Digital Pins To Byte Variable... - Arduino Forum

Digital Pins to Byte Variable... Projects Programming ArduinoTom June 10, 2016, 9:07pm 1

I have 8 digital pins on an Arduino connected to other hardware, as a way to communicate a number from 0-255 to the Arduino. What's an efficient way of checking those digital inputs and converting them into a single byte held in a variable?

I've know and can check the status of each bit one at a time and add it all up into a variable, but that seems to laborious and inefficient. Seems like there should be a way to set individual bits within a byte variable based on each digital input's status, or even read a byte value into a variable all at once based on the status of 8 digital inputs. Any ideas/help?

system June 10, 2016, 9:14pm 2

Seems like there should be a way to set individual bits within a byte variable based on each digital input's status

Like using bitWrite()?

or even read a byte value into a variable all at once based on the status of 8 digital inputs

If they are all on the same port, you can.

johnwasser June 10, 2016, 10:25pm 3

If you are using an Arduino UNO it will be a little hard to find 8 pins connected to the same port. The only port that kas all eight bits connected to pins is PORTD which covers Pin 0 through Pin 7. Unfortunately that includes Pin 0 and Pin 1 which are used for Serial. If you use them for data you can't use them for Serial.

The Arduino MEGA 2560 has several choices: 22-29 = PORTA 37-30 = PORTC (note the reversed ordering) 48-42 = PORTL (note the reversed ordering) 54-61 = PORTF 62-69 = PORTK

Arctic_Eddie June 10, 2016, 11:22pm 4

The closest I've been able to do on the 328P chip is to use the upper four bits of port D, pins 4 - 7, for the high nibble and the lower four bits of port B, pins 8 - 11, as the low nibble. By reading each port with a mask and adding them together, you will get one byte. If you need to use SPI then use port C, pins A0 - A3, for the low nibble. That also leaves the I2C pins available. It's not elegant but a lot faster than using bitRead and digitalRead.

anon57585045 June 11, 2016, 12:09am 5

ArduinoTom: I've know and can check the status of each bit one at a time and add it all up into a variable, but that seems to laborious and inefficient.

How much free time does the processor have, in your application?

tom86951 June 11, 2016, 3:24am 6

johnwasser: If you are using an Arduino UNO it will be a little hard to find 8 pins connected to the same port. The only port that kas all eight bits connected to pins is PORTD which covers Pin 0 through Pin 7. Unfortunately that includes Pin 0 and Pin 1 which are used for Serial. If you use them for data you can't use them for Serial.

The Arduino MEGA 2560 has several choices: 22-29 = PORTA 37-30 = PORTC (note the reversed ordering) 48-42 = PORTL (note the reversed ordering) 54-61 = PORTF 62-69 = PORTK

Arctic_Eddie: The closest I've been able to do on the 328P chip is to use the upper four bits of port D, pins 4 - 7, for the high nibble and the lower four bits of port B, pins 8 - 11, as the low nibble. By reading each port with a mask and adding them together, you will get one byte. If you need to use SPI then use port C, pins A0 - A3, for the low nibble. That also leaves the I2C pins available. It's not elegant but a lot faster than using bitRead and digitalRead.

Sorry, I'm missing the mechanics on this. I'm not following how to read a nibble all at once from a Port, or for that matter how to read a byte if I had an entire Port available... Could you dumb it down for me?

cattledog June 11, 2016, 5:31am 7

Sorry, I'm missing the mechanics on this.

Arduino Reference - Arduino Reference

The Arduino programming language Reference, organized into Functions, Variable and Constant, and Structure keywords.

tom86951 June 11, 2016, 4:57pm 8

Thank, that gets me into a

aarg: How much free time does the processor have, in your application?

In this part of the code I actually have plenty of time, do I could digital read each pin and add up the results, it just seemed so ham-fisted. The tips posted here about Port and bit manipulation seems to be getting me into a more efficient ball park...

sterretje June 11, 2016, 5:07pm 9

One way of doing it

struct DATARAW {   byte b0: 1;   byte b1: 1;   byte b2: 1;   byte b3: 1;   byte b4: 1;   byte b5: 1;   byte b6: 1;   byte b7: 1; }; union DATA {   DATARAW rawdata;   byte value; }; DATA data; void setup() {   ...   ... } void loop() {   data.rawdata.b0 = digitalRead(pin0);   ...   ...   data.rawdata.b7 = digitalRead(pin7);   Serial.println(data.value); }

Read up on structs, bitfields and unions

The above approach does not require bit-shifting (which would be the alternative).

Code not tested.

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