NU32v2: Digital Input and Output

Digital inputs and outputs (DIO) are the simplest of interfaces between the PIC and other electronics, sensors, and actuators. The PIC32 has many DIO pins, each of which can take one of two states: high or low. More information can be found in Chapter 12 of the Data Sheet and Chapter 12 of the Reference Manual. Some special function registers (SFRs) are listed in Chapter 4 of the Data Sheet.

Overview

The PIC32 offers many digital I/O pins, arranged into "ports" A through G. The pins are labeled Rxy, where x is the port letter and y is the pin number. For example, pin 5 of port B is named RB5. Ports B and D are "full" 16-bit ports, with pins 0-15. Other ports have a smaller number of pins, not necessarily sequentially numbered; for example, port C has pins 1-4 and 12-15. All pins labeled Rxy can be used for input or output, except for RG2 and RG3, which are input only. For more details on the available pin numbers, see chapter 1 of the Data Sheet.

Each pin configured as an output can be configured to be a typical 0 or 3.3 V output (since our PIC is powered by 3.3 V), or to be "open drain." An open drain output can take one of two states: 0 V or "floating" (disconnected). An open drain output allows you to attach an external "pull-up" resistor from the output to any positive voltage, e.g., 5 V. Then when the output is left floating by the PIC, the external pull-up resistor will pull the voltage up to 5 V. A reasonable resistance for the pull-up is 1 K to 10 K ohms.

An input pin will read low if the input voltage is close to zero, and it will read high if it is close to 3.3 V. Many input pins will tolerate voltages up to 5 V; see the figure showing the PIC pins near the beginning of the data sheet. Some input pins, those that can also be used for "change notification" (labeled CNy), can be configured to have an internal pull-up resistor to 3.3 V. If configured this way, the input will read "high" if it is disconnected (left floating). Otherwise, if an input pin is not connected to anything, we can't be certain what the input will read.

Details

The functions of the DIO pins are controlled by special function registers (SFRs). Each of these SFRs has 32 bits on the PIC32, but for many, only 16 of the bits are relevant. Depending on whether these bits are set to 0 or 1, different actions or settings are performed:

• AD1PCFG: This SFR determines which of the 16 pins with an analog input function (port B) is treated as an analog input or a DIO pin. The 16 most significant bits (high bits) AD1PCFG<31:16> are ignored, but AD1PCFG<15:0> determine which of the 16 pins is treated as an analog input. 0 indicates analog input, and 1 indicates DIO. So if the bit AD1PCFG<7> is 0, then the pin with the function AN7 is an analog input; if AD1PCFG<7> is 1, then it is a DIO. By default, AD1PCFG = 0x0000, so all 16 pins are treated as analog inputs on reset. To make AN15 and AN1 analog inputs and the rest DIO, we can use the C command AD1PCFG = 0x7FFD. To make AN15 and AN1 analog inputs but leave the other settings unchanged, we can use the C command AD1PCFG &= 0x7FFD. Note we only have to specify 16 bits, or 4 hex digits, as the 16 most significant bits have no role. Instead of acting on the entire SFR, we could instead address an individual bit, using AD1PCFGbits.PCFG15 = 1, for example, to set the value of a single bit.

• TRISx: This SFR determines which of the DIO pins of port x are inputs and which are outputs. For example, TRISBbits.TRISB5 = 0 makes RB5 an output (0 = Output), TRISBbits.TRISB5 = 1 makes RB5 an input (1 = Input), TRISB &= FF00 makes RB0..RB7 outputs while leaving the others unchanged, TRISB ^= FF00 makes RB8..RB 15 inputs while leaving the others unchanged, and TRISB = FF00 makes RB0..RB7 outputs and RB8..RB15 inputs.

• LATx: A write to the latch chooses the output for pins configured as digital outputs. For example, LATB = 0x00FF makes pins RB0..RB7 high and pins RB8..RB15 low. An individual pin can be referenced using LATBbits.LATB13, for example.

• PORTx: val = PORTB returns the current digital inputs for DIO pins configured as inputs. PORTBbits.RB6 returns the digital input for RB6.

• ODCx: ODCB = 0x00FF configures RB0..RB7 as open drain, and pins RB8..RB15 as typical buffered output. An individual pin can be referenced as ODCBbits.ODCB8, for example.

• CNPUE: The relevant bits of this SFR are CNPUE<21:0>, and they apply only to the 22 change notification pins CN0..CN21. If CNPUEbits.CNPUE2 = 1, then the pin with CN2 function has the internal pull-up resistor enabled, and if CNPUEbits.CNPUE2 = 0, then it is disabled. The command CNPUE = 0xFFFFFF enables the internal pull-up resistors for all CN pins. The default has the pull-ups disabled.

Finally, SET (set), CLR (clear), and INV (invert) registers are available for all SFRs (e.g., TRISB, LATD, PORTA, ODCC, etc.). SET means that bits are set (made equal to 1), CLR means that bits are cleared (made equal to 0), and INV means that bits are inverted. So TRISBCLR = 0x0110 will clear bits 4 and 8 and TRISBSET = 0x0110 will set bits 4 and 8. All other bits (with a 0 on the right-hand side of the = sign) are unchanged. The reason to use these functions is that they are performed with fewer CPU instructions (faster) than the read-modify-write method. For example, LATBSET = 0x0001 has the same effect as LATB ^= 0x0001, but the former is faster.

Other SFRs and their functions are described in Chapter 12 of the Reference Manual. The C names of the SFRs, and the structrues with their bit references, can be found in the file p32mx795f512l.h, which is under pic32-libs/include/proc.

Library Functions

The peripheral library offers a number of function calls to help you use the DIO pins. These functions have names such as PORTRead, PORTSetPinsDigitalIn, PORTSetPinsDigitalOut, etc. C source code can be found in pic32-libs/peripheral/ports/source/*.c, and .h header files can be found in pic32-libs/include/peripheral/ports.h. The DIO functions are simple enough, however, that we will usually just set the appropriate SFRs manually, as described above and demonstrated in the example below.

Code Sample

This example uses two pins as digital outputs and one pin as digital input. When the switch is pressed, one LED will turn on and the other will turn off. When the switch is not pressed, the opposite LED will turn on and the other one will turn off.

/*
 * Simple digital input/output.
 */

#include <plib.h>

// No need to configure the configuration bits controlling clock speed, etc.;
// these definitions are in the bootloader.

// Lines below could go into a separate .h file, since they're the same
// for all projects using the NU32v2 and the NU32v2 bootloader.
#define LED1     LATGbits.LATG12
#define LED2     LATGbits.LATG13
#define SW       PORTGbits.RG6
#define SYS_FREQ 80000000

int main(void) {
  // Turn on the pre-fetch cache, choose the maximum possible peripheral
  // bus frequency, and choose minimum flash wait states.  Could be done in
  // bootloader.  See pic32-libs/include/peripheral/system.h.
  SYSTEMConfig(SYS_FREQ, SYS_CFG_ALL);

  // Initialize LEDs as output, switch as input.  Could be done in bootloader.
  // Note we don't technically have to set switch as an input, since it's the
  // default, but let's do it anyway in case someone inserts code before this
  // line.
  TRISGCLR = 0x3000; TRISGSET = 0x0040;

  while(1) {
    if(SW) {
      LED1 = 1; LED2 = 0;
    }
    else {
      LED1 = 0; LED2 = 1;
    }
  }
  return 0;
}