[−][src]Module drone_cortex_m::reg

The Memory-Mapped Registers module.

NOTE A Drone platform crate may re-export this module with its own additions under the same name, in which case it should be used instead.

A memory-mapped register is a special location in memory. Reads and writes from/to this location produce side-effects. For example writing 1 or 0 to such location may set the related GPIO output pin to the high or low logical level. In the same way reading from such location may return 1 or 0 depending on the input level of the related GPIO input pin. The most basic way to deal with this memory is to use [core::ptr::read_volatile] and [core::ptr::write_volatile]. Here is an example:

use core::ptr::{read_volatile, write_volatile};

// The address of GPIOA_CRL register.
const GPIOA_CRL: usize = 0x4001_0800;
// The offset for MODE2 field of GPIOA_CRL register.
const GPIOA_CRL_MODE2_OFFSET: usize = 8;
// The mask for MODE2 field of GPIOA_CRL register.
const GPIOA_CRL_MODE2_MASK: u32 = 0x0000_0300;

// Read the state of GPIOA_CRL register. The function is unsafe because it
// can read from any location in memory.
let mut gpioa_crl = unsafe { read_volatile(GPIOA_CRL as *const u32) };
// Do bit arithmetic to get the value of MODE2 field.
let mut gpioa_crl_mode2 = (gpioa_crl & GPIOA_CRL_MODE2_MASK) >> GPIOA_CRL_MODE2_OFFSET;
// Toggle some bits.
gpioa_crl_mode2 ^= 0b10;
// Do bit arithmetic to update the register value with the new field value.
gpioa_crl = gpioa_crl & !GPIOA_CRL_MODE2_MASK | gpioa_crl_mode2 << GPIOA_CRL_MODE2_OFFSET;
// Update the state of GPIOA_CRL register. The function is also unsafe
// because it can write to any location in memory.
unsafe { write_volatile(GPIOA_CRL as *mut u32, gpioa_crl) };

This way has numerous disadvantages: it's obscure, verbose, error-prone, and requires unsafe blocks. Also it has less obvious problems like lack of thread-safety. This module provides safe and zero-cost abstractions to this problem. As result the above example can be written like this:

gpioa_crl.modify(|r| r.write_mode2(r.mode2() ^ 0b10));

We abstract this type of memory with zero-sized [token]s. (Familiarity with [token] module is required.) Only the code that have the token instance for a particular memory-mapped register can manipulate it safely. At the very base there is Register Field Token (like MODE2 in the above example.) Register Field Tokens for a particular register grouped in Register Token (like GPIO_CRL in the above example.) And all available Register Tokens are grouped in one Register Tokens Index.

API

The memory-mapped registers API is scattered across numerous traits. Therefore it is recommended to use [reg::prelude]:

use drone_core::reg::prelude::*;

Field Token

	Field Width	Field Mode	Register Mode
`into_unsync`
`into_sync`
`into_copy`
`as_sync`
`load_val`		read	read
`default_val`		write	write-only
`store_val`		write	write-only
`store`		write	write-only
`read`	one-bit	read	read
`read_bit`	one-bit	read	read
`set`	one-bit	write	write
`clear`	one-bit	write	write
`toggle`	one-bit	write	write
`set_bit`	one-bit	write	write-only
`clear_bit`	one-bit	write	write-only
`toggle_bit`	one-bit	write	write-only
`read`	multi-bit	read	read
`read_bits`	multi-bit	read	read
`write`	multi-bit	write	write
`write_bits`	multi-bit	write	write-only

Register Token

	Mode	Tag
`into_unsync`
`into_sync`
`into_copy`
`as_sync`
`default_val`
`default`
`hold`
`load_val`	read
`load`	read
`as_ptr`	read
`as_mut_ptr`	write
`store`	write	Urt
`store`	write	Srt, Crt
`store_val`	write	Urt
`store_val`	write	Srt, Crt
`reset`	write	Urt
`reset`	write	Srt, Crt
`modify`	read-write	Urt

Register Value

Autogenerated field methods for RegHold (foo as an example field name):

