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annotate dac.pio @ 1:b9ec45077bff

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Use MAKE_C_IDENTIFIER since it's available. Add comment about where we got the magic from. Add _end symbol.
author Daniel O'Connor <darius@dons.net.au>
date Mon, 29 Mar 2021 13:54:55 +1030
parents a55e39064a71
children 0d653f60dec8
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1 ;
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2 ; Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
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3 ;
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4 ; SPDX-License-Identifier: BSD-3-Clause
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5 ;
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6
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7 .program dac
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8
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9 ; Sample bits using an external clock, and push groups of bits into the RX FIFO.
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10 ; - IN pin 0 is the data pin
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11 ; - IN pin 1 is the clock pin
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12 ; - Autopush is enabled, threshold 8
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13 ;
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14 ; This program samples data with each rising clock edge (like mode 0 or mode 3
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15 ; SPI). The data is actually sampled one system clock cycle after the rising
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16 ; edge is observed, so a clock ratio of at least input_clk < clk_sys / 6 is
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17 ; recommended for good sampling alignment.
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18
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19 wait 0 pin 1
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20 wait 1 pin 1
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21 in pins, 1
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22
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23 % c-sdk {
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24 static inline void dac_program_init(PIO pio, uint sm, uint offset, uint pin) {
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25 pio_sm_config c = dac_program_get_default_config(offset);
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27 // Set the IN base pin to the provided `pin` parameter. This is the data
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28 // pin, and the next-numbered GPIO is used as the clock pin.
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29 sm_config_set_in_pins(&c, pin);
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30 // Set the pin directions to input at the PIO
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31 pio_sm_set_consecutive_pindirs(pio, sm, pin, 2, false);
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32 // Connect these GPIOs to this PIO block
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33 pio_gpio_init(pio, pin);
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34 pio_gpio_init(pio, pin + 1);
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35
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36 // Shifting to left matches the customary MSB-first ordering of SPI.
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37 sm_config_set_in_shift(
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38 &c,
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39 false, // Shift-to-right = false (i.e. shift to left)
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40 true, // Autopush enabled
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41 8 // Autopush threshold = 8
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42 );
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44 // We only receive, so disable the TX FIFO to make the RX FIFO deeper.
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45 sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_RX);
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46
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47 // Load our configuration, and start the program from the beginning
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48 pio_sm_init(pio, sm, offset, &c);
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49 pio_sm_set_enabled(pio, sm, true);
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50 }
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51 %}