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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
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// 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 <cinttypes>
#include "pw_preprocessor/compiler.h"
#include "pw_sys_io/sys_io.h"
namespace {
// Default core clock. This is technically not a constant, but since this app
// doesn't change the system clock a constant will suffice.
constexpr uint32_t kSystemCoreClock = 16000000;
// Base address for everything peripheral-related on the STM32F4xx.
constexpr uint32_t kPeripheralBaseAddr = 0x40000000u;
// Base address for everything AHB1-related on the STM32F4xx.
constexpr uint32_t kAhb1PeripheralBase = kPeripheralBaseAddr + 0x00020000U;
// Base address for everything APB2-related on the STM32F4xx.
constexpr uint32_t kApb2PeripheralBase = kPeripheralBaseAddr + 0x00010000U;
// Reset/clock configuration block (RCC).
// `reserved` fields are unimplemented features, and are present to ensure
// proper alignment of registers that are in use.
PW_PACKED(struct) RccBlock {
uint32_t reserved1[12];
uint32_t ahb1_config;
uint32_t reserved2[4];
uint32_t apb2_config;
};
// Mask for ahb1_config (AHB1ENR) to enable the "A" GPIO pins.
constexpr uint32_t kGpioAEnable = 0x1u;
// Mask for apb2_config (APB2ENR) to enable USART1.
constexpr uint32_t kUsart1Enable = 0x1u << 4;
// GPIO register block definition.
PW_PACKED(struct) GpioBlock {
uint32_t modes;
uint32_t out_type;
uint32_t out_speed;
uint32_t pull_up_down;
uint32_t input_data;
uint32_t output_data;
uint32_t gpio_bit_set;
uint32_t port_config_lock;
uint32_t alt_low;
uint32_t alt_high;
};
// Constants related to GPIO mode register masks.
constexpr uint32_t kGpio9PortModePos = 18;
constexpr uint32_t kGpio10PortModePos = 20;
constexpr uint32_t kGpioPortModeAlternate = 2;
// Constants related to GPIO port speed register masks.
constexpr uint32_t kGpio9PortSpeedPos = 18;
constexpr uint32_t kGpio10PortSpeedPos = 20;
constexpr uint32_t kGpioSpeedVeryHigh = 3;
// Constants related to GPIO pull up/down resistor type masks.
constexpr uint32_t kGpio9PullTypePos = 18;
constexpr uint32_t kGpio10PullTypePos = 20;
constexpr uint32_t kPullTypePullUp = 1;
// Constants related to GPIO port speed register masks.
constexpr uint32_t kGpio9AltModeHighPos = 4;
constexpr uint32_t kGpio10AltModeHighPos = 8;
// Alternate function for pins A9 and A10 that enable USART1.
constexpr uint8_t kGpioAlternateFunctionUsart1 = 0x07u;
// USART status flags.
constexpr uint32_t kTxRegisterEmpty = 0x1u << 7;
// USART configuration flags for config1 register.
// Note: a large number of configuration flags have been omitted as they default
// to reasonable values and we don't need to change them.
constexpr uint32_t kReceiveEnable = 0x1 << 2;
constexpr uint32_t kTransmitEnable = 0x1 << 3;
constexpr uint32_t kReadDataReady = 0x1u << 5;
constexpr uint32_t kEnableUsart = 0x1 << 13;
// Layout of memory mapped registers for USART blocks.
PW_PACKED(struct) UsartBlock {
uint32_t status;
// Only the lower 8 bits are valid. Read for RX data, write to TX data.
uint32_t data_register;
uint32_t baud_rate;
uint32_t config1;
uint32_t config2;
uint32_t config3;
uint32_t config4;
};
// Sets the UART baud register using the peripheral clock and target baud rate.
// These calculations are specific to the default oversample by 16 mode.
// TODO(amontanez): Document magic calculations in full UART implementation.
uint32_t CalcBaudRegister(uint32_t clock, uint32_t target_baud) {
uint32_t div_fac = (clock * 25) / (4 * target_baud);
uint32_t mantissa = div_fac / 100;
uint32_t fraction = ((div_fac - mantissa * 100) * 16 + 50) / 100;
return (mantissa << 4) + (fraction & 0xFFu);
}
// Declare a reference to the memory mapped RCC block.
volatile RccBlock& platform_rcc =
*reinterpret_cast<volatile RccBlock*>(kAhb1PeripheralBase + 0x3800U);
// Declare a reference to the 'A' GPIO memory mapped block.
volatile GpioBlock& gpio_a =
*reinterpret_cast<volatile GpioBlock*>(kAhb1PeripheralBase + 0x0000U);
// Declare a reference to the memory mapped block for USART1.
volatile UsartBlock& usart1 =
*reinterpret_cast<volatile UsartBlock*>(kApb2PeripheralBase + 0x1000U);
} // namespace
extern "C" void pw_sys_io_Init() {
// Enable 'A' GIPO clocks.
platform_rcc.ahb1_config |= kGpioAEnable;
// Enable Uart TX pin.
// Output type defaults to push-pull (rather than open/drain).
gpio_a.modes |= kGpioPortModeAlternate << kGpio9PortModePos;
gpio_a.out_speed |= kGpioSpeedVeryHigh << kGpio9PortSpeedPos;
gpio_a.pull_up_down |= kPullTypePullUp << kGpio9PullTypePos;
gpio_a.alt_high |= kGpioAlternateFunctionUsart1 << kGpio9AltModeHighPos;
// Enable Uart RX pin.
// Output type defaults to push-pull (rather than open/drain).
gpio_a.modes |= kGpioPortModeAlternate << kGpio10PortModePos;
gpio_a.out_speed |= kGpioSpeedVeryHigh << kGpio10PortSpeedPos;
gpio_a.pull_up_down |= kPullTypePullUp << kGpio10PullTypePos;
gpio_a.alt_high |= kGpioAlternateFunctionUsart1 << kGpio10AltModeHighPos;
// Initialize USART1. Initialized to 8N1 at the specified baud rate.
platform_rcc.apb2_config |= kUsart1Enable;
// Warning: Normally the baud rate register calculation is based off
// peripheral 2 clock. For this code, the peripheral clock defaults to
// the system core clock so it can be used directly.
usart1.baud_rate = CalcBaudRegister(kSystemCoreClock, /*target_baud=*/115200);
usart1.config1 = kEnableUsart | kReceiveEnable | kTransmitEnable;
}
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 OkStatus();
}
}
}
// 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 TryReadByte(std::byte* dest) {
if (!(usart1.status & kReadDataReady)) {
return Status::Unavailable();
}
*dest = static_cast<std::byte>(usart1.data_register);
return OkStatus();
}
// 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 (!(usart1.status & kTxRegisterEmpty)) {
}
usart1.data_register = static_cast<uint32_t>(b);
return OkStatus();
}
// 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