- 1.1 CHAPTER 3: INPUT OUTPUT PORTS PROGRAMMING Outcomes At the
end of this chapter, you will be able to: 1. Identify input and
output (I/O) pins and their function 2. Explain dual role ports of
data I/O 3.1 I/O Ports pins and their function PIC18F4550 belongs
to pic18f family of microcontrollers from the microchip technology.
Examine Figure 3.1 for the PIC18F4550 40-pin chip. A total of 35
pins are Input-Output pins which can be configured for general
Input or Output by setting registers associated with them. The rest
of the pins are designated as Vdd (Vcc), Vss (GND), OSC1, OSC2,
MCLR (reset) and another set of Vdd and Vss. Figure 3.1:
PIC18F4550s pin out diagram The PIC18F4550 has 5 ports. They are
PORTA, PORTB, PORTC, PORTD and PORTE. Not all port has 8 pin. Table
3.1 shows the number of pin and pin name of the port.
2. 2 Table 3.1: Number of pin and pin name Ports Number of pins
Pin Name PORTA 7 RA0-RA6 PORTB 8 RB0-RB7 PORTC 7 RC0-RC2, RC4-RC7
(Check the Pinout Diagram) PORTD 8 RD0-RD7 PORTE 4 RE0-RE3 3.2 Dual
role ports of data I/O For practical reasons, many I/O pins have
two or three functions. 1. used as a general purpose input/output
pin; 2. alternate function - can be seeing next after symbol / .
Figure 3.2 Example 3.1: I/O pin name: RA0/AN0 RA0 used as digital
I/O AN0 - alternate function (used as input for analog-to-digital
converter channel 0) 3. 3 3.2.1 Port A Port A is an 8-bit wide,
bidirectional port. All Port A pins act as digital inputs/outputs.
Five of them can also be analog inputs (denoted as AN): Because
many projects use an ADC, we do not use Port A for simple I/O
functions. Table 3.1: PORTA I/O SUMMARY 4. 4 Table 3.2: Port A
alternate function Bit Alternate Function RA0 AN0 RA1 AN1 RA2
AN2/VREF- RA3 AN3/VREF+ RA4 T0CK1 RA5 AN4/SS/LVDIN RA6 OSC2/CLKO
3.2.2 Port B Port B is an 8-bit wide, bidirectional port.. Six pins
on this port can act as analog inputs (AN). Table 3.3: Port B
alternate function Bit Alternate Function RB0 INT0 RB1 INT1 RB2
INT2/CANTX RB3 CANRX RB4 RB5 PGM RB6 PGC RB7 PGD 5. 5 3.2.3 Port C
Port C is an 8-bit wide, bidirectional port. Table 3.4: Port C
alternate function Bit Function RC0 T1OS0/T1CK1 RC1 T1OS1 RC2 CCP1
RC3 SCK/SCL RC4 SDI/SDA RC5 SDO RC6 TX/CK RC7 RX/DT 3.2.4 Port D
Port D is an 8-bit wide, bidirectional port. Table 3.4: Port D
alternate function Bit Alternate Function RD0 PSP0/C1IN+ RD1
PSP1/C1IN- RD2 PSP2/C2IN+ RD3 PSP3/C1IN- RD4 PSP4/ECCP1/PIA RD5
PSP5/P1B RD6 PSP6/P1C RD7 PSP7/P1D 6. 6 3.2.5 Port E Port E is a
4-bit wide, bidirectional port. Similar to Ports A and B, three
pins can be configured as analog inputs in this case. The ANSELH
register bits determine whether a pin will act as analog input (AN)
or digital input/output: Three pins (RE0/AN5/CK1SPP, RE1/AN6/CK2SPP
and RE2/AN7/OESPP) are individually configurable as inputs or
outputs. On a Power-on Reset, RE3 is enabled as a digital input
only if Master Clear functionality is disabled Table 3.5: Port E
alternate function Bit Alternate Function RE0 AN5/CK1SPP RE1
RE1/AN6/CK2SPP RE2 RE2/AN7/OESPP) RE3 MCLR/VPP/RE3 Activity 3.1:
Name ONE (1)of PIC18 I/O pin and identify how that pin used in I/O
control.
____________________________________________________________________
Review question 1. There are a total of _____ ports in the
PIC18f4550. 2. True or false. All of the PIC18F4550 ports have 8
pins. 3. List all ports that have 8 pins.
___________________________________________________________________
7. 7 3.3 Code PIC instruction for I/O handling To use any of I/O
ports as an input or output port, it must be programmed. Each port
in pic18f4450 is associated with three 8 bit registers for IO
operations. 1. TRISx (8 bit) 2. PORTx (8 bit) 3. LATx (8 bit) 3.3.1
TRISX: TRISX: where X is the name of the ports either of A, B, C,
D, E. Every port has its own TRISX register, thus there are 5
TRISXs to be configured: a. TRISA b. TRISB c. TRISC d. TRISD e.
