Datasheet

19 out x, 1 ; Start of bit period: always assert transition
20 jmp !x high_0 side 1 [6] ; Test the data bit we just shifted out of OSR
21 high_1:
22 nop
23 jmp initial_high side 0 [6] ; For `1` bits, also transition in the middle
24 high_0:
25 jmp initial_low [7] ; Otherwise, the line is stable in the middle
26
27 initial_low:
28 out x, 1 ; Always shift 1 bit from OSR to X so we can
29 jmp !x low_0 side 0 [6] ; branch on it. Autopull refills OSR for us.
30 low_1:
31 nop
32 jmp initial_low side 1 [6] ; If there are two transitions, return to
33 low_0:
34 jmp initial_high [7] ; the initial line state is flipped!
The .pio file also includes a helper function to initialise a state machine for differential Manchester TX, and connect it to
a chosen GPIO. We arbitrarily choose a 32-bit frame size and LSB-first serialisation (shift_to_right is true in
sm_config_set_out_shift), but as the program operates on one bit at a time, we could change this by reconfiguring the
state machine.
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/master/pio/differential_manchester/differential_manchester.pio Lines 37 - 52
37 static inline void differential_manchester_tx_program_init(PIO pio, uint sm, uint offset,
Ê uint pin, float div) {
38 pio_sm_set_pins_with_mask(pio, sm, 0, 1u << pin);
39 pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
40 pio_gpio_init(pio, pin);
41
42 pio_sm_config c = differential_manchester_tx_program_get_default_config(offset);
43 sm_config_set_sideset_pins(&c, pin);
44 sm_config_set_out_shift(&c, true, true, 32);
45 sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);
46 sm_config_set_clkdiv(&c, div);
47 pio_sm_init(pio, sm, offset + differential_manchester_tx_offset_start, &c);
48
49 // Execute a blocking pull so that we maintain the initial line state until data is
Ê available
50 pio_sm_exec(pio, sm, pio_encode_pull(false, true));
51 pio_sm_set_enabled(pio, sm, true);
52 }
The RX program uses the following strategy:
•
Wait until the initial transition at the start of the bit period, so we stay aligned to the transmit clock
•
Then wait 3/4 of the configured bit period, so that we are centred on the second half-bit-period (see Figure 54)
•
Sample the line at this point to determine whether there are one or two transitions in this bit period
•
Repeat
Pico Examples: https://github.com/raspberrypi/pico-examples/tree/master/pio/differential_manchester/differential_manchester.pio Lines 54 - 84
54 .program differential_manchester_rx
55
56 ; Assumes line is idle low
57 ; One bit is 16 cycles. In each bit period:
58 ; - A '0' is encoded as a transition at time 0
59 ; - A '1' is encoded as a transition at time 0 and a transition at time T/2
RP2040 Datasheet
3.6. Examples 377