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Serial CommunicationInterface

Chapter 3

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Objectives Upon completion of this chapter, you will

be able to: Explain serial communication protocol

Describe the serial communication features ofthe PIC18

Program the PIC18 serial port in C

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Basics of Serial Communication

(contd)

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Start and Stop Bits

Framing ASCII A (41H)

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RS-232 Standards Standard for serial comm (COM port)

1: -3V to -25V;0: +3V to +25V

Reason: for long distance wired line

Connectors Minimally, 3 wires: RxD, TxD, GND Could have 9-pin or 25-pin

Standard baud rates: 1200, 2400, 4800, 9600, 14.4k, 19.2k, 28.8k..

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RS232 Connector

DB-25

25-Pin Connector

DB-9

9-Pin Connector

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RS232 Connector (contd)

IBM PC DB-9 Signals

Pin 1 Data Carrier Detect (DCD)Pin 2 Received Data (RxD)Pin 3 Transmitted Data (TxD)

Pin 4 Data Terminal Ready (DTR)Pin 5 Signal Ground (GND)Pin 6 Data Set Ready (/DSR)Pin 7 Request to Send (/RTS)Pin 8 Clear to Send (/CTS)

Pin 9 Ring Indicator (RI)

DB-9

9-Pin Connector

Data in Data out

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PIC18 Connection to RS232

(a) Inside MAX232 (b) its Connection to the PIC18

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PIC18 Connection to RS232

(contd)

(a) Inside MAX233 (b) Its Connection to the PIC18

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Selecting Baud Rates Baud rate in PIC18 is

programmable The value loaded into

the SPBRG decides

the baud rate

338400

719200F9600

811200

402400

204800

SPBRG

(Hex Value)Baud Rate

*For XTAL = 10MHz only!

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TXSTA: Transmit Status and

Control Register

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RCSTA: Receive Status and

Control Register

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PIR: Peripheral Interrupt Register 1

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Example1

Write a C program to transfer the letter G serially at 9600,

continuously. Use 8-bit data and 1 stop bit. Assume XTAL = 10MHz.

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Example 1 (contd)

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Example2

Write a C program to transfer the message YES serially at 9600,

continuously. Use 8-bit data and 1 stop bit. Assume XTAL = 10MHz.

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Example2 (contd)

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Example 3

Write a C program to receive bytes of data serially and put them on

PORTB. Set the baud rate at 9600, 8-bit data and 1 stop bit.

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Example 3 (contd)

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End of Chapter 3