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Clean up slides.
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Jonathan Pallant (Ferrous Systems)
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1 changed files with 54 additions and 42 deletions
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@ -4,8 +4,7 @@
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## Introduction
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This crate sits at the bottom of the 'stack'. It provides access to the
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memory-mapped peripherals in your MCU.
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This crate sits at the bottom of the 'stack'. It provides access to the memory-mapped peripherals in your MCU.
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---
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@ -19,7 +18,7 @@ memory-mapped peripherals in your MCU.
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---
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## Datasheets
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## Registers
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* *Registers* are comprised of one or more *fields*.
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* Each field is at least 1 bit in length.
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@ -30,24 +29,19 @@ memory-mapped peripherals in your MCU.
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## C Code!
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Embedded Code in C often uses shifts and bitwise-AND to make up registers from
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fields.
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Embedded Code in C often uses shifts and bitwise-AND to make up registers from fields.
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```c,no_run
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```c
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#define UARTE_INTEN_CTS_SHIFT (0)
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#define UARTE_INTEN_CTS_MASK (0x00000001)
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#define UARTE_INTEN_NCTS_SHIFT (1)
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#define UARTE_INTEN_NCTS_MASK (0x00000001)
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#define UARTE_INTEN_RXRDY_SHIFT (2)
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#define UARTE_INTEN_RXRDY_MASK (0x00000001)
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// The other eight fields are skipped for brevity
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// The other nine fields are skipped for brevity
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uint32_t cts = 0;
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uint32_t ncts = 1;
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uint32_t rxrdy = 1;
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uint32_t inten_value = ((cts & UARTE_INTEN_CTS_MASK) << UARTE_INTEN_CTS_SHIFT)
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| ((ncts & UARTE_INTEN_NCTS_MASK) << UARTE_INTEN_NCTS_SHIFT)
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| ((rxrdy & UARTE_INTEN_RXRDY_MASK) << UARTE_INTEN_RXRDY_SHIFT);
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*((volatile uint32_t*) 0x40002300) = inten_value;
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@ -55,11 +49,24 @@ uint32_t inten_value = ((cts & UARTE_INTEN_CTS_MASK) << UARTE_INTEN_CTS_SHIFT)
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---
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## Rust Code
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You *could* do this in Rust if you wanted...
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```rust
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const UARTE0_INTEN: *mut u32 = 0x4000_2300 as *mut u32;
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unsafe { UARTE0_INTEN.write_volatile(0x0000_0003); }
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```
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But this still seems very error-prone. Nothing stops you putting the wrong value at the wrong address.
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---
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## Adding structure
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The various registers for a peripheral are often grouped into a `struct`
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In C, the various registers for a peripheral can also be grouped into a `struct`
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```c,no_run
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```c
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typedef volatile struct uart0_reg_t {
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uint32_t tasks_startrx; // @ 0x000
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uint32_t tasks_stoprx; // @ 0x004
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@ -67,27 +74,37 @@ typedef volatile struct uart0_reg_t {
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uint32_t inten; // @ 0x300
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uint32_t _padding[79];
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uint32_t baudrate; // @ 0x500
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} uart0_reg_t;
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} uart0_reg_t
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uart0_reg_t* const p_uart = (uart0_reg_t*) 0x40002000;
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```
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---
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## Rust Code
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## Structures in Rust
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You *could* do this in Rust if you wanted...
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We can do that too (and this is how our PAC works under the hood).
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```rust,no_run
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const UARTE0_INTEN: *mut u32 = 0x4000_2300 as *mut u32;
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unsafe { UARTE0_INTEN.write_volatile(0x0000_0003); }
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```rust
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pub struct RegisterBlock {
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pub tasks_startrx: VolatileCell<u32>, // @ 0x000
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pub tasks_stoprx: VolatileCell<u32>, // @ 0x004
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// ...
