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Prevent extra bit transition at end of data

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The initial bit transition would be set even if the queue stalled.
The state machine still takes 16 cycles per bit
The next bit starts on state 1 and 9 instead of 0 and 8 so the timing is the same as before
This commit is contained in:
Tom Messick 2021-04-21 13:42:57 -07:00 committed by Luke Wren
parent 612469d347
commit 4f64f3af80
1 changed files with 10 additions and 8 deletions
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18
pio/differential_manchester/differential_manchester.pio
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@ -7,7 +7,7 @@
.program differential_manchester_tx .program differential_manchester_tx
.side_set 1 opt .side_set 1 opt
; Transmit one bit every cycles. In each bit period: ; Transmit one bit every 16 cycles. In each bit period:
; - A '0' is encoded as a transition at the start of the bit period ; - A '0' is encoded as a transition at the start of the bit period
; - A '1' is encoded as a transition at the start *and* in the middle ; - A '1' is encoded as a transition at the start *and* in the middle
; ;
@ -16,19 +16,21 @@
public start: public start:
initial_high: initial_high:
out x, 1 side 1 ; Start of bit period: always assert transition out x, 1 ; Start of bit period: always assert transition
jmp !x high_0 [6] ; Test the data bit we just shifted out of OSR jmp !x high_0 side 1 [6] ; Test the data bit we just shifted out of OSR
high_1: high_1:
jmp initial_high side 0 [7] ; For `1` bits, also transition in the middle nop
jmp initial_high side 0 [6] ; For `1` bits, also transition in the middle
high_0: high_0:
jmp initial_low [7] ; Otherwise, the line is stable in the middle jmp initial_low [7] ; Otherwise, the line is stable in the middle
initial_low: initial_low:
out x, 1 side 0 ; Always shift 1 bit from OSR to X so we can out x, 1 ; Always shift 1 bit from OSR to X so we can
jmp !x low_0 [6] ; branch on it. Autopull refills OSR for us. jmp !x low_0 side 0 [6] ; branch on it. Autopull refills OSR for us.
low_1: low_1:
jmp initial_low side 1 [7] ; If there are two transitions, return to nop
low_0: ; initial_low on the next bit. If just one, jmp initial_low side 1 [6] ; If there are two transitions, return to
low_0:
jmp initial_high [7] ; the initial line state is flipped! jmp initial_high [7] ; the initial line state is flipped!
% c-sdk { % c-sdk {