TRISE Data direction (TRIS) register needs to be set before the I/O
operation 1 to make a port an input 0 to make a port an output
After a Reset all port pins are defined as inputs 3.3.2 PORT X PORT
X : Reads the device level, stores the Input level of the pins and
reads the input signal from the external device if the pin is
configured as Input. 3.3.3 LAT X LAT X: The latch registers reads
and modifies the write operation on the value of I/O pin and stored
the output data that is to be passed on to the external hardware. A
write to the LATx register has the same effect as a write to the
PORTx register. 8. 8 3.2.1 TRIS register role in inputting and
outputting data By clearing some bit of the TRIS register (bit=0),
the corresponding port pin is configured as output. Similarly, by
setting some bit of the TRIS register (bit=1), the corresponding
port pin is configured as input. This rule is easy to remember 0 =
Output, 1 = Input. Example 3.2 Determine the value of TRIS register
for the following circuit . Figure 3.3 Solution: Step 1: Analyze
I/O pin:- INPUT: Pin 2 and pin 4 of I/O port OUTPUT: Pin 6 and pin
7 of I/O port Step 2: Set Bit 2 and Bit 4 of TRIS register Clear
Bit 6 and Bit 7 of TRIS register The value of TRIS register in
binary is 00110101. If the external circuit is connected to Port D,
we will write TRISD=0b00110101; 9. 9 . Example 3.3: Write C program
statement to initialize I/O port for the following diagram. Figure
3.4 Solution: INPUT: RB3, RB4, RB6 and RB7 OUTPUT: RB0-RB2, and RB5
TRISB = 0b11011000; Example 3.4 Write C statement to make Port B of
PIC18F4550 become output. Solution: To make these port serves as
Output, we need to put 0s bit in the register TRISB0 to TRISB7. The
answer is: TRISB = 0b00000000;
____________________________________________________________________
Activity 3.2 1. True or false. Upon power-up, the I/O pins are
configured as output ports. 10. 10 2. To make Port B an output, we
must place __ in register _________. 3. To make Port B an input
port, we must place ___ in register __________. 4. Write the I/O
initialization for the following situation: i. SW1 connected to RB1
and LED1 connected to RD2. ii. Light sensor connected to RC0 and
lamp connected to RC3.
______________________________________________________________________
3.4 I/O Port and bit addressability All PIC18 register is 8 bit
size. We will access the register either: 1. all 8 bits - byte
addressable 2. any single bit without altering the rest. bit
addressable Access register by bit: NAMEbitx = 1/0; e.g. RB5 = 1;
//Set bit 5 of Port B Access register by byte: NAME= 0bxxxxxxxx;
e.g. PORTB= 0b00001111; //Send binary 00001111 to Port B. Example
3.5 : Write a program statement to set bit5 of Port B to logic HIGH
by: a. Bit b. Byte 11. 11 Solution: a. By bit: RB5 = 1; //Set bit 5
of Port B b. By byte: PORTB= 0b00100000; //Send logic HIGH to pin 5
of Port B. Activity 3.3: Refer the following table to write a
program statement to access LATC and TRISC by: a. Bit b. Byte 12.
12 Activity 3.4: Refer the following table to write a program
statement to access PORTD and LATD by: a. Bit b. Byte 13. 13
Hardware connections (Extra notes) Output For the purpose of
interfacing the PIC, we can consider a 'high' output to be 5V, and
a low output to be 0V. If you check the PIC datasheets, you will
find that the output pins can actually 'sink' or 'source' a
reasonable amount of current - about 25mA or so. 'Sink' means that
the chip 'sinks' current down into itself, so the load is connected
from the positive rail to the I/O pin and the load is switched ON
by the pin going 'low'. 'Source' is just the reverse, the I/O pin
is the actual source of the current, and it flows out of the pin,
through the load down to ground - in this case the load is switched
ON by the pin going 'high'. Connecting the LED without a resistor
is likely to damage both the LED and the PIC. The two examples
below show how to connect an LED in either 'sink' or 'source' modes
Sink mode example This is how to connect an LED for the PIC to SINK
current. The LED will light when the PIC pin is low. 14. 14 Source
mode example This is how to connect an LED for the PIC to SOURCE
current. The LED will light when the PIC pin is high Inputs Inputs
to a PIC have the same 5V logic requirements, so the inputs can be
active 'high' or active 'low'. Basically this is just a variation
on the same theme - but, depending on the actual input device, you
may be forced to use a particular method. Active LOW input 'Active
LOW':The resistor R1 'pulls' the PIC input high (logic '1') - when
you press the switch it pulls the voltage down to zero, changing
the input to logic '0'. 15. 15 Active HIGH input The 'Active HIGH'
example works in the opposite way, R2 is a 'pull down' resistor,
holding the PIC input at logic '0', pressing the switch connects
the PIC input to the 5V rail, forcing it to logic '1'. Example 3..5
A control system consist of a PIC microcontroller, 2 switches
(active LOW) and one LED (active HIGH). Switch 1 is connected to
pin RA3 and switch 2 connected to pin RB0. One LED connected to
RC1. a. Draw a circuit diagram for the system. b. Write I/O pin
initialization for the system c. Write a program statement to
linght on the LED 16. 16 Solution a. b. TRISA3=1; TRISB0=1;
TRISC1=0; c. RC1=1; References
http://www.matrixmultimedia.com/wiki/index.php?title=Component:_Switch_base
_(Inputs) http://www.winpicprog.co.uk/pic_tutorial_extras.htm
