PIC18F252 USART problem - Electronic Circuits Projects Diagrams Free

mikesmixes777 · 18th December 2007, 01:53 PM

Hi,
I am trying to do a small program to get the USART to work. I want to write the letter "A" out on the RS232 port. I have downloaded a sample hex file and loaded into the PIC to test the RS-232 comms to my pc. All is working except my version of code. I am getting funny asterix symbols out the comms port.

Have I left something out of the code. I have set up the registers as per the datasheet and I am only transmitting at the moment.

Code:

#include <p18cxxx.h>

void init_usart(void)		// Initialise the USART
{
	TXSTA=0x24;		// Asynch,8bits,tx enb
	RCSTA=0x90;					
	SPBRG=0x12;		// BaudRate=9600 @ 20Mhz as Table
	RCSTAbits.SPEN=1;	// Enable Serial Port
} 

void write_usart(void)		//Write to the USART
{ 
	while(TXSTAbits.TRMT==1)
	TXREG='A';
}

void main (void)

{

	TRISC=0x80;		//Set RC7=Input, RC0:6 as Output

  	init_usart();		// Initialise the USART
	write_usart();
	
}

Thanks
Mike

eng1 · 18th December 2007, 02:14 PM

Code:

        SPBRG = 0x12;

Are you sure about that value? Shouldn't it be 0x20?

mikesmixes777 · 18th December 2007, 02:23 PM

My bad....
I forgot to take out the 0x

The correct one is

Code:

SPBRG=12;		// BaudRate=9600 @ 20Mhz as Table16-5

The datasheet table says that for 9600 @ 20Mhz with +0.16error SPBRG=12 (decimal)

eng1 · 18th December 2007, 02:34 PM

Oh, the BRGH bit is set, I missed that, sorry! We must refer to table 16-5, right. The correct value is 129 (0x81) for 9600 baud, isn't it?

mikesmixes777 · 18th December 2007, 02:53 PM

ok...i need glasses.... I was looking at the 96Kbps and not 9.6Kbps. (thanks for that!!)

I have changed the SPBRG value to 129 however now i am getting nothing on the RS232, not even the symbols.

im not to sure about my "while" loop... I'm new to C.

Code:

#include <p18cxxx.h>

void init_usart(void)		// Initialise the USART
{
	TXSTA=0b00100100;	// Asynch,8bits,tx enb
	RCSTA=0b10010000;					
	SPBRG=129;		// BaudRate=9600 @ 20Mhz as Table
	RCSTAbits.SPEN=1;	// Enable Serial Port
} 

void write_usart(void)		//Write to the USART
{ 
	while(TXSTAbits.TRMT==1)
	TXREG='A';
}

void main (void)

{

	TRISC=0x80;		//Set RC7=Input, RC0:6 as Output

  	init_usart();		// Initialise the USART
	write_usart();
	
}

eng1 · 18th December 2007, 02:59 PM

Code:

void write_usart(void)     
{ 
    TXREG='A';      // copy 'A' into the buffer
    while(PIR1bits.TXIF==0); // wait until all bits are transmitted
}

mikesmixes777 · 18th December 2007, 03:10 PM

Aha!!! Thats it... its working now. I'm getting an infinite amount of A's on the screen.

How would I send a complete word "Hello" to the usart and then stop the usart from sending? Could I use an array?

THanks for your help..!!!

eng1 · 18th December 2007, 03:18 PM

Quote:

Originally Posted by mikesmixes777

How would I send a complete word "Hello" to the usart and then stop the usart from sending? Could I use an array?

Yes.

Quote:

Originally Posted by mikesmixes777

Aha!!! Thats it... its working now. I'm getting an infinite amount of A's on the screen.

Place a while(1); at the end of the program (inside the main routine).

mikesmixes777 · 18th December 2007, 03:27 PM

Thanks. I shall look into the array's....

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18th December 2007, 01:53 PM	(permalink)
mikesmixes777	PIC18F252 USART problem Hi, I am trying to do a small program to get the USART to work. I want to write the letter "A" out on the RS232 port. I have downloaded a sample hex file and loaded into the PIC to test the RS-232 comms to my pc. All is working except my version of code. I am getting funny asterix symbols out the comms port. Have I left something out of the code. I have set up the registers as per the datasheet and I am only transmitting at the moment. Code: #include <p18cxxx.h> void init_usart(void) // Initialise the USART { TXSTA=0x24; // Asynch,8bits,tx enb RCSTA=0x90; SPBRG=0x12; // BaudRate=9600 @ 20Mhz as Table RCSTAbits.SPEN=1; // Enable Serial Port } void write_usart(void) //Write to the USART { while(TXSTAbits.TRMT==1) TXREG='A'; } void main (void) { TRISC=0x80; //Set RC7=Input, RC0:6 as Output init_usart(); // Initialise the USART write_usart(); } Thanks Mike

18th December 2007, 02:14 PM	(permalink)
eng1	Code: SPBRG = 0x12; Are you sure about that value? Shouldn't it be 0x20?

18th December 2007, 02:23 PM	(permalink)
mikesmixes777	My bad.... I forgot to take out the 0x The correct one is Code: SPBRG=12; // BaudRate=9600 @ 20Mhz as Table16-5 The datasheet table says that for 9600 @ 20Mhz with +0.16error SPBRG=12 (decimal)

18th December 2007, 02:34 PM	(permalink)
eng1	Oh, the BRGH bit is set, I missed that, sorry! We must refer to table 16-5, right. The correct value is 129 (0x81) for 9600 baud, isn't it?

18th December 2007, 02:59 PM	(permalink)
eng1	Code: void write_usart(void) { TXREG='A'; // copy 'A' into the buffer while(PIR1bits.TXIF==0); // wait until all bits are transmitted }

18th December 2007, 03:10 PM	(permalink)
mikesmixes777	Aha!!! Thats it... its working now. I'm getting an infinite amount of A's on the screen. How would I send a complete word "Hello" to the usart and then stop the usart from sending? Could I use an array? THanks for your help..!!!

18th December 2007, 03:27 PM	(permalink)
mikesmixes777	Thanks. I shall look into the array's....