Control output pins using 8-bit number

Forum 2005-2010 (read only) Software Syntax & Programs
1

I would like to know how to control eight output pins using an 8-bit binary number (ie. 00001100 would turn outputs 2 and 3 on). In actual fact, it would be even better if I could control them with a decimal number (0-255) - depending on the state of a potentiometer. But they must output a binary value: my project involves reading an analog sensor (eventually, it will be serial data coming from the computer (in decimal format)), and creating another analog voltage using an Arduino and a DAC0800.

I am going to connect it like it is attached to the ROM in the following diagram:
http://www.cgs.synth.net/modules/pic/schem_cgs02_wavetable.gif

Do you know what the output rate would be (ie. changes possible per second)? I am hoping for about 100 if possible. The more the better.

Thanks.

2

Easy way is to use bitRead.
More difficult is direct port manipulation.
100Hz should be easy.

3

when you use this syntax to send the data to the dac:
(assuming that the dac is on portd == pins 0 through 7) it works fast and simple:
setup(){

DDRD=0xFF; //set portd as output
PORTD = 0; //set dac to 0
}

loop{

int potmeter = 0;

potmeter = analogRead(0); //pick a analog port..
potmeter &= 0xFF;
PORTD = portmeter;
}

Good luck!

4 
potmeter = analogRead(0); //pick an analog port..
   potmeter &= 0xFF;

maybe "potmeter /=4;" would be a little less erratic.

5

potmeter /=4; is what you want to use for another reason also. If you and with 0xFF, every time the pot goes over that value, it loops and starts counting on the pins again. Divide by 4 will give you 255 steps between 0v and 5v. However, you don't actually need the variable there:

PORTD = byte(analogRead(0)/4);

will work just fine. You may not need the byte().

6

Sorry,
Guys tour write, dividing is better than chopping bits off

7

Hi, thanks for the replies. However, the only problem is that I would like the DAC to be on pins 2-9, so I have access to the Serial Tx/Rx lines (pins 0 and 1). Is this possible to do? Any ideas? Unfortunately, there are only six data lines on Port B.
Thanks.

8

Your simplest option is probably 'bitRead', or if you're feeling advanced, a bit of port manipulation for the bulk of the bits on a single port, and bitRead for the remainder.

9

If I understand the question correctly something along the lines of:

byte    pins[2, 3, 4, 5, 6, 7, 8, 9];
byte*   p = pins;

unsigned byte   mask_bit = 0b00000001;
do
{
    digitalWrite(*p++, (value & mask_bit) ? HIGH : LOW);
} while ( mask_bit <<= 1 );

EDIT: Changed per below

10

Your 'while' is too long - make the mask a byte.

11

Thanks I thought I'd corrected that before the post but apparently not.

12

lloyddean and Groove, could you please format your example below in a way a beginner like me can use it :slight_smile:

I presume it's something like this:

void setup() {
  Serial.begin(9600);
}

void loop() {
  byte p = analogRead(0);

  byte pins[2, 3, 4, 5, 6, 7, 8, 9]; //an array

  unsigned byte mask_bit = 0b00000001;
  do
  {
    digitalWrite(*p++, (value & mask_bit) ? HIGH : LOW); 
  } 
  while ( mask_bit <<= 1 );
}

Unfortunately, the compiler doesn't like this. Could you help? I just want to read a pot, and output it's value to the DAC for now.

I would eventually like to control the DAC through MATLAB as seen at Legacy MATLAB and Simulink Support for Arduino: Slides and Examples - File Exchange - MATLAB Central

Thanks.

13

Your pins array declaration/initialisation is incorrect.
Look for some examples to see how it should be.
also I don't know what your doing withe the *p++, but you should be indexing the pins array, not trying to dereference a non-existent pointer.
Sorry, on train, posting via phone.

14 
const  byte pins [8] = {2, 3, 4, 5, 6, 7, 8, 9};

void setup() {
  Serial.begin(9600);
}

void loop() {
  byte p = analogRead(0) / 4;
  for (int i = 0, unsigned byte mask_bit = 1; i < 8; ++i, mask_bit<<=1) {  {
    digitalWrite(pins [i],  !!(p & mask_bit));
  }
}

You should be aware that because you're not writing the DAC in a single operation, the output will wander around a bit, though if you're driving it with a pot, you probably won't notice this.

15

Hi Guys, thanks very much for your help so far. I have got a prototype up and running, however, the issue is at the moment how the 8-bit number updates one bit at a time. It is causing the DACs output to swing all over the place when the update speed is high (ie. 100 times per second).

Is there any way to output the 8-bit number at once? Perhaps I may have to use an Arduino Mega instead so I have full access to one of the PORT outputs, and the serial outputs.