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mtm_computer: Add DAC audio out module
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‎ports/raspberrypi/boards/mtm_computer/module/DACOut.c‎

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// This file is part of the CircuitPython project: https://circuitpython.org
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//
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// SPDX-FileCopyrightText: Copyright (c) 2026 Tod Kurt
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//
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// SPDX-License-Identifier: MIT
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//
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// MCP4822 dual-channel 12-bit SPI DAC driver for the MTM Workshop Computer.
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// Uses PIO + DMA for non-blocking audio playback, mirroring audiobusio.I2SOut.
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#include <stdint.h>
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#include <string.h>
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#include "mpconfigport.h"
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#include "py/gc.h"
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#include "py/mperrno.h"
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#include "py/runtime.h"
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#include "boards/mtm_computer/module/DACOut.h"
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#include "shared-bindings/microcontroller/Pin.h"
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#include "shared-module/audiocore/__init__.h"
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#include "bindings/rp2pio/StateMachine.h"
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// ─────────────────────────────────────────────────────────────────────────────
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// PIO program for MCP4822 SPI DAC
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// ─────────────────────────────────────────────────────────────────────────────
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//
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// Pin assignment:
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// OUT pin (1) = MOSI — serial data out
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// SET pins (N) = MOSI through CS — for CS control & command-bit injection
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// SIDE-SET pin (1) = SCK — serial clock
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//
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// On the MTM Workshop Computer: MOSI=GP19, CS=GP21, SCK=GP18.
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// The SET group spans GP19..GP21 (3 pins). GP20 is unused and driven low.
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//
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// SET PINS bit mapping (bit0=MOSI/GP19, bit1=GP20, bit2=CS/GP21):
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// 0 = CS low, MOSI low 1 = CS low, MOSI high 4 = CS high, MOSI low
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//
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// SIDE-SET (1 pin, SCK): side 0 = SCK low, side 1 = SCK high
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//
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// MCP4822 16-bit command word:
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// [15] channel (0=A, 1=B) [14] don't care [13] gain (1=1x)
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// [12] output enable (1) [11:0] 12-bit data
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//
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// DMA feeds unsigned 16-bit audio samples. RP2040 narrow-write replication
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// fills both halves of the 32-bit PIO FIFO entry with the same value,
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// giving mono→stereo for free.
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//
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// The PIO pulls 32 bits, then sends two SPI transactions:
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// Channel A: cmd nibble 0b0011, then all 16 sample bits from upper half-word
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// Channel B: cmd nibble 0b1011, then all 16 sample bits from lower half-word
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// The MCP4822 captures exactly 16 bits per CS frame (4 cmd + 12 data),
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// so only the top 12 of the 16 sample bits become DAC data. The bottom
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// 4 sample bits clock out harmlessly after the DAC has latched.
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// This gives correct 16-bit → 12-bit scaling (effectively sample >> 4).
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//
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// PIO instruction encoding with .side_set 1 (no opt):
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// [15:13] opcode [12] side-set [11:8] delay [7:0] operands
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//
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// Total: 26 instructions, 86 PIO clocks per audio sample.
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// ─────────────────────────────────────────────────────────────────────────────
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static const uint16_t mcp4822_pio_program[] = {
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// side SCK
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// 0: pull noblock side 0 ; Get 32 bits or keep X if FIFO empty
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0x8080,
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// 1: mov x, osr side 0 ; Save for pull-noblock fallback
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0xA027,
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// ── Channel A: command nibble 0b0011 ──────────────────────────────────
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// Send 4 cmd bits via SET, then all 16 sample bits via OUT.
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// MCP4822 captures exactly 16 bits per CS frame (4 cmd + 12 data);
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// the extra 4 clocks shift out the LSBs which the DAC ignores.
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// This gives correct 16→12 bit scaling (top 12 bits become DAC data).
