Example of using i2c with the bme280 air sensor (stm32-rs#313)

* Example of useing i2c with the bme280 air sensor

.cargo/config

@@ -1,5 +1,5 @@
 [target.thumbv7m-none-eabi]
-runner = 'gdb-multiarch'
+runner = 'arm-none-eabi-gdb'
 rustflags = [
   "-C", "link-arg=-Tlink.x",
 ]

examples/i2c-bme280/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "i2c-bme280"
+version = "0.1.0"
+license = "Apache-2.0"
+description = "I2C example for real peripheral"
+repository = "https://github.com/stm32-rs/stm32f1xx-hal"
+edition = "2018"
+publish = false
+
+[dependencies]
+bme280 = "0.2.1"
+cortex-m-semihosting = "0.3.3"
+panic-semihosting = "0.5.2"
+cortex-m-rt = "0.6.8"
+cortex-m = "0.6.0"
+
+[dependencies.stm32f1xx-hal]
+path = "../.."
+version = "0.7.0"
+features = ["stm32f103", "rt", "stm32-usbd"]
+
+[profile.dev]
+incremental = false
+codegen-units = 1
+
+[profile.release]
+codegen-units = 1
+debug = true
+lto = true

examples/i2c-bme280/memory.x

@@ -0,0 +1,6 @@
+/* Linker script for the STM32F103C8T6 */
+MEMORY
+{
+  FLASH : ORIGIN = 0x08000000, LENGTH = 64K
+  RAM : ORIGIN = 0x20000000, LENGTH = 20K
+}

examples/i2c-bme280/src/main.rs

@@ -0,0 +1,103 @@
+//! Reads data from a BME280 over i2c
+//!
+//! This assumes that a BME280 is connected with clk on PB6 and data on PB7.
+//!
+//! For the Adafruit breakout boards PB6 should be connected to SCK and PB7 to SDI
+//!
+//! This program writes the sensor values to the debug output provided by semihosting
+//! you must enable semihosting in gdb with `monitor arm semihosting enable` I have it
+//! added to my `.gdbinit`. Then the debug infomation will be printed in your openocd
+//! terminal.
+//!
+//! This program dose not fit on my blue pill unless compiled in release mode
+//! eg. `cargo run --example i2c-bme280 --features "stm32f103 bme280 rt" --release`
+//! However as noted above the debug output with the read values will be in the openocd
+//! terminal.
+
+#![deny(unsafe_code)]
+#![no_std]
+#![no_main]
+
+use cortex_m_semihosting::hprintln;
+use panic_semihosting as _;
+
+use bme280::BME280;
+use cortex_m_rt::entry;
+use stm32f1xx_hal::{
+    delay::Delay,
+    i2c::{BlockingI2c, DutyCycle, Mode},
+    pac,
+    prelude::*,
+};
+
+#[entry]
+fn main() -> ! {
+    // Get access to the core peripherals from the cortex-m crate
+    let cp = cortex_m::Peripherals::take().unwrap();
+    // Get access to the device specific peripherals from the peripheral access crate
+    let dp = pac::Peripherals::take().unwrap();
+
+    // Take ownership over the raw flash and rcc devices and convert them into the corresponding
+    // HAL structs
+    let mut flash = dp.FLASH.constrain();
+    let mut rcc = dp.RCC.constrain();
+    let mut afio = dp.AFIO.constrain();
+    // Freeze the configuration of all the clocks in the system and store the frozen frequencies in
+    // `clocks`
+    let clocks = if 1 == 1 {
+        rcc.cfgr.use_hse(8.mhz()).freeze(&mut flash.acr)
+    } else {
+        // My blue pill with a stm32f103 clone dose not seem to respect rcc so will not compensate its pulse legths
+        // with a faster clock like this. And so the sensor dose not have time to respond to the START pulse.
+        // I would be interested if others with real stm32f103's can use this program with the faster clocks.
+        rcc.cfgr
+            .use_hse(8.mhz())
+            .sysclk(48.mhz())
+            .pclk1(6.mhz())
+            .freeze(&mut flash.acr)
+    };
+
+    // Acquire the GPIOB peripheral
+    let mut gpiob = dp.GPIOB.split();
+
+    let scl = gpiob.pb6.into_alternate_open_drain(&mut gpiob.crl);
+    let sda = gpiob.pb7.into_alternate_open_drain(&mut gpiob.crl);
+
+    let i2c = BlockingI2c::i2c1(
+        dp.I2C1,
+        (scl, sda),
+        &mut afio.mapr,
+        Mode::Fast {
+            frequency: 400_000.hz(),
+            duty_cycle: DutyCycle::Ratio16to9,
+        },
+        clocks,
+        1000,
+        10,
+        1000,
+        1000,
+    );
+
+    // The Adafruit boards have address 0x77 without closing the jumper on the back, the BME280 lib connects to 0x77 with `new_secondary`, use
+    // `new_primary` for 0x76 if you close the jumper/solder bridge.
+    let mut bme280 = BME280::new_secondary(i2c, Delay::new(cp.SYST, clocks));
+    bme280
+        .init()
+        .map_err(|error| {
+            hprintln!("Could not initialize bme280, Error: {:?}", error).unwrap();
+            panic!();
+        })
+        .unwrap();
+    loop {
+        match bme280.measure() {
+            Ok(measurements) => {
+                hprintln!("Relative Humidity = {}%", measurements.humidity).unwrap();
+                hprintln!("Temperature = {} deg C", measurements.temperature).unwrap();
+                hprintln!("Pressure = {} pascals", measurements.pressure).unwrap();
+            }
+            Err(error) => {
+                hprintln!("Could not read bme280 due to error: {:?}", error).unwrap();
+            }
+        }
+    }
+}