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Tighten up timing on bit cells

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Signed-off-by: David Schmidt <1110325+david-schmidt@users.noreply.github.com>
This commit is contained in:
David Schmidt 2023-09-26 10:47:31 -04:00
parent 6c327f4283
commit d4efb9f581
1 changed files with 23 additions and 23 deletions
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46
src/bitbangr.asm
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@ -22,17 +22,16 @@ PRERR = $FF2D
PB0 = $C061 ; Paddle 0 PushButton: HIGH/ON if > 127, LOW/OFF if < 128.
start:
lda #$09 ; We'll be watching for 8 bits plus one stop bit
sta bits
clc
jsr pb0_recv ; Pull a byte from PB0
bcs printit ; Carry set means we got a '1' stop bit, so we're good
jsr CROUT
jsr PRERR ; Else we got a framing error
jsr CROUT
jmp start
printit:
ora #$80
jsr COUT ; PRBYTE
jsr COUT
lda $25
cmp #$17
bne start
@ -42,20 +41,22 @@ printit:
done: rts
pb0_recv:
; State is unknown
; State is currently unknown
lda #$09 ; We'll be watching for 8 bits plus one stop bit
sta bits
clc
poll_for_1:
; sample state
; Sample PB0's state
lda PB0
bpl poll_for_1 ; if not negative, branch to wait_for_1
bpl poll_for_1 ; if not negative, branch to poll_for_1
; State is now 1
poll_for_0:
; sample state
; Sample PB0's state
lda PB0
bmi poll_for_0 ; if negative, branch to wait_for_0
bmi poll_for_0 ; if negative, branch to poll_for_0
; State just became 0 (start bit)
@ -64,13 +65,13 @@ poll_for_0:
; When falling through to here, the above branch was not taken - consuming 2 cycles to get here
ldx #$14 ; 2 loop count
: nop ; 2 \
dex ; 2 |-- 7 * loop count
dex ; 2 |-- 7 * loop count - 1
bne :- ; 3 / final exit of the loop adds 2, branch not taken
; $90 cycles to get here; need to burn 7 more
beq :+ ; 3
: nop ; 2
nop ; 2
; $97 cycles to get here; save 2 for upcoming clc
; $8F cycles to get here
beq :+ ; 3 burn
: beq :+ ; 3 baby
: nop ; 2 burn
; $97 cycles to get here; final 2 will be consumed by clc below
pull_byte:
; We now have one bit time (104.2us at 9600 baud) to process this bit
; Approximately 106.6 ($6B) CPU cycles
@ -86,23 +87,22 @@ push_bit: ; We now have a bit in the carry
lda ring ; 4
ror ; 2
sta ring ; 4
; $1C/$1D cycles to get here (since center of bit time)
; We are now done with processing that bit; we need to cool our heels for the rest ($6B - $1C = $4F) of the
; $1D cycles to get here (since center of bit time)
; We are now done with processing that bit; we need to cool our heels for the rest ($6B - $1D = $4E) of the
; bit time in order to get into the middle of the next bit
ldx #$0A ; 2 loop count
: nop ; 2 \
dex ; 2 |-- 7 * loop count
dex ; 2 |-- 7 * loop count - 1
bne :- ; 3 / final exit of the loop adds 2, branch not taken
; $48 cycles to get here
; $47 cycles to get here, burn $07 more (includes our jump back to top of loop)
nop ; 2
nop ; 2
jmp pull_byte ; 3 Loop around for another bit - we burned $4F cycles
; $6B
jmp pull_byte ; 3 Loop around for another bit - we burned $4E cycles
; $6A
byte_complete:
; Carry now holds stop bit (clear/0 indicates framing error, because we end with set/1)
lda ring ; Exit with the assembled byte in A
rts
pb_state: .byte $00
ring: .byte $55
bits: .byte $00