add files

.github/workflows/build.yml

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+name: PlatformIO CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cache/pip
+            ~/.platformio/.cache
+          key: ${{ runner.os }}-pio
+      - uses: actions/setup-python@v4
+        with:
+          python-version: '3.9'
+      - name: Install PlatformIO Core
+        run: pip install --upgrade platformio
+
+      - name: Build PlatformIO Project
+        run: pio run
+
+      - name: "Upload Firmware"
+        uses: actions/upload-artifact@v3
+        with:
+          name: Uno Firmware (Hex)
+          path: .pio/build/uno/firmware.hex

.gitignore

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+.pio
+.vscode

include/README

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+
+This directory is intended for project header files.
+
+A header file is a file containing C declarations and macro definitions
+to be shared between several project source files. You request the use of a
+header file in your project source file (C, C++, etc) located in `src` folder
+by including it, with the C preprocessing directive `#include'.
+
+```src/main.c
+
+#include "header.h"
+
+int main (void)
+{
+ ...
+}
+```
+
+Including a header file produces the same results as copying the header file
+into each source file that needs it. Such copying would be time-consuming
+and error-prone. With a header file, the related declarations appear
+in only one place. If they need to be changed, they can be changed in one
+place, and programs that include the header file will automatically use the
+new version when next recompiled. The header file eliminates the labor of
+finding and changing all the copies as well as the risk that a failure to
+find one copy will result in inconsistencies within a program.
+
+In C, the usual convention is to give header files names that end with `.h'.
+It is most portable to use only letters, digits, dashes, and underscores in
+header file names, and at most one dot.
+
+Read more about using header files in official GCC documentation:
+
+* Include Syntax
+* Include Operation
+* Once-Only Headers
+* Computed Includes
+
+https://gcc.gnu.org/onlinedocs/cpp/Header-Files.html

lib/README

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+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link into executable file.
+
+The source code of each library should be placed in a an own separate directory
+("lib/your_library_name/[here are source files]").
+
+For example, see a structure of the following two libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional, custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |
+|  |- README --> THIS FILE
+|
+|- platformio.ini
+|--src
+   |- main.c
+
+and a contents of `src/main.c`:
+```
+#include <Foo.h>
+#include <Bar.h>
+
+int main (void)
+{
+  ...
+}
+
+```
+
+PlatformIO Library Dependency Finder will find automatically dependent
+libraries scanning project source files.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html

platformio.ini

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+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:uno]
+platform = atmelavr
+board = uno

src/main.c

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+#include <stdio.h>
+#include <avr/io.h>
+#include <stdint.h>
+#include "uart.h"
+#include "swio.h"
+
+#define TARGET_NRESET_PORT  PORTB
+#define TARGET_NRESET_DDR   DDRB
+#define TARGET_NRESET_BIT   1
+
+void target_reset(int x) {
+    if (x)
+        TARGET_NRESET_PORT &= ~_BV(TARGET_NRESET_BIT);
+    else
+        TARGET_NRESET_PORT |= _BV(TARGET_NRESET_BIT);
+}
+
+#define PROTOCOL_START     '!'
+#define PROTOCOL_ACK       '+'
+#define PROTOCOL_TEST      '?'
+#define PROTOCOL_RESET     'a'
+#define PROTOCOL_NORES     'A'
+#define PROTOCOL_WRITE_REG 'w'
+#define PROTOCOL_READ_REG  'r'
+
+int main() {
+    uint8_t reg;
+    uint32_t val;
+
+    // Make the target reset pin an output.
+    TARGET_NRESET_DDR |= _BV(TARGET_NRESET_BIT);
+
+    uart_init();
+    swio_init();
+
+    fputc(PROTOCOL_START, uart);
+    while (1) {
+        switch (fgetc(uart)) {
+            case PROTOCOL_TEST:
+                fputc(PROTOCOL_ACK, uart);
+                break;
+            case PROTOCOL_RESET:
+                target_reset(1);
+                fputc(PROTOCOL_ACK, uart);
+                break;
+            case PROTOCOL_NORES:
+                target_reset(0);
+                fputc(PROTOCOL_ACK, uart);
+                break;
+            case PROTOCOL_WRITE_REG:
+                fread(&reg, sizeof(uint8_t), 1, uart);
+                fread(&val, sizeof(uint32_t), 1, uart);
+                swio_write_reg(reg, val);
+                fputc(PROTOCOL_ACK, uart);
+                break;
+            case PROTOCOL_READ_REG:
+                fread(&reg, sizeof(uint8_t), 1, uart);
+                val = swio_read_reg(reg);
+                fwrite(&val, sizeof(uint32_t), 1, uart);
+                break;
+        }
+    }
+}

