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Arduino-like pin definitions in C++

From Just in Time

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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:

  • 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.

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. In short, it turns code like this:

#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);

into code like the following, while maintaining the performance of the C-style original:

PIN_TYPE( D, 6) led1;

make_output( led1);
set( led1);
clear( led1);

Pin definitions, the state of the art

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, 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:

    PORTC |= 1 << 5; // set pin 5 in port C

In other words: If I want to manipulate the output state of a pin, I need to know the port (the address) of the pin and the bit position of that pin in the port. In Arduino land, instead of the or-expression above you would typically use the digitalWrite()-function to manipulate pins if performance is not your highest priority. I'll get to the Arduino style functions later, but this section assumes either raw AVR or performance critical Arduino.

For most projects, especially for libraries, hardcoding the pins is not a good idea. You may want to re-route your PCB, or at some later point decide to add extra hardware that requires you to shift your pins a few places to the left or right. Additionally, it would be convenient to give some meaningful name to the pins, so that the reader of the code knows which pin controls the LED and which pin detonates the fireworks—which you will agree, is a useful distinction to make. Most developers define their pin positions in a separate section of their code, often in some header file:

#define LED_PORT       PORTC
#define LED_PIN        5

#define FIREWORKS_PORT PORTB
#define FIREWORKS_PIN  1

And then use it in their code:

    LED_PORT |= 1 << LED_PIN; // this is probably safe

If you're writing or using a library for a piece of connected hardware, then typically you'll use defines like the above to tell the library which pins are connected to the hardware. See for example how a semi-randomly chosen library[1] (for driving HD44780 LCD displays) defines which pins have which function:

#define LCD_PORT       PORTA
#define LCD_DATA0_PORT LCD_PORT
#define LCD_DATA1_PORT LCD_PORT
#define LCD_DATA2_PORT LCD_PORT
#define LCD_DATA3_PORT LCD_PORT
#define LCD_DATA0_PIN  0
#define LCD_DATA1_PIN  1
#define LCD_DATA2_PIN  2
#define LCD_DATA3_PIN  3

#define LCD_RS_PORT    LCD_PORT
#define LCD_RS_PIN     4

#define LCD_RW_PORT    LCD_PORT
#define LCD_RW_PIN     5

#define LCD_E_PORT     LCD_PORT
#define LCD_E_PIN      6

This is not all of the story. Before we can write to output pins, we need to configure them as such. This is done by setting the appropriate bit in the appropriate data direction (DDR) register. In addition to the output port, we should really define the data direction port for our pins as well. Some libraries, like the one above, work around this extra definition by making use of the fact that there is an easy relation between the address of the PORT register (e.g. PORTB) and its data direction register (DDRB, the relation is &DDRB == &PORTB - 1). But for this discussion, I'm going to define the data direction port separately:

#define LED_DDR        DDRC
#define LED_PORT       PORTC
#define LED_PIN        5

#define FIREWORKS_DDR  DDRB
#define FIREWORKS_PORT PORTB
#define FIREWORKS_PIN  1

void setup()
{
   FIREWORKS_PORT &= ~(1<<FIREWORKS_PIN); // make sure that the output pin is low
   LED_DDR        |= 1 << LED_PIN;
   FIREWORKS_DDR  |= 1 << FIREWORKS_PIN;
}

void warn_user() 
{
   LED_PORT |= 1 << LED_PIN; // this is probably safe
}

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)).

Arduino

On Arduino, defining a pin function becomes a lot easier and more readable:

int led1 = 13;                 // LED connected to digital pin 13, port B, pin 5

void setup()
{
  // make the pin for led1 an output
  pinMode(led1, OUTPUT);
  // do other outputs as well...

}

void f()
{
  // ...
  // flash led 1
  digitalWrite(led1, LOW);
  delay(50);
  digitalWrite(led1, HIGH);
  // ...
}

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[2] we can see that cost:

void digitalWrite(uint8_t pin, uint8_t val)
{
        uint8_t timer = digitalPinToTimer(pin);
        uint8_t bit = digitalPinToBitMask(pin);
        uint8_t port = digitalPinToPort(pin);
        volatile uint8_t *out;

        if (port == NOT_A_PIN) return;

        // If the pin that support PWM output, we need to turn it off
        // before doing a digital write.
        if (timer != NOT_ON_TIMER) turnOffPWM(timer);

        out = portOutputRegister(port);

        if (val == LOW) {
                uint8_t oldSREG = SREG;
                cli();
                *out &= ~bit;
                SREG = oldSREG;
        } else {
                uint8_t oldSREG = SREG;
                cli();
                *out |= bit;
                SREG = oldSREG;
        }
}

This will easily take 50 clock cycles![3] 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.

