[SOLVED] Digitial ciruit input doubt

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brunofunchas

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Hi!
I would like to confirm if I have understood corretly the circuit:
  1. SW05 Connector:
    • One pin is connected to ground.
    • The other pin is connected to the cathode of D7 (marked A62, 462, or A6).
  2. Voltage Measurement:
    • Measuring between the two pins with a multimeter gives you 5V.
  3. Microcontroller Behavior:
    • The MC68HC908 microcontroller likely has internal pull-up resistors to 5V.
    • When you short the connector, it pulls the voltage down to ground, which the microcontroller detects.
  4. Single Wire to Ground:
    • In one connector, a single wire goes to ground (static reel stand).
    • When this wire is grounded, the board detects it.
    • The detection happens because the wire connects to the cathode of a diode, allowing current to flow to ground without needing to close the circuit through the other pin of the connector.

 

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I have the board mounted on a vehicle. The pictures were taken by me. The manufacturer went into bankruptcy, so I have to develop an alternative. I traced back the connector and found it goes into one pin of the diode, and the other pin is connected to the ground. I was puzzled by measuring 5V on the connector and when I read the manual of the microcontroller I saw that it can work at 5V. Prior to guessing that the diode is just preventing the current from flowing in the wrong direction, I posted this question to check if the assumption that I had from the circuit was correct.
 

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I think the question is only about digital inputs, and the example is the SW05 connector.
There is only a series diode between the connector and the MCU.
I think the conclusions in post #1 are correct.
The internal pull-up in the 68HC908 is 20-65 kOhm, so the current with a shorted connector is in the order of 100 uA.
This doesn't meet the design rules used in automotive circuits. If connectors are not gold plated (tin plated instead), the minimum current is in the range 5-10 mA. The open circuit voltage must be at least 4V (OK for this design). These rules can probably be relaxed for circuits not subjected to vibrations and temperature cycles. I have investigated very high quality tin plated connectors failing at 20 uA after a year or so in a difficult environment.
 
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Thanks for the feedback.
 

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