Interfacing with Arduino I2C LCD

[EarlyBird2]

I also found this code written by norohs difficult to work out and eventually I decided that it works more by accident than design and that there has to be a better way of doing this. This is what I worked out.

The LCD works in four bit mode which makes every byte sent a four bit control nibble plus a nibble of data. To send a byte of data two bytes have to be sent consisting of a control nibble plus high data nibble then control nibble plus low data nibble.

To send "H" which is $48

requires two LCDWRITE instructions

LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $48
GOSUB LCDWRITE

In each instruction the first 4 after the $ sets the R/W bit to write and $48 is made up from the 4 and 8 in red.

and "D" would be

LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $44
GOSUB LCDWRITE

and to print "DHD" would be

LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $48
GOSUB LCDWRITE
LCD_CMD = $44
GOSUB LCDWRITE
LCD_CMD = $44
GOSUB LCDWRITE

Looking at the LCDWRITE

LCDWRITE:
I2CWRITE D,C,ADDR,[LCD_CMD]
E=1
I2CWRITE D,C,ADDR,[LCD_CMD]
E=0
I2CWRITE D,C,ADDR,[LCD_CMD]
RETURN

each command is sent three times with E=0 then E=1 then E=0 which I think is an error, the first line is not needed.

LCDWRITE:
E=1
I2CWRITE D,C,ADDR,[LCD_CMD]
E=0
I2CWRITE D,C,ADDR,[LCD_CMD]
RETURN

but you can test this.

Why send it twice? On the LCD with the E bit set to 1 the data is loaded into a buffer and E has to change from 1 to 0 for the data to be transferred to the screen which is why the second instruction is required.

I decided to look for a better solution as I was not totally confident that the above was correct.

Darrel Taylor came to the rescue here

http://support.melabs.com/threads/98...F8574-20x4-LCD

As you can see from this not all Arduino LCDs are wired the same. The control and data nibbles can be either data/control or control/data.

johncouture worked this out for us in the thread above.

[doctordhd]

EarlyBird2, your reply has helped me understand this a bit more. I was able to get the norohs code to work without the initial I2CWRITE command as you suggested.

Darrel Taylor's program seems to be a work of art but I think with the INCLUDE, it's just too big for my 12F629 as I am getting, "Unable to fit variable" errors.

I have been able to use the lcdout command to write data to my LCD1602. My goal is to do the same with the i2cwrite command.

I will try to get a better understanding of how to send the control commands by studying Darrel's code.

I think what I am trying to figure out is...

Hey, you need to initialize the LCD to nibble / 4 bit mode by sending I2CWRITE commands with code $xx, ...
then to turn on the backlight you need to I2CWRITE $yy (mabe needs 2 commands for high and low nibbles)
to send data you need to I2CWRITE highNibble, then I2CWRITE lowNibble...

I will reread the thread you referenced, http://support.melabs.com/threads/98...F8574-20x4-LCD

[richard]

Darrel Taylor's program seems to be a work of art but I think with the INCLUDE, it's just too big for my 12F629 as I am getting, "Unable to fit variable" errors.

I did not realise you were using a pic12f the dt code won't work on those chips because

ARRAYWRITE is not supported on 12-bit core PIC MCUs due to RAM and stack

constraints

[doctordhd]

I believe Darrel Taylor was using a 12F1822. Doesn't that have the same "bit core" as my 12F629?

According to the Microchip datasheet on the 12F1822,

"This family of devices contains an enhanced mid-range 8-bit CPU core"

[doctordhd]

The PIC numbering system was getting me confused so I spoke to an engineer at Microchip and this is what I was told...

The 12 in the 12F629 means it is an 8 pin chip. (I didn't ask why 12 means 8 or why they just didn't just call it an 8Fxxx)

The 6 after the F means that it has a 14 bit memory core, since anything other (? # > 5) has a 14 bit memory core.

A 12F5xx chip would be an 8 pin chip with a 12 bit memory core...

So it does not seem to a 12-bit core problem but rather an overall memory problem.

The 12F629 has 1k x 14 memory (there's that 14 bit core again), where as
the 12F1822 has 2K 14 memory.

I ordered some 12F1822s to see if I can get Darrel's program working.

[doctordhd]

Here is the thread in which I believe Darrel was using a 12F1822.

Post #3

http://support.melabs.com/threads/98...F8574-20x4-LCD

[doctordhd]

Hmmm, unfortunately, I just figured out that my Silver version of PBP3.0 doesn't support the 12F1822...

[richard]

a pic12f1xxx is an enhanced midrange device . they have twice the stack capacity of a 12f6xx device , apart from more memory etc ....