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// 2: set pins, 0 side 0 ; CS low, MOSI=0 (bit15=0: channel A)
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0xE000,
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// 3: nop side 1 ; SCK high — latch bit 15
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0xB042,
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// 4: set pins, 0 side 0 ; MOSI=0 (bit14=0: don't care)
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0xE000,
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// 5: nop side 1 ; SCK high
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0xB042,
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// 6: set pins, 1 side 0 ; MOSI=1 (bit13=1: gain 1x)
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0xE001,
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// 7: nop side 1 ; SCK high
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0xB042,
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// 8: set pins, 1 side 0 ; MOSI=1 (bit12=1: output active)
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0xE001,
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// 9: nop side 1 ; SCK high
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0xB042,
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// 10: set y, 15 side 0 ; Loop counter: 16 sample bits
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0xE04F,
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// 11: out pins, 1 side 0 ; Data bit → MOSI; SCK low (bitloopA)
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0x6001,
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// 12: jmp y--, 11 side 1 ; SCK high, loop back
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0x108B,
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// 13: set pins, 4 side 0 ; CS high — DAC A latches
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0xE004,
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// ── Channel B: command nibble 0b1011 ──────────────────────────────────
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// 14: set pins, 1 side 0 ; CS low, MOSI=1 (bit15=1: channel B)
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0xE001,
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// 15: nop side 1 ; SCK high
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0xB042,
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// 16: set pins, 0 side 0 ; MOSI=0 (bit14=0)
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0xE000,
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// 17: nop side 1 ; SCK high
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0xB042,
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// 18: set pins, 1 side 0 ; MOSI=1 (bit13=1: gain 1x)
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0xE001,
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// 19: nop side 1 ; SCK high
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0xB042,
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// 20: set pins, 1 side 0 ; MOSI=1 (bit12=1: output active)
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0xE001,
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// 21: nop side 1 ; SCK high
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0xB042,
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// 22: set y, 15 side 0 ; Loop counter: 16 sample bits
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0xE04F,
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// 23: out pins, 1 side 0 ; Data bit → MOSI; SCK low (bitloopB)
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0x6001,
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// 24: jmp y--, 23 side 1 ; SCK high, loop back
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0x1097,
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// 25: set pins, 4 side 0 ; CS high — DAC B latches
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0xE004,
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};
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// Clocks per sample: 2 (pull+mov) + 42 (chanA) + 42 (chanB) = 86
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// Per channel: 8(4 cmd bits × 2 clks) + 1(set y) + 32(16 bits × 2 clks) + 1(cs high) = 42
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#define MCP4822_CLOCKS_PER_SAMPLE 86
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void common_hal_mtm_hardware_dacout_construct(mtm_hardware_dacout_obj_t *self,
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const mcu_pin_obj_t *clock, const mcu_pin_obj_t *mosi,
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const mcu_pin_obj_t *cs) {
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// SET pins span from MOSI to CS. MOSI must have a lower GPIO number
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// than CS, with at most 4 pins between them (SET count max is 5).
