Archon wrote on 2023-05-08, 19:58:
Here we go:
1. CE on RTC goes high for a very short while after power on, and then goes low. After turning power off it goes hig […]
Here we go:
1. CE on RTC goes high for a very short while after power on, and then goes low. After turning power off it goes high and then slowly goes to zero.
- pin 1 and 4 are high all the time
- pin 2 - high for a short period of time and then stays low
- pin 3 - 24MHz, Vpp 3.5V
- pin 2 is not connected with pin 9, at least there's no continuity between them
- pin 5 is also high for a while, but then goes down to ~2V, which I observed earlier.
So what's next? Should I unsolder the sucker and try a new one?
Pin 2 behaves as expected. Pin 5 does not. This can mean one of two things: Either something else is sourcing way too much current, so the 74ALS74 is unable to pull the output low enough, or the 74ALS is broken. To find out whether something is sourcing too much current, put your meter into 200mA current measurement mode, red wire to 74ALS74 pin 5 (the "2V" pin), and black wire to GND.
If the current is below 5mA, the 74ALS74 should be able to sink that current, and likely is broken, so replace it.
If the current is above 10mA, some other chip is broken and pulls the line high strongly. In that case, looking for the culprit can be tedious, because there are so many chips connected to that signal. First try removing all socketed pins that connect to 74ALS74, pin 5, and see whether the voltage goes down.