Support for I2C (stm32-rs#26)

* Add support for I2C.

Most of this code is taken from stm32g0xx-hal, with only minor changes to
the way pins are initialized and to the interaction with RCC.

* Improve the I2C documentation.

* Fix compiler error for devices without I2C4.

* Add an I2C/MPU6050 example (the timing is still broken).

* Fix the I2C bit rate.

The std::max() meant that scll was always set to 255, resulting in bit
rates that were far too slow. Also, the documentation states that the
clock is divided by PRESC+1, not by 2^(PRESC+1).

Cargo.toml

@@ -53,6 +53,7 @@ cortex-m-log = { version = "0.7", features = ["log-integration"] }
 cfg-if = "0.1.10"
 rtt-target = { version = "0.3.0", features = ["cortex-m"] }
 panic-rtt-target = { version = "0.1.1", features = ["cortex-m"] }
+mpu6050 = "0.1.4"
 
 [features]
 default = ["rt"]

examples/i2c-mpu6050.rs

@@ -0,0 +1,44 @@
+#![deny(warnings)]
+#![deny(unsafe_code)]
+#![no_main]
+#![no_std]
+
+use hal::i2c::Config;
+use hal::prelude::*;
+use hal::stm32;
+use stm32g4xx_hal as hal;
+
+use cortex_m_rt::entry;
+use log::info;
+use mpu6050::*;
+
+#[macro_use]
+mod utils;
+
+#[entry]
+fn main() -> ! {
+    utils::logger::init();
+
+    let dp = stm32::Peripherals::take().expect("cannot take peripherals");
+    let cp = cortex_m::Peripherals::take().expect("cannot take core peripherals");
+
+    let mut rcc = dp.RCC.constrain();
+    let gpiob = dp.GPIOB.split(&mut rcc);
+
+    let sda = gpiob.pb9.into_alternate_open_drain();
+    let scl = gpiob.pb8.into_alternate_open_drain();
+
+    let i2c = dp.I2C1.i2c(sda, scl, Config::new(100.khz()), &mut rcc);
+
+    let mut mpu = Mpu6050::new(i2c);
+    let mut delay = cp.SYST.delay(&rcc.clocks);
+    mpu.init(&mut delay).expect("cannot initialize the MPU6050");
+
+    loop {
+        let acc = mpu.get_acc().expect("cannot read accelerometer");
+        let gyro = mpu.get_gyro().expect("cannot read gyro");
+        let temp = mpu.get_temp().expect("cannot read temperature");
+        info!("acc: {:?}, gyro: {:?}, temp: {:?}C", acc, gyro, temp);
+        delay.delay(250.ms());
+    }
+}

examples/i2c.rs

@@ -0,0 +1,42 @@
+#![deny(warnings)]
+#![deny(unsafe_code)]
+#![no_main]
+#![no_std]
+
+use hal::i2c::Config;
+use hal::prelude::*;
+use hal::stm32;
+use stm32g4xx_hal as hal;
+
+use cortex_m_rt::entry;
+use log::info;
+
+#[macro_use]
+mod utils;
+
+#[entry]
+fn main() -> ! {
+    utils::logger::init();
+
+    let dp = stm32::Peripherals::take().expect("cannot take peripherals");
+    let mut rcc = dp.RCC.constrain();
+    let gpiob = dp.GPIOB.split(&mut rcc);
+
+    let sda = gpiob.pb9.into_alternate_open_drain();
+    let scl = gpiob.pb8.into_alternate_open_drain();
+
+    let mut i2c = dp.I2C1.i2c(sda, scl, Config::new(40.khz()), &mut rcc);
+    // Alternatively, it is possible to specify the exact timing as follows (see the documentation
+    // of with_timing() for an explanation of the constant):
+    //let mut i2c = dp
+    //  .I2C1
+    //   .i2c(sda, scl, Config::with_timing(0x3042_0f13), &mut rcc);
+
+    let buf: [u8; 1] = [0];
+    loop {
+        match i2c.write(0x3c, &buf) {
+            Ok(_) => info!("ok"),
+            Err(err) => info!("error: {:?}", err),
+        }
+    }
+}