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// Copyright 2019 The Pigweed Authors
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#include <Arduino.h>
#include <cinttypes>
#include <cstdint>
#include "pw_preprocessor/compiler.h"
#include "pw_sys_io/sys_io.h"
extern "C" void pw_sys_io_Init() { Serial.begin(115200); }
namespace pw::sys_io {
// Wait for a byte to read on USART1. This blocks until a byte is read. This is
// extremely inefficient as it requires the target to burn CPU cycles polling to
// see if a byte is ready yet.
Status ReadByte(std::byte* dest) {
while (true) {
if (TryReadByte(dest).ok()) {
return Status::Ok();
Status TryReadByte(std::byte* dest) {
if (!Serial.available()) {
return Status::Unavailable();
*dest = static_cast<std::byte>(;
return Status::Ok();
// Send a byte over USART1. Since this blocks on every byte, it's rather
// inefficient. At the default baud rate of 115200, one byte blocks the CPU for
// ~87 micro seconds. This means it takes only 10 bytes to block the CPU for
// 1ms!
Status WriteByte(std::byte b) {
// Wait for TX buffer to be empty. When the buffer is empty, we can write
// a value to be dumped out of UART.
while (Serial.availableForWrite() < 1) {
return Status::Ok();
// Writes a string using pw::sys_io, and add newline characters at the end.
StatusWithSize WriteLine(const std::string_view& s) {
size_t chars_written = 0;
StatusWithSize result = WriteBytes(std::as_bytes(std::span(s)));
if (!result.ok()) {
return result;
chars_written += result.size();
// Write trailing newline.
result = WriteBytes(std::as_bytes(std::span("\r\n", 2)));
chars_written += result.size();
return StatusWithSize(result.status(), chars_written);
} // namespace pw::sys_io