Bandwidth Calculation And Saving; Bus Overhead Considerations - Motorola DragonBall MC68328 User Manual

LCD Controller

4.8 BANDWIDTH CALCULATION AND SAVING

Because LCD screen refresh is a periodic task, the load LCDC puts on the host data bus
becomes an important consideration to the high-performance handheld system designer.

4.8.1 Bus Overhead Considerations

The following example illustrates the issues involved in the estimation of bandwidth
overhead to the data bus.
Consider a typical case scenario:
Screen size: 320 x 240 pixels
Bits per pixel: 2 bits / pixel
Screen refresh rate: 60 Hz
System clock = 16.67 MHz
Host bus size: 16 bit
DMA access cycle: 2 cycles per 16-bit word
The period, T ,, that LCDC must update one line of the screen is,
l
At the same period, the line buffer must be filled. The duration, T
will take up the bus is,
Thus, the percentage of host bus time taken up by the LCDC DMA is P
4-20 MC68328 DRAGONBALL PROCESSOR USER'S MANUAL
1
T = ------------ -
l
60Hz
= 69.4 s
320pixels 2bitperpixel 2clock
T = --------------------------------------------------------------------------------------
DMA
16.67MHz 16bitbus
= 4.8 s
4.8` s
P = ------------------ -
DMA
69.4` s
%
= 6.92
1
------------------- -
240lines
DMA
(EQ 1)
, which the DMA cycle
(EQ 2)
,
DMA
(EQ 3)
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