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CFAG14432 144x32 Vdd, Vo and PSB signals

aleks

New member
Hello,

I need some clarification about the CFAG14432 144x32 displays:

1. Looking on the ST7920 data sheet the VDD is : 2.7V to 5.5V, but CFAG14432D-YYH-TT data sheet (page 5) states:
Supply Voltage For Logic VDD-VSS - 4.5 to 5.5 V
The question is: may I use the CFAG14432 display with 3.3 Vdd for logic supply?

2. If I order LCD for serial interface will "PSB" signal already set correctly (with pull down ressitors) or it is user responsebility to do it? If it is user responsebility where I can find instructions which resistor has to be populated on the back side of the LCD?

3. According to CFAG14432D-YYH-TT data sheet (page 7) the pin 3 designated as (Vo) is Supply voltage for LCD, but on page 8 on the block diagram it is shown as output. From the other hand the data sheet for ST7920 on page 47 (reference design) does not use this pin. Please clarify:
Is VO not an output?
Does the contrast pot connect to VDD and VSS with the wiper going to VO?

Thanks.
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I am not privy to the circuit on the 14432 board, but I can offer some opinions.

1. You can use 3.3 v for the logic supply, but the LCD bias requires about 4.5, referenced to Vdd. So you would have to use a negative supply to achieve that (-1.2 v below ground).

2. ??

3. Vo is normally considered an input, but in behavior it is a current sink output. The Vo circuit is typically referred to Vdd on the board, and an external voltage/resistance circuit is used to pull current from the Vo pin to a voltage below Vdd. In the diagram on p47 of the data sheet, it may be R10 acting as the contrast control.

Perhaps CFA can give you better information.
 

aleks

New member
Update on question #1:

1. You can use 3.3 v for the logic supply, but the LCD bias requires about 4.5, referenced to Vdd. So you would have to use a negative supply to achieve that (-1.2 v below ground).
I was able use 3.3 Vdc for the logic supply with external 4.7 Vdc referenced to Vss for LCD bias, connected to Vo (pin 3). This means we do not need negative voltage. All we need is 4.5 – 4.8 Vdc referenced to Vss.:)
 
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