src/swio.c

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+#include "swio.h"
+
+// Timings.
+// T = 1 / 8 MHz
+// 0 = T-4T low, T-16T high
+// 1 = 6T-64T low, T-16T high
+
+// Note: bitwise ops take up 2 clock cycles (1T).
+// nop are 1 cycle (0.5T).
+
+static inline void swio_send_one() {
+    /* 0.0T */ SWIO_DDR |= _BV(SWIO_BIT);
+    /* 1.0T */ SWIO_PORT &= ~_BV(SWIO_BIT);
+    /* 2.0T */ asm("nop"); asm("nop");
+    /* 3.0T */ SWIO_PORT |= _BV(SWIO_BIT);
+    /* 4.0T */ SWIO_DDR &= ~SWIO_BIT;
+}
+
+static inline void swio_send_zero() {
+    /* 0.0T */ SWIO_DDR |= _BV(SWIO_BIT);
+    /* 1.0T */ SWIO_PORT &= ~_BV(SWIO_BIT);
+    /* 2.0T */ asm("nop"); asm("nop");
+    /* 3.0T */ asm("nop"); asm("nop");
+    /* 4.0T */ asm("nop"); asm("nop");
+    /* 5.0T */ asm("nop"); asm("nop");
+    /* 6.0T */ asm("nop"); asm("nop");
+    /* 7.0T */ asm("nop"); asm("nop");
+    /* 8.0T */ SWIO_PORT |= _BV(SWIO_BIT);
+    /* 9.0T */ SWIO_DDR &= ~SWIO_BIT;
+}
+
+static inline char swio_recv_bit() {
+    char x;
+    /* 0.0T */ SWIO_DDR |= _BV(SWIO_BIT);
+    /* 1.0T */ SWIO_PORT &= ~_BV(SWIO_BIT);
+    /* 2.0T */ SWIO_PORT |= _BV(SWIO_BIT); // Precharge when the line is floating.
+    /* 3.0T */ SWIO_DDR &= ~_BV(SWIO_BIT);
+    /* 4.0T */ asm("nop"); asm("nop");
+    /* 5.0T */ asm("nop"); asm("nop");
+    /* 6.0T */ x = SWIO_PIN;
+
+    // Wait for the line to come back up if it's down.
+    while (!(SWIO_PIN & _BV(SWIO_BIT)))
+        ;
+
+    return x & _BV(SWIO_BIT);
+}
+
+// Write a register.
+void swio_write_reg(uint8_t addr, uint32_t val) {
+    char i;
+
+    // Start bit.
+    swio_send_one();
+
+    // Send the address.
+    for (i = 0; i < 7; i++) {
+        if (addr & 0x40)
+            swio_send_one();
+        else
+            swio_send_zero();
+
+        addr <<= 1;
+    }
+
+    // Start bit.
+    swio_send_one();
+
+    // Send the word.
+    for (i = 0; i < 32; i++) {
+        if (val & 0x80000000)
+            swio_send_one();
+        else
+            swio_send_zero();
+
+        val <<= 1;
+    }
+
+    // Stop bit.
+    _delay_us(10);
+}
+
+// Read a register.
+uint32_t swio_read_reg(uint8_t addr) {
+    char i;
+    uint32_t x = 0;
+
+    // Start bit.
+    swio_send_one();
+
+    // Send the address.
+    for (i = 0; i < 7; i++) {
+        if (addr & 0x40)
+            swio_send_one();
+        else
+            swio_send_zero();
+
+        addr <<= 1;
+    }
+
+    // Start bit.
+    swio_send_zero();
+
+    // Receive the word.
+    for (i = 0; i < 32; i++) {
+        x <<= 1;
+
+        if (swio_recv_bit())
+            x |= 1;
+    }
+
+    // Stop bit.
+    _delay_us(10);
+
+    return x;
+}
+
+void swio_init() {
+    SWIO_PORT |= _BV(SWIO_BIT);
+    SWIO_DDR |= _BV(SWIO_BIT);
+    _delay_ms(5);
+    SWIO_PORT &= ~_BV(SWIO_BIT);
+    _delay_ms(20);
+    SWIO_PORT |= _BV(SWIO_BIT);
+    SWIO_DDR &= ~_BV(SWIO_BIT);
+}

src/swio.h

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+#ifndef SWIO_H
+#define SWIO_H
+#include <avr/io.h>
+#include <util/delay.h>
+
+// SWIO on PB0
+#define SWIO_DDR  DDRB
+#define SWIO_PORT PORTB
+#define SWIO_PIN  PINB
+#define SWIO_BIT  0
+
+void swio_init();
+void swio_write_reg(uint8_t addr, uint32_t val);
+uint32_t swio_read_reg(uint8_t addr);
+
+#endif

src/uart.c

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+#include <avr/io.h>
+#include "uart.h"
+
+int _uart_putchar(char c, FILE *unused);
+int _uart_getchar(FILE *unused);
+FILE uart_file = FDEV_SETUP_STREAM(_uart_putchar, _uart_getchar, _FDEV_SETUP_READ | _FDEV_SETUP_WRITE);
+FILE *uart = &uart_file;
+
+void uart_init() {
+    // Enable 2X clock.
+    UCSR0A |= _BV(U2X0);
+
+    // Set the baud rate divider.
+    // 16M / 8 / (16 + 1) = 117647 bps (2% error)
+    UBRR0L = 16;
+
+    // Enable TX.
+    UCSR0B |= _BV(TXEN0) | _BV(RXEN0);
+}
+
+int _uart_putchar(char c, FILE *unused) {
+    (void)unused;
+
+    // Wait for the UART to become ready.
+    while (!(UCSR0A & _BV(UDRE0)))
+        ;
+
+    // Send.
+    UDR0 = c;
+
+    return 0;
+}
+
+int _uart_getchar(FILE *unused) {
+    (void)*unused;
+
+    // Wait for a character.
+    while (!(UCSR0A & _BV(RXC0)))
+        ;
+
+    return UDR0;
+}

src/uart.h

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+#ifndef UART_H
+#define UART_H
+
+#include <stdio.h>
+extern FILE *uart;
+
+void uart_init();
+
+#endif