I2C vs 4bit LCD operation

[aliyesami]

on a 20Hhz clock driven Atmega1284 board will I2C be faster or 4bit LCD operation ?
I already notice screen refreshing in 4 bit mode so wouldnt I2C which is serial will make it slower?
also whats the max freq of I2C signal ?

 

[barry]

I2C can run as fast as 3.4MHz, I don't know if the ATMega (or your LCD) supports that speed. The question is 'how fast is your 4 bit rate?'. I2C would have to run at least four times faster than that to improve your performance.

 

[FvM]

There might be a problem of a partly unclear question. I guess the OP is not asking about an I2C interfaced LCD in general rather than driving the LCD through an I2C port extender. In this case, the overhead of the bus emulation through I2C must be considered. You'll end up with three I2C write transactions for each 4-bit bus write.

Usual port extenders are at maximum using fast (400 kHz) I2C.

 

[aliyesami]

yes FvM that's what I meant.
and how about comparing I2C to SPI ? I think SPI should be faster since it can run on much higher clock ?
I am looking for a protocol to drive the TFT color display and I think I2C wont be able to handle it.

 

[Gorgon]

The real question here is 'how fast is Your LCD display controller'? Most LCDs I've used has some latency when executing some of the commands available. This is communicated by Reading 'busy' flags or data bus toggles. It's of no use to bang on with a lot of data, if parts of it disappears. You need to start with the display, and then evaluate the communication protocol. You can do a lot with streamlining the LCD driver, and run it in the background, if you need the processing speed in the foreground.

Most LCDs are inherently slow in operation.

 

[embpic]

actually SPI is faster than I2C. but i think you want to drive lcd with using such port expander having such protocols.
if you want to use 16x2 or 16x4 type lcd go for I2C protocol.
and if you wanna do some graphical presentation then use SPI.

 

[FvM]

I am looking for a protocol to drive the TFT color display and I think I2C wont be able to handle it.

TFT color display? Time to specify intended data throughput and interface properties!

The problem with I2C port extender on 4-bit LCD interface is that you need to send about 12 bytes on I2C to transmit one data byte to the LCD, so you'll end up with about 1kbyte/s with fast I2C and 1/4 of it with standard speed.

A fast serial interface, either I2C or SPI, preferably uses some hardware logic to reduce the overhead.