LCD Frame Buffer and Power Consumption (Windows Embedded CE 6.0)
1/5/2010
Displaying an image on an LCD while the target device is otherwise idle can consume a large amount of power.
You can reduce the amount of power consumed for image display by following these guidelines:
- Use an LCD that can display a static image without a constant, externally supplied, refresh signal from the system frame buffer. This reduces power consumption by enabling the display controller to be disabled and the associated memory accesses to be eliminated.
- Ensure that the system frame buffer that the display controller uses is located in the CPU or display refresh control hardware, and not in system SDRAM. This eliminates cross-chip memory accesses during display refresh. It also avoids the need to access SDRAM to refresh the display when the system would otherwise be idle.
If neither of these goals can be met, the best solution could be to automatically turn off the display after a period of user inactivity. While the LCD needs constant refreshing from a frame buffer in SDRAM, the target device continues to access the SDRAM while it would otherwise be idle. This consumes power in two ways:
- Normal SDRAM power usage when the SDRAM is accessed to refresh the LCD.
- By preventing the SDRAM from entering its reduced-power self-refresh mode. The SDRAM instead stays in standby mode between LCD refresh accesses. The following table shows sample differences in power consumption between these modes for common SDRAM configurations.
| SDRAM configuration | Self-refresh current (mA) | Standby current (Icc2NS, mA) | Idle power increase (mW) |
|---|---|---|---|
16mb (x16) |
0.8 |
10 |
30 |
32mb (x16) |
1.5 |
10 |
28 |
64mb (x16) |
3.0 |
10 |
23 |
32mb (x32) |
1.6 |
20 |
61 |
64mb (x32) |
3.0 |
20 |
56 |
128mb (x32) |
6.0 |
20 |
46 |