The mysterious D7

desijays said:

Hello everyone,

I am using a PIC 16f877a for interfacing with an LCD. Typically, an LCD spends some time occasionally to deal with its own internal processing and during that time, trying to send any commands or data to the LCD will be futile cos the LCD is busy.

So before I send any commands or data to the LCD, I would like to check if it is busy or not. Could someone please tell me how to do that?

In my book here, it says I should check the D7 data bit of the LCD. And that if D7 = 0, then LCD is not busy but otherwise it is busy.

This is where the problem arises. When Im initializing the LCD, whichever port is used for the D0-D7 bits is set as output. So the LCD will not be able to pull the D7 bit high even if it is busy since D7 is configured for output. If that is the case, how is it possible to check to see if the LCD is busy or not.

I am aware of other solutions like creating a constant delay. But I would like to know if the same, can be accomplished using the D7 pin as it is mentioned in my book.

Thank you.

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ericgibbs said:

hi,
When initialising the LCD, for the first 2 or 3 commands the BUSY is not read, its not active.

During normal operation, you set the PORT on the PIC to INPUT.
You then test BUSY [D7] to check the LCD is ready for a WRITE, when the D7 becomes ready,
the PIC PORT is set as OUTPUT and you write to the LCD.

You repeat this cycle whenever you need to WRITE a command or character to the LCD.

Do you plan to use 4 or 8 lines for the data connection from PIC to LCD.?

Do you follow.?

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Pommie said:

To add to what Eric said, as well as setting the data lines to input you have to set the RW line to 1 and the RS line to 0 before you take the Enable line high. It is permissible to leave the Enable line high until D7 goes low and you don't need to read the second nibble.

Mike.

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desijays said:

Actually, to read the value on the D7 bit, the enable pulse must be a low to high pulse.

For all other commands the pulse is high to low though.

Click to expand...

void lcd_busy(void)
{
	RW_TrisBit = 1;		//make R/W input (read)
	D7_TrisBit = 1;		//make D7 input
	RS = 0;			//set RS low
	RW = 1;			//set R/W high
	E = 1;			//set E high
	while(busyflag);	//wait for busy flag to go low
	E = 0;			//set E low
	RW = 0;			//set R/W low
	TRISB = 0;		//make D7 output
	RW_TrisBit = 0;		//make R/W output (write)
}

desijays said:

Actually, to read the value on the D7 bit, the enable pulse must be a low to high pulse.

For all other commands the pulse is high to low though.

Thank you

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futz said:

Not true. Here's a typical (working) busy check routine in C:

Code:

void lcd_busy(void)
{
	RW_TrisBit = 1;		//make R/W input (read)
	D7_TrisBit = 1;		//make D7 input
	RS = 0;			//set RS low
	RW = 1;			//set R/W high
	E = 1;			//set E high
	while(busyflag);	//wait for busy flag to go low
	E = 0;			//set E low
	RW = 0;			//set R/W low
	TRISB = 0;		//make D7 output
	RW_TrisBit = 0;		//make R/W output (write)
}

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Pommie said:

How does that work?

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If you make the R/W pin an input then you must have the weak pullups on or something!

I see you do the permissible things.

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Pommie said:

How does that work? If you make the R/W pin an input then you must have the weak pullups on or something!

Click to expand...

ericgibbs said:

hi des,
If you say which PORT pins the LCD is connected too, we could post a code sample for that configuration.

If you use the 4 bit LCD option, this will free up 4 PIC pins for other jobs.

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ericgibbs said:

hi Des,
Sorry, there's no way that program is going to work the LCD.

Would you like me to post a working sample.? or do you prefer to have a go yourself.

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desijays said:

Hello Eric,

Could you please tell why that program would not work? Obviously there is some error in it. It would help me learn if you could tell me in which part of the program I have erred!!

One other thing; what is the necessity to use portA in your program?

Thank you

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