How to switch of a LCD screen

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How to switch of a LCD screen

Topic last updated 29 May 2015, by David Smart. 1 reply

# 19 May 2015

I need to power up a LCD screen sometime after I have powered up the mbed board. Also it would be good to be able to switch off the screen to save power.

I have a 24V supply and need 3.3V for the display and about 800mA

What would be a good way of doing this ?

Thanks Kim

David Smart

# 29 May 2015

I Kim, I had replied by private email when the forum would not accept my post.

RA8875 Display module

For the RA8875 display, which I think you are experimenting with, there are software controls to turn the display off (this is not the same as sleep mode). There are additional settings that are not currently exposed by the driver library. I'll come back to those.

For the RA8875 chip, the electrical specifications indicate 50 mA max operating, and 320 uA typical when in sleep mode. Sleep mode seems to keep the power and oscillator on. The data sheet for the 4.3" module also indicates 320 uA sleep, but now the power-on also includes the other circuitry, including the backlight (230 mA max at 3.3v). The back light can be controlled by the driver library, which has a "Power( )" API that presently includes only an on/off state, not a sleep state).

Sleep mode

Sleep mode is controlled by the same Power and Display Control Register - and it looks easy to activate, to get to that 320 uA level. But take care when bringing it out of sleep mode, there are some particular steps listed to wake it back up correctly. It looks like there are quite a few steps involved, but they appear to be well described. Note that even at the specified 320 uA, which is a typical rating for the chip, other circuitry on the module may also be drawing some power.

Hardware Power Control

I put this section last, because in my mind you may not need it, and you will need to take care if you apply it. In this concept, you could control the power to the display module, using some mbed pin, and a power control chip, perhaps like the FPF2123, which you will find on the schematic for the LPC1768 mbed module. This part is switching the 5v supply - I don't know if it is rated for 3.3v, so there may be similar parts that would work.

My main concern with this concept is that I think you will need to take care that all the pins between the host micro and the RA8875 are first a logic low, and then cut the power to the module. If you do not do this, the module may try to source power from the communication pins, which won't work at best, and may cause damage at worst. If you are connected to the SPI port, you would not be able to use the SPI port for any other shared purpose - for example. In powering up the display, you would first apply power to the module, and then you can enable communications on the IO pins.

I Kim, I had replied by private email when the forum would not accept my post. == RA8875 Display module == For the RA8875 display, which I think you are experimenting with, there are software controls to turn the display off (this is not the same as sleep mode). There are additional settings that are not currently exposed by the driver library. I'll come back to those. For the RA8875 __chip__, the electrical specifications indicate 50 mA max operating, and 320 uA typical when in sleep mode. Sleep mode seems to keep the power and oscillator on. The data sheet for the 4.3" __module__ also indicates 320 uA sleep, but now the power-on also includes the other circuitry, including the backlight (230 mA max at 3.3v). The back light can be controlled by the driver library, which has a "Power( )" API that presently includes only an on/off state, not a sleep state). == Sleep mode == Sleep mode is controlled by the same Power and Display Control Register - and it looks easy to activate, to get to that 320 uA level. But take care when bringing it out of sleep mode, there are some particular steps listed to wake it back up correctly. It looks like there are quite a few steps involved, but they appear to be well described. Note that even at the specified 320 uA, which is a typical rating for the chip, other circuitry on the module may also be drawing some power. == Hardware Power Control == I put this section last, because in my mind you may not need it, and you will need to take care if you apply it. In this concept, you could control the power to the display module, using some mbed pin, and a power control chip, perhaps like the FPF2123, which you will find on the schematic for the LPC1768 mbed module. This part is switching the 5v supply - I don't know if it is rated for 3.3v, so there may be similar parts that would work. My main concern with this concept is that I think you will need to take care that all the pins between the host micro and the RA8875 are first a logic low, and then cut the power to the module. If you do not do this, the module may try to source power from the communication pins, which won't work at best, and may cause damage at worst. If you are connected to the SPI port, you would not be able to use the SPI port for any other shared purpose - for example. In powering up the display, you would first apply power to the module, and then you can enable communications on the IO pins.

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