VII LCD CONTROLLER BLOCK: LCD CONTROLLER
Power Save
The LCD controller has two types of power-save modes. Use LPSAVE[1:0] (D[1:0])/LCDC mode register 2 (0x39FFE3) to set power-save modes.
Table 2.17 Settings of Power-Save Modes
LPSAVE1 | LPSAVE0 | Mode |
0 | 0 | Power-save mode |
0 | 1 | Reserved |
1 | 0 | Doze mode |
1 | 1 | Normal operation |
Power-save mode
When the LCD controller enters this mode, all LCD signal output pins, including the LCDPWR pin, are dropped low, with the LCD panel placed in power-down mode. All operations of the LCD controller, other than accessing of its control registers, are disabled. The look-up tables cannot be accessed.
The LCD controller is placed in power-save mode by setting LPSAVE to "00", thereby executing a power- down sequence. The LCDPWR signal goes low a one-frame period later, and LCD signals are deasserted.
Note: Because the bus clock is turned off in HALT2 or SLEEP mode, the one-frame period described above must elapse before the chip can be placed in standby mode. The number of frames can be counted by reading the VNDPF (D7)/vertical non-display period register (0x39FFEA) repeatedly. VNDPF is set to "1" during the vertical non-display period (set to "0" during the display period).
The LCD controller is taken out of power-save mode by setting LPSAVE to "11", thereby executing a power- up sequence. The LCD signal output is enabled and the LCDPWR signal goes high a one-frame period after power-save mode is released.
The above power-up/power-down sequences can be controlled with a user’s desired timing by using LPWREN (D4)/LCDC mode register 2 (0x39FFE3). For details on the control procedure, refer to "Controlling LCD Power Up/Down".
Doze mode
Doze mode is a power-save mode designed for use with Epson’s MLS LCD drivers. When MLS LCD drivers are used, there is no need to send data constantly in order to refresh the display of the same image. The LCD controller can be set in doze mode during this period. In doze mode, the FPDAT and FPSHIFT signals are fixed low so that no access to the display memory occurs. Although the power-saving effects are not as significant as in power-save mode, this mode helps reduce the current consumption in the LCD panel while keeping the display on.
Comparison of power-save modes
The differences between power-save modes are summarized in Table 2.18.
Table 2.18 Differences between Power-Save Modes
Item | Doze mode | Power-save mode | Normal |
Accessing IO registers | Enabled | Enabled | Enabled |
Accessing look-up table | Enabled | Disabled | Enabled |
Sequence controller in LCDC | Run | Stop | Run |
Display | Display | Blank | Display |
LCDPWR signal | Active | Inactive | Active |
FPDAT[7:0], FPSHIFT signals | Forced low | Forced low | Active |
FPLINE, FPFRAME, DRDY signals | Active | Forced low | Active |
