woensdag 25 november 2009

Project: Rolling the dice, expanding the Arduino (PCF8574P)

Last time we looked at I2C to support communication with other ICs from our Arduino. As I2C is only a communication method, we need an IC to expand the number of I/O pins  available to us, which communicates on I2C. The PCF8574P will just do that for us. Today we are going to blink a LED on distance with our Arduino.

The pin layout for the PCF8574P is as follows:

SYMBOL PIN                               DESCRIPTION
A0         1  address input 0
A1         2  address input 1
A2         3  address input 2
P0         4  quasi-bidirectional I/O 0
P1         5  quasi-bidirectional I/O 1
P2         6  quasi-bidirectional I/O 2
P3         7  quasi-bidirectional I/O 3
VSS       8  supply ground
P4         9  quasi-bidirectional I/O 4
P5        10  quasi-bidirectional I/O 5
P6        11  quasi-bidirectional I/O 6
P7        12  quasi-bidirectional I/O 7
INT       13  interrupt output (active LOW)
SCL       14  serial clock line
SDA       15  serial data line
VDD       16  supply voltage

As we have learned, each slave IC communicating with I2C needs an address. The address is one byte long, but we have only seven bits to send as the eighth bit is an read/write indication. For the PCF8574P the address always starts with 0100, leaving us 3 bits to set. This allows us to have 8 PCF8574P slaves connected in one I2C network, because the bits 1,1,1 = 7. So we have 0 - 7 = 8 possible settings. The three bits are set by connecting the pins a0 - a2 on either Vcc (1) or ground (0). For example if I have set  A0 = 1, A1=0, A2=0 my address to control the PCF8574P  would be: 0100001.

P0 through P7 are the input/output pins much like the ones on the Arduino. VSS is the ground connection pin, while Vdd connects to the voltage supply. INT allows you to disable the PCF8574P by placing a HIGH voltage on the pin. SCL and SDA are the familiar I2C pins which should connect to the Arduino.

The following circuit will  require the following components:
1 x Arduino Duemilanove
1 x Red LED
1 x PCF8574P
1 x 680 Ohm resistor (blue, gray, brown, gold)
2 x 1.5 KOhm resistor (brown, green, red, gold) You can use up to 2 KOhm. I put 1 KOhm + 680 Ohm in series instead.

Note the two resistor connected to the 5 volt pin on the Arduino.The following code will blink the LED:

#define CARD_ADR (0x4 << 3 | 0x0) // address 0100000

/* Include the wire header */

void setup ()
  /* setup mwiring */

void loop ()
  /* set all pins off */
  /* set all pins on */

That's it for today, next time we will be completing the project!

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