Datasheet

20 jmp !x high_0 [6] ; Test the data bit we just shifted out of OSR
21 high_1:
22 jmp initial_high side 0 [7] ; For `1` bits, also transition in the middle
23 high_0:
24 jmp initial_low [7] ; Otherwise, the line is stable in the middle
25
26 initial_low:
27 out x, 1 side 0 ; Always shift 1 bit from OSR to X so we can
28 jmp !x low_0 [6] ; branch on it. Autopull refills OSR for us.
29 low_1:
30 jmp initial_low side 1 [7] ; If there are two transitions, return to
31 low_0: ; initial_low on the next bit. If just one,
32 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/pre_release/pio/differential_manchester/differential_manchester.pio Lines 35 - 50
35 static inline void differential_manchester_tx_program_init(PIO pio, uint sm, uint offset,
Ê uint pin, float div) {
36 pio_sm_set_pins_with_mask(pio, sm, 0, 1u << pin);
37 pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
38 pio_gpio_select(pio, pin);
39
40 pio_sm_config c = differential_manchester_tx_program_get_default_config(offset);
41 sm_config_set_sideset_pins(&c, pin);
42 sm_config_set_out_shift(&c, true, true, 32);
43 sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);
44 sm_config_set_clkdiv(&c, div);
45 pio_sm_init(pio, sm, offset + differential_manchester_tx_offset_start, &c);
46
47 // Execute a blocking pull so that we maintain the initial line state until data is
Ê available
48 pio_sm_exec(pio, sm, pio_encode_pull(false, true));
49 pio_sm_enable(pio, sm, true);
50 }
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 52)
•
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/pre_release/pio/differential_manchester/differential_manchester.pio Lines 52 - 82
52 .program differential_manchester_rx
53
54 ; Assumes line is idle low
55 ; One bit is 16 cycles. In each bit period:
56 ; - A '0' is encoded as a transition at time 0
57 ; - A '1' is encoded as a transition at time 0 and a transition at time T/2
58 ;
59 ; The IN mapping and the JMP pin select must both be mapped to the GPIO used for
60 ; RX data. Autopush must be enabled.
RP2040 Datasheet
3.6. Examples 358