Difference between revisions of "Arduino-like pin definitions in C++"
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[[File:Lotsapins.jpg|right|300px]] | [[File:Lotsapins.jpg|right|300px]] | ||
| − | * Hard coding pin definitions is evil | + | In all non-trivial AVR projects, at some point in the software you need to define which pin is connected to what other device. Let's start with a manifesto: |
| − | * Raw AVR pin definitions, using | + | * Hard coding pin definitions–just using bit numbers, and port names at the places in code where you use pins–is evil |
| + | * Raw AVR pin definitions, using the preprocessor to define ports and bit indexes can become cumbersome, makes for less readable ''set'' and ''reset'' functions and blocks optimization opportunities | ||
* Arduino style pin definitions using functions like ''digitalWrite'' facilitate readable code, but are really, really slow. | * Arduino style pin definitions using functions like ''digitalWrite'' facilitate readable code, but are really, really slow. | ||
| − | This page describes our pin-definition library. This library allows for Arduino-like syntax for pin usage, while keeping the performance of 'manual' pin manipulations in C. | + | This page describes our [https://github.com/DannyHavenith/avr_utilities pin-definition library]. This library allows for Arduino-like syntax for pin usage, while keeping the performance of 'manual' pin manipulations in C. In short, it turns code like this: |
| + | <source lang="cpp"> | ||
| + | #define LED_DDR DDRC | ||
| + | #define LED_PORT PORTC | ||
| + | #define LED_PIN 5 | ||
| + | // set up | ||
| + | LED_DDR |= _BV(LED_PIN); | ||
| + | |||
| + | // assert pin | ||
| + | LED_PORT |= _BV(LED_PIN); | ||
| + | |||
| + | // clear pin | ||
| + | LED_PORT &= ~_BV(LED_PIN); | ||
| + | </source> | ||
| + | into code like the following, while maintaining the performance of the C-style original: | ||
| + | <source lang="cpp"> | ||
| + | PIN_TYPE( D, 6) led1; | ||
| + | |||
| + | make_output( led1); | ||
| + | set( led1); | ||
| + | clear( led1); | ||
| + | </source> | ||
==Pin definitions, the state of the art== | ==Pin definitions, the state of the art== | ||
===Raw AVR=== | ===Raw AVR=== | ||
| − | Most microcontroller projects revolve in some way around toggling signals on the output pins or reading values from input pins in some fashion. Concentrating on output for now, changing the output signals is done by manipulating individual bits in the AVR memory space that have been mapped onto these pins. So for example, if I wanted to make sure that pin 28 (the "top right" pin) of my atmega88 is in a "high" state, I would have to know that that pin is mapped on bit 5 in port C and write the following code: | + | Most microcontroller projects revolve in some way around toggling signals on the output pins or reading values from input pins in some fashion. Concentrating on output for now, changing the output signals is done by manipulating individual bits in the AVR memory space that have been mapped onto these pins. So for example, if I wanted to make sure that pin 28 (the "top right" pin, which–confusingly–is pin 19 in Arduino's digital pin functions) of my atmega88 is in a "high" state, I would have to know that that pin is mapped on bit 5 in port C and write the following code: |
<source lang="cpp"> | <source lang="cpp"> | ||
PORTC |= 1 << 5; // set pin 5 in port C | PORTC |= 1 << 5; // set pin 5 in port C | ||
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</source> | </source> | ||
| − | In summary, for each pin function we need to define the pin port, the bit position of that pin in the port and typically also the data direction register. As can be seen above, setting or resetting pins requires code that is not the easiest to read, although I guess you get used to statements like ''FIREWORKS_PORT &= ~(1<<FIREWORKS_PIN)'' (or the slightly more readable ''FIREWORKS_PORT &= ~_BV(FIREWORKS_PIN)''). | + | In summary, for each pin function we need to define the pin port, the bit position of that pin in the port and typically also the data direction register. As can be seen above, setting or resetting pins requires code that is not the easiest to read, although I guess you get used to statements like <code>''FIREWORKS_PORT &= ~(1<<FIREWORKS_PIN)''</code> (or the slightly more readable <code>''FIREWORKS_PORT &= ~_BV(FIREWORKS_PIN)''</code>). |
| − | |||
===Arduino=== | ===Arduino=== | ||
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Changing the led pin is a matter of just adapting the initialization of led1. At the same time, making the output pin high or low is about as readable as it could get: ''digitalWrite(led1, LOW)''. You just know what that line does. | Changing the led pin is a matter of just adapting the initialization of led1. At the same time, making the output pin high or low is about as readable as it could get: ''digitalWrite(led1, LOW)''. You just know what that line does. | ||
| − | However, this readability comes at a significant cost. If we look at the implementation of digitalWrite<ref>[https:// | + | However, this readability comes at a significant cost. If we look at the implementation of digitalWrite<ref>[https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/wiring_digital.c#L138 wiring_digital.c], arduino sources at GitHub</ref> we can see that cost: |
<source lang="cpp"> | <source lang="cpp"> | ||
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</source> | </source> | ||
| − | This will easily take 50 clock cycles<ref>[http://billgrundmann.wordpress.com/2009/03/03/to-use-or-not-use-writedigital/ "To use or not use digitalWrite"]</ref> | + | This will easily take 50 clock cycles!<ref>[http://billgrundmann.wordpress.com/2009/03/03/to-use-or-not-use-writedigital/ "To use or not use digitalWrite"]</ref> That's a bit steep, if all we wanted is to change a single bit value. If we know which pin we're going to set at compile time, changing a single pin value should take at most 2 clock cycles (1 on an Attiny). |
It would be nice if we could combine the readability and the single pin definition of Arduino's digitalWrite()-function with the performance of the raw AVR approach. pin_definitions is a library that offers just that. | It would be nice if we could combine the readability and the single pin definition of Arduino's digitalWrite()-function with the performance of the raw AVR approach. pin_definitions is a library that offers just that. | ||
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// declare a single pin | // declare a single pin | ||
| − | + | PIN_TYPE( D, 6) led1; | |
| − | + | PIN_TYPE( B, 0) led2; | |
// declare a consecutive range of pins in a single register | // declare a consecutive range of pins in a single register | ||
DECLARE_PIN_GROUP( counter, D, 2, 3); // D2, D3 and D4 are a counter | DECLARE_PIN_GROUP( counter, D, 2, 3); // D2, D3 and D4 are a counter | ||
| − | DECLARE_PIN_GROUP( rotary_encoder, D, 5, 2); // D5, D6 are some | + | DECLARE_PIN_GROUP( rotary_encoder, D, 5, 2); // D5, D6 are some input, e.g. from a quadrature rotary encoder |
void do_stuff() | void do_stuff() | ||
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==Introducing pin_definitions.hpp== | ==Introducing pin_definitions.hpp== | ||
| − | With AVR-GCC we have a fully functional ''C++'' compiler at our disposal. It should be possible to use this fact to create a library that allows Arduino's intuitive digitalWrite function with native C performance. | ||