Read data input from shift register

Question

I am having some troubles getting the readouts from the shift register.

The 74HC_HCT164 controls a EADCD040YA2 (or alike) two digit 7 segment display.

Is there any way that I could reliably read the Serial input data from the shift register.

I have got a pin-out of the shift register to the display, so that shouldn't be any problem.

What might be useful to know, is that the shift register circuit is not only used for the display, but also for some of the LED's.

The data is shifted into the shift register, then the first common anode is turned on by the microprocessor, then the shiftregister latches, shift register clears, second diigit data get's shifted in, second common anode is turned on, shift register latches and shift register clears.

I don't know if there is happening anything between the two digit 'shift's' and what's the clock cycle of the whole circuit. I don't have access to an ocsilloscope, well, I do if you call my Arduino one.

I am using an Arduino to read out the code (using the following sketch: https://pastebin.com/8fWA6qUP)

I would appriciate it very much if anyone could help me out with this problem!

So to be clear: this is NOT about using a shift register the normal way, but to basically hack shift register and read what's being send to it!

Tim

mcb1 · Answer

tim687 The 74HCT164 is a serial to parallel converter. Basically you set the state you want on that particular output using the two inputs and then raise the clock. You need to do this 8 times to replace/set all 8 outputs You need to read the value of the two data lines, and note them when the clock goes high. You'll also need to check if the MR line is able to change, since it will reset the outputs. Most 7 segment displays only need 4 bits, which means they could easily use the other 4 outputs for something else. Mark

mcb1 · Answer

Is this related to this. Detect whether a pin is connected to GND I posted a link where a local person has been able to detect the serial output from an Aircon unit. https://nicegear.co.nz/blog/hacking-a-mitsubishi-heat-pump-air-conditioner/ It seems that many of them have a serial feed and I was wondering if your might have the same. Mark

michaelkellett · Answer

I don't think you'll ever pull this off without a scope or a logic analyzer. A scope may well be outside your budget but you may be able to afford a cheap logic analyzer. hobby components usb 8ch 24mhz 8 channel logic analyser. compatible with saleae software. This one is typical of the cheapest - it's limited to 24MHz which might be a problem (the clock to the shift register might be faster than this). I haven't used it so it may be rubbish - look for reviews. To sort out how the display drive is working you need to monitor the clock, data and anode signals - so you need at least 4 channels - once you know the protocol you may be able to decode what is sent with less. MK

Read data input from shift register

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