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if (cs->number <= mosi->number || (cs->number - mosi->number) > 4) {
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mp_raise_ValueError(
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MP_COMPRESSED_ROM_TEXT("cs pin must be 1-4 positions above mosi pin"));
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}
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uint8_t set_count = cs->number - mosi->number + 1;
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// Initial SET pin state: CS high (bit at CS position), others low
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uint32_t cs_bit_position = cs->number - mosi->number;
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pio_pinmask32_t initial_set_state = PIO_PINMASK32_FROM_VALUE(1u << cs_bit_position);
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pio_pinmask32_t initial_set_dir = PIO_PINMASK32_FROM_VALUE((1u << set_count) - 1);
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common_hal_rp2pio_statemachine_construct(
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&self->state_machine,
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mcp4822_pio_program, MP_ARRAY_SIZE(mcp4822_pio_program),
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44100 * MCP4822_CLOCKS_PER_SAMPLE, // Initial frequency; play() adjusts
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NULL, 0, // No init program
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NULL, 0, // No may_exec
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mosi, 1, // OUT: MOSI, 1 pin
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PIO_PINMASK32_NONE, PIO_PINMASK32_ALL, // OUT state=low, dir=output
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NULL, 0, // IN: none
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PIO_PINMASK32_NONE, PIO_PINMASK32_NONE, // IN pulls: none
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mosi, set_count, // SET: MOSI..CS
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initial_set_state, initial_set_dir, // SET state (CS high), dir=output
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clock, 1, false, // SIDE-SET: SCK, 1 pin, not pindirs
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PIO_PINMASK32_NONE, // SIDE-SET state: SCK low
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PIO_PINMASK32_FROM_VALUE(0x1), // SIDE-SET dir: output
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false, // No sideset enable
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NULL, PULL_NONE, // No jump pin
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PIO_PINMASK_NONE, // No wait GPIO
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true, // Exclusive pin use
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false, 32, false, // OUT shift: no autopull, 32-bit, shift left
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false, // Don't wait for txstall
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false, 32, false, // IN shift (unused)
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false, // Not user-interruptible
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0, -1, // Wrap: whole program
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PIO_ANY_OFFSET,
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PIO_FIFO_TYPE_DEFAULT,
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PIO_MOV_STATUS_DEFAULT,
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PIO_MOV_N_DEFAULT
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);
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self->playing = false;
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audio_dma_init(&self->dma);
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}
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bool common_hal_mtm_hardware_dacout_deinited(mtm_hardware_dacout_obj_t *self) {
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return common_hal_rp2pio_statemachine_deinited(&self->state_machine);
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}
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void common_hal_mtm_hardware_dacout_deinit(mtm_hardware_dacout_obj_t *self) {
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if (common_hal_mtm_hardware_dacout_deinited(self)) {
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return;
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}
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if (common_hal_mtm_hardware_dacout_get_playing(self)) {
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common_hal_mtm_hardware_dacout_stop(self);
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}
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common_hal_rp2pio_statemachine_deinit(&self->state_machine);
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audio_dma_deinit(&self->dma);
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}
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void common_hal_mtm_hardware_dacout_play(mtm_hardware_dacout_obj_t *self,
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mp_obj_t sample, bool loop) {
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if (common_hal_mtm_hardware_dacout_get_playing(self)) {
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common_hal_mtm_hardware_dacout_stop(self);
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}
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uint8_t bits_per_sample = audiosample_get_bits_per_sample(sample);
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if (bits_per_sample < 16) {
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bits_per_sample = 16;
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}
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uint32_t sample_rate = audiosample_get_sample_rate(sample);
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uint8_t channel_count = audiosample_get_channel_count(sample);
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if (channel_count > 2) {
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mp_raise_ValueError(MP_COMPRESSED_ROM_TEXT("Too many channels in sample."));
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}
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// PIO clock = sample_rate × clocks_per_sample
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common_hal_rp2pio_statemachine_set_frequency(
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&self->state_machine,
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(uint32_t)sample_rate * MCP4822_CLOCKS_PER_SAMPLE);
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common_hal_rp2pio_statemachine_restart(&self->state_machine);
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// DMA feeds unsigned 16-bit samples. The PIO discards the top 4 bits
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// of each 16-bit half and uses the remaining 12 as DAC data.
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// RP2040 narrow-write replication: 16-bit DMA write → same value in
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// both 32-bit FIFO halves → mono-to-stereo for free.
