BCD format in Assembler in AVR Atmega

BCD, Atmega

Decimal numbers machine language may be represented in the form of eight-bit sequences of ones and zeros, that is bytes. However, there are two types of presentation formats decimal numbers. It is packed format (BCD), and unpacked.

Uncompressed format

In this format, one digit of a decimal number is placed in one byte. Namely LSnibble bytes, whereas the most significant nibble (referred to as a zone) is filled units. For example, the number 157 would be as follows.

Packed Format - BCD

The packed byte format includes two digits decimal number and may also include a character code. As in the following example below, which shows the number of "-1574".

Converting numbers in BCD to hexadecimal and back

When working with Atmega microcontrollers may need to convert the number into hexadecimal format packed decimal number, that is, in BCD format:

 ;======= Converting from BCD to HEX ===================================================
 BCD_to_HEX:
    push    r16              ; We put the converted values on the stack
    andi    r16,0b11110000   ; Mask the register r16, as the MSB
    swap    r16              ; Change tetrad places
    ldi     r17,10           ; Write to the auxiliary register 10 value
    mul     r16,r17          ; Multiply the value of r16 register 10, the result is stored in the registers r1, r0
    pop     r16              ; get the stored value from the stack
    andi    r16,0b00001111   ; Mask the register r16, as the LSB
    add     r16,r0           ; Adds the r16 register with register r0, the result is a binary number from 0 to 99
    ret

And the inverse transform:

;======= Converting from HEX to BCD ====================================================
 HEX_to_BCD:
    clr     r17            ; clear register help
 HEX_to_BCD_l: 
    subi    r16,10         ; temp = temp — 10
    brcs    HEX_to_BCD_2   ; Abort if the transfer set
    inc     r17            ; Increment r17  
    rjmp    HEX_to_BCD_l   ; We pass on the testing cycle again
 HEX_to_BCD_2: 
    subi    r16,-10        ; compensate a negative value in r16
    swap    r17            ; Change tetrad places
    or      r16,r17        ; Combine the received values in a packed BCD
    ret

As you can see, the work is done in these programs with four registers: r0, r1, r16, r17. Trust register in this case is the register r16. Also, these programs from working with numbers from 0 to 99.

Attention. Also for the library you want to initialize the stack of the microcontroller. For example as follows:

 ;======= Макросы ========================================================================
 ; Output macro to the port or Register
 .macro outi             
    ldi     r16, @1 
    out     @0,R16
 .endm
 ;========================================================================================
 RESET:
    outi    SPL,Low(RAMEND)     ; Init stack
    outi    SPH,High(RAMEND)