```
; Demonstrates floating point conversion
; in Assembler, (C)2003 www.avr-asm-tutorial.net
;
; The task: You read in an 8-bit result of an
; analogue-digital-converter, number is in the
; range from hex 00 to FF.
; You need to convert this into a floating point
; number in the range from 0.00 to 5.00 Volt
;
; The program scheme:
; 1. Multiplication by 502 (hex 01F6).
; That step multiplies by 500, 256 and
; divides by 255 in one step!
; 2. Round the result and cut the last byte
; of the result.
; This step divides by 256 by ignoring the
; last byte of the result. Before doing
; that, bit 7 is used to round the result.
; 3. Convert the resulting word to ASCII and set
; the correct decimal sign
; The resulting word in the range from 0 to
; 500 is displayed in ASCII-characters as
; 0.00 to 5.00.
;
; The registers used:
; The routines use the registers R8..R1 without
; saving these before. Also required is a multi-
; purpose register called rmp, located in the
; upper half of the registers. Please take care
; that these registers don't conflict with the
; register use in the rest of your program.
;
; When entering the routine the 8-bit number is
; expected in the register R1.
; The multiplication uses R4:R3:R2 to hold
; the multiplicator 502 (is shifted left
; max. eight times during multiplication).
; The result of the multiplication is calculated
; in the registers R7:R6:R5.
; The result of the so called division by 256
; by just ignoring R5 in the result, is in
; R7:R6. R7:R6 is rounded, depending on the
; highest bit of R5, and the result is copied to
; R2:R1.
; Conversion to an ASCII-string uses the input
; in R2:R1, the register pair R4:R3 as a divisor
; for conversion, and places the ASCII result
; string to R5:R6:R7:R8 (R6 is the decimal char).
;
; Other conventions:
; The conversion uses subroutines and the stack.
; The stack must work fine for the use of three
; levels (six bytes SRAM).
;
; Conversion times:
; The whole routine requires 228 clock cycles
; maximum (converting $FF), and 79 clock cycles
; minimum (converting $00). At 4 MHz the times
; are 56.75 microseconds resp. 17.75 microseconds.
;
; Definitions:
; Registers
.DEF rmp = R16 ; used as multi-purpose register
;
; AVR type
; Tested for type AT90S8515, only required for
; stack setting, routines work fine with other
; AT90S-types also
.NOLIST
.INCLUDE "8515def.inc"
.LIST
;
; Start of test program
;
; Just writes a number to R1 and starts the
; conversion routine, for test purposes only
;
.CSEG
.ORG $0000
rjmp main
;
main:
ldi rmp,HIGH(RAMEND) ; Set the stack
out SPH,rmp
ldi rmp,LOW(RAMEND)
out SPL,rmp
ldi rmp,$FF ; Convert $FF
mov R1,rmp
rcall fpconv8 ; call the conversion routine
no_end: ; unlimited loop, when done
rjmp no_end
;
; Conversion routine wrapper, calls the different conversion steps
;
fpconv8:
rcall fpconv8m ; multiplicate by 502
rcall fpconv8r ; round and divide by 256
rcall fpconv8a ; convert to ASCII string
ldi rmp,'.' ; set decimal char
mov R6,rmp
ret ; all done
;
; Subroutine multiplication by 502
;
; Starting conditions:
; +--+
; |R1| Input number, example is $FF
; |FF|
; +--+
; +--+--+--+
; |R4|R3|R2| Multiplicant 502 = $00 01 F6
; |00|01|F6|
; +--+--+--+
; +--+--+--+
; |R7|R6|R5| Result, as example 128,010
; |01|F4|0A|
; +--+--+--+
;
fpconv8m:
clr R4 ; set the multiplicant to 502
ldi rmp,$01
mov R3,rmp
ldi rmp,$F6
mov R2,rmp
clr R7 ; clear the result
clr R6
clr R5
fpconv8m1:
or R1,R1 ; check if the number is all zeros
brne fpconv8m2 ; still one's, go on convert
ret ; ready, return back
fpconv8m2:
lsr R1 ; shift number to the right (div by 2)
brcc fpconv8m3 ; if the lowest bit was 0, then skip adding
add R5,R2 ; add the number in R6:R5:R4:R3 to the result
adc R6,R3
adc R7,R4
fpconv8m3:
lsl R2 ; multiply R4:R3:R2 by 2
rol R3
rol R4
rjmp fpconv8m1 ; repeat for next bit
;
; Round the value in R7:R6 with the value in bit 7 of R5
;
fpconv8r:
clr rmp ; put zero to rmp
lsl R5 ; rotate bit 7 to carry
adc R6,rmp ; add LSB with carry
adc R7,rmp ; add MSB with carry
mov R2,R7 ; copy the value to R2:R1 (divide by 256)
mov R1,R6
ret
;
; Convert the word in R2:R1 to an ASCII string in R5:R6:R7:R8
;
; +--+--+
; +R2|R1| Input value 0..500
; +--+--+
; +--+--+
; |R4|R3| Decimal divider value
; +--+--+
; +---+---+---+---+
; | R5| R6| R7| R8| Resulting ASCII string (for input value 5,00)
; |'5'|'.'|'0'|'0'|
; +---+---+---+---+
;
fpconv8a:
clr R4 ; Set the decimal divider value to 100
ldi rmp,100
mov R3,rmp
rcall fpconv8d ; get ASCII digit by repeated subtraction
mov R5,rmp ; set hundreds string char
ldi rmp,10 ; Set the decimal divider value to 10
mov R3,rmp
rcall fpconv8d ; get the next ASCII digit
mov R7,rmp ; set tens string char
ldi rmp,'0' ; convert the rest to an ASCII char
add rmp,R1
mov R8,rmp ; set ones string char
ret
;
; Convert binary word in R2:R1 to a decimal digit by substracting
; the decimal divider value in R4:R3 (100, 10)
;
fpconv8d:
ldi rmp,'0' ; start with decimal value 0
fpconv8d1:
cp R1,R3 ; Compare word with decimal divider value
cpc R2,R4
brcc fpconv8d2 ; Carry clear, subtract divider value
ret ; done subtraction
fpconv8d2:
sub R1,R3 ; subtract divider value
sbc R2,R4
inc rmp ; up one digit
rjmp fpconv8d1 ; once again
;
; End of floating point conversion routines
;
;
; End of conversion test routine
;
```

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