hm_fa_da
Full Member level 5
Hi,
Recently i'm facing a problem in a device (many ones) that the input Key of it seems to be pressed.
the device has 4 arrow keys as inputs. the malfunction is like one or more than one of the keys are pressed. i checked mechanical structure of the keys and found no problem, the problem exist even after removing cable between board and keys.
then i checked input circuit, one 470K pull up resistor + 10nF bypass cap is used in the input circuit as below:
I measured the voltage on input pin of mcu, in the inputs without problem the voltage is 3.3V (through pull up resistor). but in the inputs with problem, the voltage is very low, near 0V. so the MCU senses it as SW is pressed.
The only route to 0V can be the 10nF MLCC.
it was right, i tested in many devices (same device):
1- When i use soldering Iron on the cap by just touching the solder pads and melt the solder, the problem is fixed !
2- When i remove the cap by breaking it by a tool (I didn't want to use soldering heat to remove the cap), the problem is fixed !
3- When i cut the route between the cap and input, the voltage on MCU input pin rises to 3.3V (through pull up resistor).
4- I measured capacitance of the cap by meter, it shows around 10nF. but when i use ohm meter on the cap (which is electronically isolated from other components) in the failed caps, it shows around 1K resistance !
The point is that all these problem will be fixed if i only use (not even change or remove the cap) soldering iron on two pads of the cap ...
seems there is locked malfunction in the cap that will be removed by soldering iron toughing it. note that there is no dead soldering problem, even if there was, it would make disconnection not short or low resistance connection.
Another point is that the all devices with this problem are installed in outdoor conditions for +5 years. in a humid weather area.
I doubt for moisture problems ...
What do you think about the problem reasons ?
Recently i'm facing a problem in a device (many ones) that the input Key of it seems to be pressed.
the device has 4 arrow keys as inputs. the malfunction is like one or more than one of the keys are pressed. i checked mechanical structure of the keys and found no problem, the problem exist even after removing cable between board and keys.
then i checked input circuit, one 470K pull up resistor + 10nF bypass cap is used in the input circuit as below:
I measured the voltage on input pin of mcu, in the inputs without problem the voltage is 3.3V (through pull up resistor). but in the inputs with problem, the voltage is very low, near 0V. so the MCU senses it as SW is pressed.
The only route to 0V can be the 10nF MLCC.
it was right, i tested in many devices (same device):
1- When i use soldering Iron on the cap by just touching the solder pads and melt the solder, the problem is fixed !
2- When i remove the cap by breaking it by a tool (I didn't want to use soldering heat to remove the cap), the problem is fixed !
3- When i cut the route between the cap and input, the voltage on MCU input pin rises to 3.3V (through pull up resistor).
4- I measured capacitance of the cap by meter, it shows around 10nF. but when i use ohm meter on the cap (which is electronically isolated from other components) in the failed caps, it shows around 1K resistance !
The point is that all these problem will be fixed if i only use (not even change or remove the cap) soldering iron on two pads of the cap ...
seems there is locked malfunction in the cap that will be removed by soldering iron toughing it. note that there is no dead soldering problem, even if there was, it would make disconnection not short or low resistance connection.
Another point is that the all devices with this problem are installed in outdoor conditions for +5 years. in a humid weather area.
I doubt for moisture problems ...
What do you think about the problem reasons ?