# LCD refresh rate

How many hertz will the LCD run?

Does this differ at the different resolutions and wil the speed be above 60 hertz?

The refresh depends on what other things you are doing with the CPU. There is 1 chip taking care of everything: if you do a lot of calculations, refresh rate is slower.

LCD bus is running at 8 MHz, meaning 8 million pixels per second. This would be 206 fps, but its not possible in real world (the LCD can not communicate at that speed).

20-30 Hz should be easy normal situation in most games - depends on the other CPU workload. Lots of floating point calculations etc. will slow down the CPU significantly.

the LCD can not communicate at that speed

Let’s say there is no cpu limitation. What would the max speed be then?

This question requires a story, here goes:

The origin of the Pokitto was when I saw the NXP LPC11U68 introduced. It has a very interesting feature called “single cycle GPIO”. I realized there and then that THIS was my chip of choice to create a low-cost simple as hell color DIY Gameboy.

Single cycle GPIO means the chip can toggle the I/O pins in a single CPU step. So in theory, you could push data out at 48 MHz, which is the speed the chip is running.

However, this is not a realistic scenario. You also need to set up the data, that you will send.

I have been sending the data at 8Mhz. Even at this speed, I need to insert NOPs (no operations) in some places to slow the bus down, because the LCD can not read the data fast enough.

The graphics bus is 16-bit parallel MCU bus with a couple of additional lines and was made according to our spec. So, in one cycle at 8 Mhz, 1 pixel can be sent.

8Mhz = 8 million pixels per second, divided by resolution of 220x176 = 206 frames per second.

But remember: this is THEORETICAL max. In actuality, the CPU has to do other things as well, such as looping jumps and so on. The rendering loops in Pokitto are very optimized and unrolled. You can easily hit 20-30fps in your games which is perfectly playable. I am sure someone comes up with ideas to make it go even faster.

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