled byte by byte using the API boot_page_fill (address, data). 

Fig. 5: Operation of Data Transfer to temporary page buffer using AVR’s API boot_page_fill

The parameter data in the API boot_page_fill (address, data) is actually read from the first location of the flash memory itself with the help of another API which is available in the header file <avr/pgmspace.h>.

pgm_read_byte (address)

Step: 3 Program the filled temporary buffer into the already erased flash page

This is the final step in which the filled temporary buffer is flashed using an API into already erased page of the flash memory. The API which helps in this step is;

boot_page_write (address)

Fig. 6: Temporary Buffer Data transferred in AVR’s Flash Memory using API

The code in this project which is written for the BLS can copy 300bytes from the flash memory into the temporary buffer starting from the address 0x0000. These bytes are then flashed into the flash memory page starting from the address 0x0100. After doing this the code from the BLS will make a jump to the address 0x0100 so that the re-written binary can be executed next. With this boot loader code, whatever program we flash into the address starting from 0x0000 will get rewritten at ox 0x0100 and executed. A simple LED blinking test application can be written into the flash memory starting from 0x0000 to test the working. Flash the code for the BLS first and then the LED application code using the steps explained in the previous project on LED blinking from BLS of AVR. When the led blinks, it means that the code has been re-written from one section of the flash memory to another and is executing from there.

Fig. 7: LED Blinking using SPM of AVR circuit set up on breadboard

Project Source Code

###


#define F_CPU 8000000
#include <avr/io.h>
#include <util/delay.h>
#include <avr/boot.h>
#include <inttypes.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <stdio.h>
 
int main ( void )
{
uint16_t i;
uint8_t A [ 300 ];
uint8_t sreg;
uint32_t page = 256;
unsigned char *buf = A;
 
 
//-------------------------------------------------------------------------------
DDRD |= 0x80;
PORTD |= 0x7F; //led on. bootloader ok.
_delay_ms ( 2000 );
PORTD |= 0x80; //turn off led now. We hope the application will turn on the LED again
_delay_ms ( 2000 );
//--------------------------------------------------------------------------------
 
// storing the bytes from ith location of flash memory to ith variable of array A//
for ( i = 0; i < 300; i ++ )
A [ i ] = pgm_read_byte ( i );
// storing the bytes from ith location of flash memory to ith variable of array A//
 
//================================= SPM ==========================================//
        sreg = SREG; // store the current interrupt status in sreg
        cli(); // clear interrupts
        eeprom_busy_wait (); // wait till the eeprom is free.
        boot_page_erase (page); // erase the page in the flash which we are about to write into
        boot_spm_busy_wait ();      // Wait until the memory is erased.
 
//---- fill the bytes of the page into temperory page buffer before wriying into flash ----//
        for (i=0; i<SPM_PAGESIZE; i+=2)
        {
            //convert the bytes to little-endian word//
            uint16_t w = *buf++;
            w += (*buf++) << 8;
       //convert the bytes to little-endian word//
 
            boot_page_fill (page + i, w); // fill the temperory page buffer byte by byte
        }
//---- fill the bytes of the page into temperory page buffer before wriying into flash ----//
 
//--------------------------------------------------------------------//
        boot_page_write (page);     // Store buffer in flash page.
//--------------------------------------------------------------------//
 
        boot_spm_busy_wait();       // Wait until the memory is written.
        boot_rww_enable (); // Reenable RWW-section again
        SREG = sreg; // Re-enable interrupts
//================================= SPM ==========================================//
 
asm ( "jmp 0x0100" ); // jump to application programmed at 0x0100
 
}
 
###
 

Project Source Code

###


#define F_CPU 8000000
#include <avr/io.h>
#include <util/delay.h>
 
int main ( void )
{
DDRD |= 0x80;
 
while(1)
{
PORTD &= 0x7F;
_delay_ms ( 2000 );
PORTD |= 0x80;
_delay_ms ( 2000 );
}
 
}
 
###
 

Circuit Diagrams

Circuit-Diagram-of-How-to-Use-SPM-for-Flash-to-Flash-Programming

Project Components

Project Video