Pin_definitions TL;DR

Pin-definitions is a header-only library, you can find the sources on GitHub, in file pin_definitions.hpp. This library is intended to declare AVR pins for functions in an intuitive way. Typical usage is as follows:

#include "avr_utilities/pin_definitions.hpp"

// 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_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 input, e.g. from a quadrature rotary encoder

void do_stuff()
{
	// initialize data direction for the output pins.
	init_as_output(led1 | led2 | counter);

	// start by setting both leds
	set( led1 | led2);

	while (true)
	{
		// read the two input bits of the rotary encoder and
		// output the result to the bits of the counter
		write( counter, read(rotary_encoder));
		if (read(rotary_encoder) == 0b10)
		{
			set( led1);
			reset( led2);
		}
		else
		{
			set( led2);
			reset( led1);
		}
	}
}

On top of this, the library aims at exactly the same performance as hand-written bit manipulations. This means that the line

set( led1 | led2)

has the same performance as

    LED_PORT |= ( _BV( LED1_PIN)| _BV(LED2_PIN));

...if led1 and led2 are on the same port. If the leds are on different ports, it should have performance equivalent to

    LED1_PORT |= _BV( LED1_PIN);
    LED2_PORT |= _BV( LED2_PIN);

It will even go one step further. For a line like

set( led1 | led2 | led3)

if led1 and led3 are on the same port, but led2 is on a different port, it will generate code equivalent to:

    LED13_PORT |= (_BV( LED1_PIN)| _BV( LED3_PIN));
    LED2_PORT  |= _BV( LED2_PIN);

The library will perform this type of optimization regardless the number of pins or pin groups that are combined in a single call. Of course it will also perform the same optimizations for the functions reset(), make_output() and initialize_as_output(), minimizing the clock ticks for the given arguments.

In order to do this, the library makes heavy use of C++ template metaprogramming. This makes the library itself less readable to those that are not familiar with template metaprogramming, but it sure makes (re-)declaration and usage of pins a lot easier to read.

The next Section will go into the details of the pin_definitions library and how it can be used.

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. pin_definitions.hpp was written with that goal in mind. Although it makes heavy use of template metaprogramming, if you only use the library, you should find it very intuitive—plus I've done my stinking best to not let any compilation errors end up deep inside the source code of this library...

Usage

Usage is simple. First include the library header file:

#include "avr_utilities/pin_definitions.hpp"

Then you can declare your pins and pin groups. A pin is a single input- or output pin on the AVR, for example pin B5 (that would be pin number 13 on an Arduino). A pin is identified by a port (depending on your AVR this is any port from port A to port F) and a bit number (0-7) on that port. A pin group is a range of consecutive pins on the same port, for example B0-B4. Pin groups are identified by the port and bit number of the first pin and the pin count:

DECLARE_PIN( led1, D, 6); // allocate led1 to bit 6 of port D
DECLARE_PIN( button1, B, 1); // button1 is connected to B1
DECLARE_PIN_GROUP( counter, D, 2, 3); // D2-D4 form a three bit counter output
DECLARE_PIN_GROUP( rotary_encoder, D, 5, 2); // D5, D6 are some input, e.g. from a quadrature rotary encoder

Whenever you declare a pin, you are declaring a variable. The type of this variable is constructed from some C++ template. Both the port and the bit number are encoded in the type, so that the variable itself can be completely empty. In fact, the compiler allocates zero bytes data space for the variable itself (we'll take a look under the hood later on).

Now it's time to configure the data direction registers for those pins that you'll use as outputs this can be done with the make_output() function, somewhat comparable to the pinMode() function on Arduino.

	make_output( led1);
	make_output( counter);

This would be a good time to introduce pin lists. A pin list is a list of pins or pin groups. Most functions that accept a pin or pin group as parameter will also accept a list. Moreover, these functions will optimize their operations for those pins and pin groups that are in the same port. So not only can you use lists as shorthand notation, but it can actually result in shorter or faster code! There are two ways to create pin lists: the list_of() function and, as a shorthand notation, the bitwise-or operator:

    // list_of
    some_function( list_of( led1)(counter)(led2) /* etc... */);

    // bitwise-or operator
    some_function( led1 | counter | led2 /* etc... */);

make_output() is one of those functions that accepts lists, so the two earlier function calls can be rewritten as:

	make_output( led1 | counter); // turn the led1 pin and the pins of counter into output pins.

make_output() takes care not to change other bits in the DDR-registers. To squeeze the last drop of performance out of your initialization you could also use the init_as_output() function, which isn't so squeemish and will just set all other bits to zero in each of the DDR registers it touches. Use init_as_output() only if you know what you're doing. It should typically be used only once in a program.

