LCD Bandwidth Questions and Explanation of the Calculator

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by Martin84 on Mon Oct 07 10:46:27 MST 2013
Hi everybody,

I'm now trying to understand the LCD Bandwidth Calculator for the 18xx/43xx and the theory of the AHB-Bus.

The calculation for the framebuffer size is easy, it is the product of the resolution times the color depth in bytes. To determine the LCD Data Rate I take the Pixels for one frame and multiply this value with the refresh rate. So I get e.g. 640x480x60Hz = 18432000 Pixels/s = 18,432MPixel/s.

To get the Mbit/s value, we multiply the MPixel/s with the size of one Pixel. So 18,432 MPixel/s x 16 Bit/Pixel = 294,912 Mbit/s.
To get the MWords/s, we have to know that 1 Word = 32 bit. -> 294,912Mbit/s / 32bit = 9,216 MWords/s.

With the Note in the calculator sheet, we know that 1 Burst = 4 words. So we get 9,216 MWords/s / 4 Words = 2,304MBursts/s. Here I got the first question, but I could answer this question with the help of the LPC4357-User Manual and the EMC-Section.
A: For a 32-bit external memory bus the burst size is 4 words.

Now the next thing in the calculator sheet is to determine the duration of one burst in clock cycles. So the formula in the sheet is

Duration[clocks] = 7 + trp + RAS_Latency + CAS_Latency - 2 + 4 * 32bit/32bit

What do the given values (7 and - 2) mean?
4*32bit means the length of 1 Burst and the other 32 bit are the EMC Bandwith, or am I wrong?

So: Duration[clocks] = 7 + trp + RAS_Latency + CAS_Latency - 2 + 1Burst / EMC_Width?

The Busload is determined with (MBursts/s * Duration[clocks]) / EMC_frequency[MHz].

So the busload can be determined with the 'duration of all Burst per seconds' referred to 'EMC_frequency'?

The calculated busload is only the portion of the AHB Bus which is needed by LCD Controller reading from SDRAM. Additional to that there will be a portion used for writing pixel data from EMC-Controller to SDRAM and for reading image data from SD-Card?