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audio_dma_result result = audio_dma_setup_playback(
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&self->dma,
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sample,
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loop,
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false, // single_channel_output
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0, // audio_channel
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false, // output_signed = false (unsigned for MCP4822)
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bits_per_sample, // output_resolution
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(uint32_t)&self->state_machine.pio->txf[self->state_machine.state_machine],
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self->state_machine.tx_dreq,
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false); // swap_channel
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if (result == AUDIO_DMA_DMA_BUSY) {
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common_hal_mtm_hardware_dacout_stop(self);
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mp_raise_RuntimeError(MP_COMPRESSED_ROM_TEXT("No DMA channel found"));
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} else if (result == AUDIO_DMA_MEMORY_ERROR) {
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common_hal_mtm_hardware_dacout_stop(self);
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mp_raise_RuntimeError(MP_COMPRESSED_ROM_TEXT("Unable to allocate buffers for signed conversion"));
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} else if (result == AUDIO_DMA_SOURCE_ERROR) {
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common_hal_mtm_hardware_dacout_stop(self);
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mp_raise_RuntimeError(MP_COMPRESSED_ROM_TEXT("Audio source error"));
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}
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self->playing = true;
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}
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void common_hal_mtm_hardware_dacout_pause(mtm_hardware_dacout_obj_t *self) {
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audio_dma_pause(&self->dma);
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}
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void common_hal_mtm_hardware_dacout_resume(mtm_hardware_dacout_obj_t *self) {
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audio_dma_resume(&self->dma);
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}
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bool common_hal_mtm_hardware_dacout_get_paused(mtm_hardware_dacout_obj_t *self) {
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return audio_dma_get_paused(&self->dma);
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}
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void common_hal_mtm_hardware_dacout_stop(mtm_hardware_dacout_obj_t *self) {
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audio_dma_stop(&self->dma);
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common_hal_rp2pio_statemachine_stop(&self->state_machine);
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self->playing = false;
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}
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bool common_hal_mtm_hardware_dacout_get_playing(mtm_hardware_dacout_obj_t *self) {
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bool playing = audio_dma_get_playing(&self->dma);
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if (!playing && self->playing) {
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common_hal_mtm_hardware_dacout_stop(self);
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}
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return playing;
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}

‎ports/raspberrypi/boards/mtm_computer/module/DACOut.h‎

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// This file is part of the CircuitPython project: https://circuitpython.org
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//
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// SPDX-FileCopyrightText: Copyright (c) 2026 Tod Kurt
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//
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// SPDX-License-Identifier: MIT
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#pragma once
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#include "common-hal/microcontroller/Pin.h"
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#include "common-hal/rp2pio/StateMachine.h"
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#include "audio_dma.h"
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#include "py/obj.h"
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typedef struct {
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mp_obj_base_t base;
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rp2pio_statemachine_obj_t state_machine;
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audio_dma_t dma;
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bool playing;
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} mtm_hardware_dacout_obj_t;
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void common_hal_mtm_hardware_dacout_construct(mtm_hardware_dacout_obj_t *self,
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const mcu_pin_obj_t *clock, const mcu_pin_obj_t *mosi,
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const mcu_pin_obj_t *cs);
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void common_hal_mtm_hardware_dacout_deinit(mtm_hardware_dacout_obj_t *self);
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bool common_hal_mtm_hardware_dacout_deinited(mtm_hardware_dacout_obj_t *self);
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void common_hal_mtm_hardware_dacout_play(mtm_hardware_dacout_obj_t *self,
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mp_obj_t sample, bool loop);
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void common_hal_mtm_hardware_dacout_stop(mtm_hardware_dacout_obj_t *self);
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bool common_hal_mtm_hardware_dacout_get_playing(mtm_hardware_dacout_obj_t *self);
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void common_hal_mtm_hardware_dacout_pause(mtm_hardware_dacout_obj_t *self);
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void common_hal_mtm_hardware_dacout_resume(mtm_hardware_dacout_obj_t *self);
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bool common_hal_mtm_hardware_dacout_get_paused(mtm_hardware_dacout_obj_t *self);
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extern const mp_obj_type_t mtm_hardware_dacout_type;

‎ports/raspberrypi/boards/mtm_computer/mpconfigboard.mk‎

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@@ -11,3 +11,7 @@ EXTERNAL_FLASH_DEVICES = "W25Q16JVxQ"
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CIRCUITPY_AUDIOEFFECTS = 1
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CIRCUITPY_IMAGECAPTURE = 0
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CIRCUITPY_PICODVI = 0
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SRC_C += \
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boards/$(BOARD)/module/mtm_hardware.c \
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boards/$(BOARD)/module/DACOut.c

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