Now that you've correctly configured the data direction registers, it's time to start reading from and writing to your pins:

	// set all bits of led1 and counter to LOW
	reset( led1 | counter);

	// set the three bits of counter to some value
	write( counter, 0b101);

	// set the led1 pin to HIGH
	write( led1, true);

	// same as previous led1 to HIGH
	set( led1);

	// read the two rotary encoder bits and assign to 'value'.
	uint8_t value = read( rotary_encoder);

	uint8_t count = 0;
	for (;;)
	{
		// increase counter if button input is high
		if (is_set( button1)) ++count;

		// same as above
		if (read( button1)) ++count;

		write( counter, count); // since counter is 3-bit, value will be truncated
	}

Pins & Libraries

Being able to associate a symbol with a pin is convenient when creating self-contained pieces of firmware, but it becomes a necessity when creating re-usable components, such as the HD44780 LCD display library described earlier. There are many libraries out there that can be used to control devices like LCD-displays, NRF24L01(+) transceivers, WS2811(B) LED strings etc., and all of these somehow need the user of the library to specify the pins to which these devices are connected.

As an example, consider a bit-banging SPI library. Although most AVRs have hardware SPI support, sometimes it's convenient to use other pins for SPI signalling than the one designated pin that is also used by the AVR programmer. SPI is a dead-simple protocol to implement, but if we're moving the bits about ourselves, then it would be nice if it would be done as fast as possible, without the 50-clock delays between pin transitions. An excellent case therefore to demonstrate the use of a fast pin-twiddling library.

This is also a good opportunity to showcase C++'s excellent features for writing re-usable components for constrained devices. The SPI functions are implemented as static member functions of a C++ class template. All member functions are static because a SPI device that is associated with certain pins is always a singleton and we don't want to burden all member function calls with an unnecessary (hidden) this-pointer argument. In fact, the spi class acts more like a templated namespace than a class from which objects would be instantiated.

This class template takes a single template argument: a struct that defines which pins are associated with the three pin functions controlled by this class (mosi, miso and clk). Its use is as follows:

#include "avr_utilities/devices/bitbanged_spi.h"
#include "avr_utilities/pin_definitions.hpp"

struct spi_pins {
	DECLARE_PIN( mosi, B, 0);
	DECLARE_PIN( miso, B, 1);
	DECLARE_PIN( clk,  B, 2);
};

typedef bitbanged_spi<spi_pins> spi; // spi is now a class that uses B0, B1 and B2 as signal lines

// declare chip select lines
DECLARE_PIN( select_some_spi_device, C, 5);
DECLARE_PIN( select_some_other_spi_device, C, 4);

void init()
{
    // initialize all spi pins
    spi::init();

    // Initialize all select pins. These are typically active-low.
    set( select_some_spi_device | select_other_spi_device);
    make_output( select_some_spi_device | select_other_spi_device);
}


void do_stuff( uint8_t value)
{
    reset( select_some_spi_device);
    spi::transmit_receive( value);
    set( select_some_spi_device);
}

We already saw that the DECLARE_PIN macro declares a variable. By using this macro inside a struct, we're declaring member variables of a struct type. This struct type is used to tell the bitbanged_spi template which pins are associated with miso, mosi and clk. The spi library expects the user to set or reset the appropriate chip select lines before sending or receiving data (but it could be extended to do this for the user).

Now let's take a look at the implementation of bitbanged_spi. As stated, this is a class template that takes one argument: a struct that defines its pins:

template< typename pin_definitions>
struct bitbanged_spi
{
private:
    static pin_definitions pins;

    /// send a byte via mosi, while at the same time listening for a byte at miso.
    /// This implements a mode 0 spi protocol, i.e. clock polarity is 0 (positive) and phase is 0.
    static uint8_t exchange_byte( uint8_t out)
    {
        uint8_t receive = 0;
        for (uint8_t mask = 0x80; mask; mask >>= 1)
        {
            write( pins.mosi, out&mask);
            set( pins.clk);
            if (is_set( pins.miso))
            {
                receive |= mask;
            }
            reset( pins.clk);
        }
        return receive;
    }
public:
    static void init()
    {
        reset( pins.clk);
        make_output( pins.mosi | pins.clk);
    }

    /// send and receive one byte at the same time.
    static uint8_t transmit_receive( uint8_t transmit)
    {
        uint8_t receive = exchange_byte( transmit);
        return receive;
    }
    /// send a buffer of bytes, replacing the contents with bytes received.
    static void transmit_receive( uint8_t *inout_buffer, uint8_t length)
    {
        while (length--)
        {
            *inout_buffer = exchange_byte( *inout_buffer);
            ++inout_buffer;
        }
    }

    /// send a buffer of bytes, not receiving any bytes.
    static void transmit( const uint8_t *out_buffer, uint8_t length)
    {
        while (length--)
        {
            exchange_byte( *out_buffer++);
        }
    }

    static void receive( uint8_t *in_buffer, uint8_t length)
    {
        while (length--)
        {
            *in_buffer++ = exchange_byte( 0);
        }
    }

    /// transmit a 16 bit value, msb first
    static void transmit( uint16_t value)
    {
    	exchange_byte( value >> 8);
    	exchange_byte( value & 0xff);
    }

    // transmit a zero-terminated string of characters.
    static void transmit( const char *text)
    {
        while (*text)
        {
            exchange_byte( *text--);
        }
    }

};

References