Ebay HD44780 LCD with PCF8574 expander

I have an Ebay HD44780 LCD equipped with a piggybacked PCF8574 expander chip. I have been able to make it work in 4-bit mode thanks to many of the related threads on this forum. Unfortunately 4-bit mode requires some awkward programming every time you display a message.

So my question is, why does everyone seem to operate this board in 4-bit mode? It is advertised a 4-bit or 8-bit and by examining the connections to the display it appears that all eight bits are interconnected. So it seems like 8-bit mode should be feasible and yet 4-bit mode seems to be the common implementation.

I have done some Googling and I have not found an answer to this question...anybody have some insight before I spin my wheels trying to implement 8-bit mode?

Thanks,
Frank

In 4-bit mode an LCD requires 6 signals; RS, E and 4 data lines.

In 8-bit mode an LCD requires 10 signals; RS, E and 8 data lines.

The PCF8574 can handle 6 signals but not 10.

Hi Frank,

Most of those I2C-LCD driver boards do actually "use" all 8 lines of the expander. In addition to the 6 lines described by hippy, the 7th is connected to the LCD "Read/Write" line (although it's doubtful if Reading in 4-bit mode is practical via an I2C expander) and the 8th line is connected to the backlight, which can be turned On/Off or even dimmed using (software) PWM.

Cheers, Alan.

hippy said:

In 4-bit mode an LCD requires 6 signals; RS, E and 4 data lines.

In 8-bit mode an LCD requires 10 signals; RS, E and 8 data lines.

The PCF8574 can handle 6 signals but not 10.

Click to expand...

Thanks, Hippy, I thought that might be the situation. However there are 16 interconnections between the two boards with labels suggesting that 8 data transfer lines and independent control lines are implemented, hence my question. Although the PCF8574 data sheet only shows 8 controllable outputs so I think that you are correct.

I guess I could move control of the LCD RS and E pins from the PCF8574 onto the Picaxe basically leaving the 8 data lines strictly for data. Do you think that might work?

Frank

Edit by Frank: I did some experimenting trying to determine if moving control of the RS and E pins could work and I think it is not practical . That's because I would also have to also rewire four PCF8574 output lines to the appropriate LCD input pins and the very small size of the unit I have makes that very difficult. Maybe it can be done, but I don't think my eyesight or my motivation is good enough to try it.

Thanks, Alan, I agree.