Add bitband module

The code is just copied from tm4c123x-hal.

See-Also: rust-embedded-community/tm4c-hal#9

src/bitband.rs

@@ -0,0 +1,29 @@
+//! Bit-banding access for peripheral registers
+//!
+//! The addresses for this are derived from the Cortex-M4 generic guide, but the EFM32-Cortex-M3
+//! guide uses the same addresses.
+//!
+//! The implementation is copied over from the tm4c123x-hal crate, pending generalization.
+//!
+//! (The original code is licensed under the same terms and available on
+//! <https://github.com/thejpster/tm4c123x-hal>)
+
+use core::ptr::write_volatile;
+
+/// Sets/Clears a bit at the given address atomically, using the bit-banding
+/// feature. We take a const pointer and mutate it, but that's because the
+/// svd2rust crate will only give us const pointers.
+pub unsafe fn change_bit<T>(address: *const T, bit: u8, value: bool) {
+    let address = address as u32;
+    let bit_word = ref_to_bitband(address, bit);
+    write_volatile(bit_word, if value { 0x01 } else { 0x00 });
+}
+
+/// Address must be >= 0x2000_0000 and <= 0x2007_FFFC. Bit must be < 32.
+fn ref_to_bitband(address: u32, bit: u8) -> *mut u32 {
+    let prefix = address & 0xF000_0000;
+    let byte_offset = address & 0x0FFF_FFFF;
+    let bit_word_offset = (byte_offset * 32) + (bit as u32 * 4);
+    let bit_word_addr = bit_word_offset + prefix + 0x0200_0000;
+    bit_word_addr as *mut u32
+}

src/lib.rs

@@ -9,3 +9,5 @@ pub mod time_util;
 pub mod gpio;
 pub mod cmu;
 pub mod systick;
+
+mod bitband;