Unable to understand the Backplane pin of the AME7106

[arbj2]

Hi,

I am trying to hack a digital multimeter (MAS730) to connect the digit pins to a micro-controller. The schematic is attached.

I am unable to locate the ground reference for the mcu. The datasheet lists pin 28 (BP/GND) as the ground ref for the AME7107 decoder IC. However this does not seem to be connected anywhere in the circuit

I would like the mcu to detect high or low on each of the digits pins. Using this I can determine the digits being displayed.

From the schematic I notice that the BP pin is not connected to the ground anywhere. But as the mcu will be powered from the same power supply as the meter, how can I derive a suitable ground for the micro-controller ?

One solution would be to connect the mcu across the +V (pin 8) and the BP pin of the AME7107. But the IC works on 2.6V DC, also with an oscilloscope across these pins I noticed that the supply to the AME7107 chip is pulsing, Is this how it is supposed to work ??

The schematic and the datasheet is given below.

View attachment ame7106.pdf

thanks
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[FvM]

Suggest to read an ICL7106 (=AME7106) datasheet and learn how a (non-mux) LCD display works. Segment and BP pins are toggled between V+ and internal digital ground (at about V+ - 6V). To recover the segment data, the pin state must be xor-ed with BP.

 

[arbj2]

thanks for the tip,

As per the pic you posted, there is no ground on the chip, the internal ground is not connected to the outside pin ?

The problem I am facing now is what ground ref. I should use, the ground from the battery supply (input) does not seem to be connected at the LCD end, whereas when I use the V+ and BP pin of the 7106 IC, the entire supply seems to be pulsing when checked by a scope

 

[FvM]

whereas when I use the V+ and BP pin of the 7106 IC, the entire supply seems to be pulsing when checked by a scope

"learn how a LCD display works" is still the next step. LCD is driven with AC voltage and BP isn't a ground. As the datasheet shows, the Test pin can be used as ground reference, but it must be probably buffered. You may need level conversion to connect 6V swing LCD pins to µC input

 

[arbj2]

thanks, I just read about LCD backplanes and frontplanes on the internet,

thanks for pointing out the TEST pin connections though, I missed that part.