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bandwidth and hence an application could potentially run faster in a system with a smaller LCD
display or a display with a lower refresh rate. Also, DMA channels can influence the performance
of applications. This section describes how different sub-systems can be optimized for improving
system performance.
3.5.1
LCD Controller Optimization
The LCD controller provides an interface between the PXA27x processor and a LCD module. The
LCD module can be passive (STN), active (TFT), or an LCD panel with internal frame buffering.
3.5.1.1
Bandwidth and Latency Requirements for LCD
The LCD controller may have up to 7 DMA channels running depending on the mode of operation.
Therefore the LCD can potentially consume the majority of the bus bandwidth when used with
large panels. Bandwidth requirements for each plane (that is: base, overlay1, overlay 2, etc.) must
be considered when determining LCD bandwidth requirements. The formula for each plane is:
Plane Bandwidth =
Length and width are the number of lines per panel and pixels per line, respectively. Refresh rate is
in frames per second. BPP is bits per pixel in physical memory, that is: 16 for 16 BPP, 32 for 18
BPP unpacked, 24 for 18 BPP packed (refer to the Intel® PXA27x Processor Family Developer's
Manual for more info).
Depending on where the overlay planes are placed, there might be variable data bandwidth
requirement during a refresh cycle of the LCD. The sections on the screen with overlaps between
overlay 1 and 2 require fetching data at the highest rate. It is important to understand both the long
term average and the peak bandwidth. The average bandwidth is a long term average of the
consumed bandwidth over the entire frame. The peak bandwidth is the highest (instantaneous) data
rate that the LCD consumes - which occurs when fetching data for the overlapped section of the
frame.
The average bandwidth can be calculated as:
The formula for peak bandwidth is:
Peak Bandwidth
Intel® PXA27x Processor Family Optimization Guide
×
×
Length
Width
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Average Bandwidth
=
=
Maximum Plane Overlap
System Level Optimization
×
Bytes / Second
Refresh Rate
BPP
8
∑
(
)
Plane Bandwidths
X
Base Plane Bandwidth
3-11
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