	Field Width	Mode
`foo()` (`read`)	one-bit	read
`foo()` (`read`)	multi-bit	read
`set_foo()` (`set`)	one-bit	write
`clear_foo()` (`clear`)	one-bit	write
`toggle_foo()` (`toggle`)	one-bit	write
`write_foo(bits)` (`write`)	multi-bit	write

Mappings

We define concrete register mappings in platform crates. Usually the user doesn't need to map registers themselves. But lets have a look to an example of how it could be organized for STM32 platform:


use core::mem::size_of_val;
use drone_core::{reg::prelude::*, token::Token};

use drone_core::reg;

// ----- this is drone_cortex_m crate -----

// Registers belong to blocks. Here we declare CTRL register in STK block.
reg! {
    // The output of this macro is `pub mod stk_ctrl { ... }`.
    /// SysTick control and status register.
    pub mod STK CTRL;
    0xE000_E010 // the register address in memory
    0x20        // size of the register in bits
    0x0000_0000 // reset value of the register
    // Traits to implement for the register token. The most common sets are:
    //     RReg RoReg - read-only register
    //     RReg WReg  - read-write register
    //     WReg WoReg - write-only register
    RReg WReg;

    // Here we define register fields.
    /// Counter enable.
    ENABLE {
        0 // the offset of the field
        1 // the width of the field
        // Traits to implement for the field token. The most common sets are:
        //     RRRegField RoRRegField - read-only field
        //     RRRegField WWRegField  - read-write field
        //     WWRegField WoWRegField - read-write field
        RRRegField WWRegField
    }
}

// Here we define the register tokens index. Actually the result of this macro
// is another macro, which can be used to define the final register token index
// or to extend with another registers in downstream crates. It will become
// clearer below.
reg::tokens! {
    // The result of this macro is
    // `macro_rules! cortex_m_reg_tokens { ... }`.
    /// Defines an index of core ARM Cortex-M register tokens.
    pub macro cortex_m_reg_tokens;
    // Path prefix to reach registers.
    crate;
    // Absolute path to the current module.
    crate;

    // Here we declare all register blocks. This produces `pub mod stk { ... }`
    /// SysTick timer.
    pub mod STK {
        // Declare all registers for this block. This produces:
        // pub mod stk {
        //     pub use crate::stk_ctrl as ctrl;
        // }
        CTRL;
    }
}

// ----- this is drone_stm32 crate -----
// This crate parses SVD files provided by the manufacturer and generates more
// registers.

// Same as above, except it will reuse the upstream macro, resulting in a
// combined register tokens index. Note `use macro cortex_m_reg_tokens`.
reg::tokens! {
    /// Defines an index of STM32F103 register tokens.
    pub macro stm32_reg_tokens;
    use macro cortex_m_reg_tokens;
    crate;
    crate;
}

// ----- this is an application crate -----

// This macro defines the concrete register tokens index for STM32 MCU. The
// index is a sum of `drone_cortex_m` and `drone_stm32` registers. The result
// of this macro is `pub struct Regs { ... }`.
stm32_reg_tokens! {
    /// Register tokens.
    pub struct Regs;
}

// Your entry point.
fn main() {
    // It's unsafe because we can accidentally create more than one instance
    // of the index.
    let reg = unsafe { Regs::take() };
    // The index doesn't really exist in memory.
    assert_eq!(size_of_val(&reg), 0);
    assert_eq!(size_of_val(&reg.stk_ctrl), 0);
    assert_eq!(size_of_val(&reg.stk_ctrl.enable), 0);
    // Pass the index to your safe entry point.
    trunk(reg);
}

fn trunk(reg: Regs) {}

Modules

field	Memory-mapped register fields module.
marker	Marker traits representing properties of memory-mapped registers.
prelude	The Memory-Mapped Registers prelude.
tag	Register token tags and their traits.

Macros

tokens

	Atomic	Affine
`Urt` (Unsynchronized)	-	+
`Srt` (Synchronized)	+	+
`Crt` (Copyable)	+	-