Displaying Memory Values on the LCD

• ASCII is a mapping between integers and characters.
• For example,
  • '0' has the ASCII value 0x30.
  • '1' has ASCII value 0x31.
  • '9' has ASCII value 0x39.
  • 'A' has ASCII value 0x41.
  • 'B' has ASCII value 0x42.
• The LCD panel accepts ASCII values for input.

Binary to ASCII Conversions

• Suppose we want to convert a one-byte hexadecimal value into a two-byte ASCII sequence.
• For example, 0xA5 = 0b10100101. Converted to ASCII would be 'A' = 0x41 and '5' = 0x35.
• In order to convert this to ASCII, we'll need to deal with the lower four bits (lower nibble) and upper four bits (upper nibble) independently.
• We can get the lower nibble by masking the upper four bits to zero.
• If 0xA5 is in R20,

       andi   r20, 0x0f
  masks the upper nibble to produce 0x05 = 0b00000101.
* Next we need to add 0x30 (since '0' = 0x30):
       subi   r20, -'0'

• We use SUBI because the ATmega32 does not have an ADDI instruction.
• Adding 0x30 to 0x05 gets us: 0x35 which is '5'.
• This works as long as the result is between '0' and '9', but if the result is greater than '9', then we need a letter.
• We can get this by adding 7... 'A' = 0x41 = 0x40 + 7.

       cpi    r20, '9' + 1
       brlo   done
       subi   r20, -7
done:
       ret

Example Program

• The following instructions may be useful:
  • SWAP Rd swaps the upper and lower nibbles — this can be used to isolate the upper nibble.
  • SUBI Rd, value — subtracts value from Rd (there is no add immediate instruction.
  • CPI Rd, value — compares value and the value stored in Rd.
  • BRSH address — (BRanch if Same or Higher) branch to address if value in Rd is greater than or equal (unsigned arithmetic) to value in the preceeding CPI instruction.
  • Don't use BRGE (BRanch if Greater than or Equal) which uses signed arithmetic.

Flowchart for Conversion Function

• The following generates a two byte ASCII sequence that represents the value stored in r17 and stores the ASCII values in r16 (most significant) and r17 (least significant).
• It makes use of a convert_lower_nibble function shown below.

Flowchart for Function that Converts a Nibble to an ASCII value

; Converts lower nibble of r20 into its ASCII equivalent
; The result is stored in r20
convert_lower_nibble:
       andi   r20, 0x0f     ; masking: clear upper nibble
       ; Convert to ASCII
       subi   r20, -'0'     ; add '0' to r20
       ; Correct conversion if digit is greater than '9'
       cpi    r20, '9' + 1
       brlo   done
       subi   r20, -7
done:
       ret