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Add ADC DMA example
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‎adc/CMakeLists.txt

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if (NOT PICO_NO_HARDWARE)
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add_subdirectory(adc_console)
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add_subdirectory(dma_capture)
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add_subdirectory(hello_adc)
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add_subdirectory(joystick_display)
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endif ()

‎adc/dma_capture/CMakeLists.txt

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add_executable(adc_dma_capture
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dma_capture.c
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)
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pico_generate_pio_header(adc_dma_capture ${CMAKE_CURRENT_LIST_DIR}/resistor_dac.pio)
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target_link_libraries(adc_dma_capture
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pico_stdlib
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hardware_adc
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hardware_dma
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# For the dummy output:
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hardware_pio
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pico_multicore
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)
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# create map/bin/hex file etc.
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pico_add_extra_outputs(adc_dma_capture)
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# add url via pico_set_program_url
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example_auto_set_url(adc_dma_capture)

‎adc/dma_capture/dma_capture.c

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/**
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* Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
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*
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* SPDX-License-Identifier: BSD-3-Clause
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*/
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#include <stdio.h>
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#include "pico/stdlib.h"
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// For ADC input:
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#include "hardware/adc.h"
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#include "hardware/dma.h"
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// For resistor DAC output:
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#include "pico/multicore.h"
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#include "hardware/pio.h"
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#include "resistor_dac.pio.h"
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// This example uses the DMA to capture many samples from the ADC.
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//
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// - We are putting the ADC in free-running capture mode at 0.5 Msps
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//
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// - A DMA channel will be attached to the ADC sample FIFO
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//
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// - Configure the ADC to right-shift samples to 8 bits of significance, so we
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// can DMA into a byte buffer
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//
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// This could be extended to use the ADC's round robin feature to sample two
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// channels concurrently at 0.25 Msps each.
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//
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// It would be nice to have some analog samples to measure! This example also
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// drives waves out through a 5-bit resistor DAC, as found on the reference
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// VGA board. If you have that board, you can take an M-F jumper wire from
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// GPIO 26 to the Green pin on the VGA connector (top row, next-but-rightmost
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// hole). Or you can ignore that part of the code and connect your own signal
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// to the ADC input.
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// Channel 0 is GPIO26
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#define CAPTURE_CHANNEL 0
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#define CAPTURE_DEPTH 1000
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uint8_t capture_buf[CAPTURE_DEPTH];
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void core1_main();
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int main() {
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stdio_init_all();
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// Send core 1 off to start driving the "DAC" whilst we configure the ADC.
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multicore_launch_core1(core1_main);
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// Init GPIO for analogue use: hi-Z, no pulls, disable digital input buffer.
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adc_gpio_init(26 + CAPTURE_CHANNEL);
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adc_init();
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adc_select_input(CAPTURE_CHANNEL);
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adc_fifo_setup(
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true, // Write each completed conversion to the sample FIFO
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true, // Enable DMA data request (DREQ)
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1, // DREQ (and IRQ) asserted when at least 1 sample present
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false, // We won't see the ERR bit because of 8 bit reads; disable.
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true // Shift each sample to 8 bits when pushing to FIFO
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);
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// Divisor of 0 -> full speed. Free-running capture with the divider is
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// equivalent to pressing the ADC_CS_START_ONCE button once per `div + 1`
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// cycles (div not necessarily an integer). Each conversion takes 96
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// cycles, so in general you want a divider of 0 (hold down the button
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// continuously) or > 95 (take samples less frequently than 96 cycle
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// intervals). This is all timed by the 48 MHz ADC clock.
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adc_set_clkdiv(0);
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printf("Arming DMA\n");
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sleep_ms(1000);
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// Set up the DMA to start transferring data as soon as it appears in FIFO
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uint dma_chan = dma_claim_unused_channel(true);
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dma_channel_config cfg = dma_channel_get_default_config(dma_chan);
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// Reading from constant address, writing to incrementing byte addresses
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channel_config_set_transfer_data_size(&cfg, DMA_SIZE_8);
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channel_config_set_read_increment(&cfg, false);
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channel_config_set_write_increment(&cfg, true);
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// Pace transfers based on availability of ADC samples
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channel_config_set_dreq(&cfg, DREQ_ADC);
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dma_channel_configure(dma_chan, &cfg,
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capture_buf, // dst
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&adc_hw->fifo, // src
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CAPTURE_DEPTH, // transfer count
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true // start immediately
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);
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printf("Starting capture\n");
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adc_run(true);
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// Once DMA finishes, stop any new conversions from starting, and clean up
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// the FIFO in case the ADC was still mid-conversion.
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dma_channel_wait_for_finish_blocking(dma_chan);
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printf("Capture finished\n");
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adc_run(false);
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adc_fifo_drain();
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// Print samples to stdout so you can display them in pyplot, excel, matlab
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for (int i = 0; i < CAPTURE_DEPTH; ++i) {
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printf("%-3d, ", capture_buf[i]);
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if (i % 10 == 9)
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printf("\n");
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}
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}
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// ----------------------------------------------------------------------------
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// Code for driving the "DAC" output for us to measure
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// Core 1 is just going to sit and drive samples out continously. PIO provides
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// consistent sample frequency.
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#define OUTPUT_FREQ_KHZ 5
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#define SAMPLE_WIDTH 5
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// This is the green channel on the VGA board
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#define DAC_PIN_BASE 6
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void core1_main() {
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PIO pio = pio0;
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uint sm = pio_claim_unused_sm(pio0, true);
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uint offset = pio_add_program(pio0, &resistor_dac_5bit_program);
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resistor_dac_5bit_program_init(pio0, sm, offset,
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OUTPUT_FREQ_KHZ * 1000 * 2 * (1 << SAMPLE_WIDTH), DAC_PIN_BASE);
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while (true) {
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// Triangle wave
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for (int i = 0; i < (1 << SAMPLE_WIDTH); ++i)
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pio_sm_put_blocking(pio, sm, i);
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for (int i = 0; i < (1 << SAMPLE_WIDTH); ++i)
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pio_sm_put_blocking(pio, sm, (1 << SAMPLE_WIDTH) - 1 - i);
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}
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}
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‎adc/dma_capture/resistor_dac.pio

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;
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; Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
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;
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; SPDX-License-Identifier: BSD-3-Clause
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;
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.program resistor_dac_5bit
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; Drive one of the 5-bit resistor DACs on the VGA reference board. (this isn't
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; a good way to do VGA -- just want a nice sawtooth for the ADC example!)
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out pins, 5
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% c-sdk {
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#include "hardware/clocks.h"
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static inline void resistor_dac_5bit_program_init(PIO pio, uint sm, uint offset,
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uint sample_rate_hz, uint pin_base) {
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pio_sm_set_pins_with_mask(pio, sm, 0, 0x1fu << pin_base);
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pio_sm_set_pindirs_with_mask(pio, sm, ~0u, 0x1fu << pin_base);
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for (int i = 0; i < 5; ++i)
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pio_gpio_init(pio, pin_base + i);
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pio_sm_config c = resistor_dac_5bit_program_get_default_config(offset);
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sm_config_set_out_pins(&c, pin_base, 5);
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// Shift to right, autopull threshold 5
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sm_config_set_out_shift(&c, true, true, 5);
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// Deeper FIFO as we're not doing any RX
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sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);
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float div = (float)clock_get_hz(clk_sys) / sample_rate_hz;
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sm_config_set_clkdiv(&c, div);
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pio_sm_init(pio, sm, offset, &c);
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pio_sm_set_enabled(pio, sm, true);
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}
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%}

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