I saw a topic on 6502.org about converting byte to hex string. I remembered Lee Davison's code shorts about converting 0 to 15 to ascii 0 to 15 by using decimal mode. I know Lee passed along this year so RIP Lee, I didn't know you but you seemed like a genius to me.

Using his basic code I came up with this for converting a byte:

;HEX to ASCII
;  A = entry value
  sed        ;2  @2
  tax        ;2  @4
  and #$0F   ;2  @6
  cmp #9+1   ;2  @8
  adc #$30   ;2  @10
  tay        ;2  @12
  txa        ;2  @14
  lsr        ;2  @16
  lsr        ;2  @18
  lsr        ;2  @20
  lsr        ;2  @22
  cmp #9+1   ;2  @24
  adc #$30   ;2  @26
  cld        ;2  @28
;  A = MSN ASCII char
;  Y = LSN ASCII char

I ran a short test program for all values and it passed. Twenty eight cycles is not too bad. It's kind of a cool way to convert a byte like this. This could make for a nice project for communicating out of the 2600 I/O port and displaying to a LCD panel in useless but fun way. Maybe you could have two 2600's with the LCD in between them to see the bytes fly by.

Just need to make a conversion routine from ASCII back to hex.

Edit: Came up with this off the top of my head. Tested it for a few values and seems to work. It can only handle ASCII '0' to '9', and 'A' to 'F' (capitals).

;ASCII to HEX
;  A = MSN ASCII char
;  Y = LSN ASCII char

    cmp #$39+1      ;2  @2
    bcc .highNybble ;2³ @4/5
    sbc #7          ;2  @6
.highNybble:
    asl             ;2  @8
    asl             ;2  @10
    asl             ;2  @12
    asl             ;2  @14
    sta temp        ;3  @17
    tya             ;2  @19
    cmp #$39+1      ;2  @21
    bcc .LowNibble  ;2³ @23/24
    sbc #7          ;2  @25
.LowNibble:
    and #$0F        ;2  @27
    eor temp        ;3  @30 worse case, 28 best case