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pub inten: VolatileCell<u32>, // @ 0x300
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_reserved12: [u32; 79],
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pub baudrate: VolatileCell<u32>, // @ 0x500
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}
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let p_uart: &RegisterBlock = unsafe { &*(0x40002000 as *const RegisterBlock) };
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```
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But it seems like a lot of reading PDFs and re-typing everything?
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We use the [`VolatileCell`](https://docs.rs/vcell/0.1.3/vcell/struct.VolatileCell.html) to ensure reads/writes on the structure fields are always performed with volatile pointer read/writes.
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---
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## CMSIS-SVD Files
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A CMSIS-SVD (or just SVD) file is an XML description of all the peripherals,
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registers and fields on an MCU.
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A CMSIS-SVD (or just SVD) file is an XML description of all the peripherals, registers and fields on an MCU.
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We can use `svd2rust` to turn this into a Peripheral Access Crate.
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@ -100,12 +117,6 @@ graph LR
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## The `svd2rust` generated API
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* The crate has a top-level `struct Peripherals` with members for each *Peripheral*
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* Each *Peripheral* gets a `struct`, like `UARTE0`, `SPI1`, etc.
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* Each *Peripheral* `struct` has members for each *Register*
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* Each *Register* gets a `struct`, like `BAUDRATE`, `INTEN`, etc.
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* Each *Register* `struct` has `read()`, `write()` and `modify()` methods
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```mermaid
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graph TB
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Peripherals --> uarte1[.UARTE1: <b>UARTE1</b>]
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@ -116,6 +127,12 @@ graph TB
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uarte2 --> uart2_inten[.inten: <b>INTEN</b>]
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```
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* The crate has a top-level `struct Peripherals` with members for each *Peripheral*
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* Each *Peripheral* gets a `struct`, like `UARTE0`, `SPI1`, etc.
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* Each *Peripheral* `struct` has members for each *Register*
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* Each *Register* gets a `struct`, like `BAUDRATE`, `INTEN`, etc.
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* Each *Register* `struct` has `read()`, `write()` and `modify()` methods
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---
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## The `svd2rust` generated API (2)
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@ -129,14 +146,14 @@ graph TB
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---
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## An example
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## Using a PAC
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```rust,no_run
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```rust
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// nrf52840 is the PAC
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let p = nrf52840::Peripherals::take().unwrap();
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// This register has only one field
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// Reading the 'baudrate' field
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let current_baud_rate = p.UARTE1.baudrate.read().baudrate();
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// This register has multiple fields
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// Modifying multiple fields in one go
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p.UARTE1.inten.modify(|_r, w| {
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w.cts().enabled();
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w.ncts().enabled();
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@ -157,15 +174,11 @@ p.UARTE1.inten.modify(|_r, w| {
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## Let's take it in turns
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* I, the callee, need to set some stuff up
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* You, the caller, need to do a bit of work
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* I, the callee, need to clean everything up
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- I, the callee, need to set some stuff up
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- You, the caller, need to do a bit of work
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- I, the callee, need to clean everything up
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We can use a closure to insert the caller-provided code in the middle of our
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function. We see this used
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[all](https://doc.rust-lang.org/core/iter/trait.Iterator.html#method.map)
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[over](https://doc.rust-lang.org/core/primitive.str.html#method.matches)
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[the](https://doc.rust-lang.org/std/thread/fn.spawn.html) Rust standard library!
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We can use a closure to insert the caller-provided code in the middle of our function. We see this used [all (1)](https://doc.rust-lang.org/core/iter/trait.Iterator.html#method.map) [over (2)](https://doc.rust-lang.org/core/primitive.str.html#method.matches) [the (3)](https://doc.rust-lang.org/std/thread/fn.spawn.html) Rust standard library!
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---
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@ -190,5 +203,4 @@ Docs can be generated from the source code.
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See <https://docs.rs/nrf52840>
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Note that `uarte0` is a *module* and `UARTE0` could mean either a `struct` type,
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or a field on the `Peripherals` struct.
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Note that `uarte0` is a *module* and `UARTE0` could mean either a `struct` type, or a field on the `Peripherals` struct.
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