I don't understand how this circuit is supposed to work.
(I verified that pulling up base of 2Q2 to +5V turns on the fan though.)
2U8 has no way of talking to 2U14 except through the pin 10.
(It has 3 incoming digital lines thath seem to be CS/CLK/DATA but all of them come from the outputs of the level shifter 2U13 (SN74LVC07A), so these cannot be used for output).
Upd: this might not be true, see next message.2Q2 controls the fan. It is attached to the pin 10 of the 2U8 (MSP430F1101A,
https://www.ti.com/lit/ds/symlink/msp430f1101a.pdf) which can be used as comparator output.
So it's reasonable to assume that it is indeed wired to the comparator output.
MSP430 has two comparator inputs: 11,12 which both could also be used for GPIO.
Pin 13 could be either auxilary clock output or GPIO.
Comparator is explained in the chaptor 16 of the user guide
https://www.ti.com/lit/ug/slau049f/slau049f.pdf .
It can compare either its two external inputs, or compare one of them to the internal reference (selectable [0.25 x Vcc], [0.5 x Vcc], or "or a fixed transistor threshold voltage of ~ 0.55 V").
Vcc in our case is 2.5V, so it should be 0.625V, 1.25V, and 0.55V.
I tried to simulate the circuit for the temperatures 25C and 85C as two possible edge cases:
For all pins 11,12,13 input voltages decreases when temperature rises.
Threshold 0.5 x Vcc = 1.25V is too large for any simulated input.
Threshold 0.25 x Vcc = 625mV is barely crossed by pin 13 input around 25C (which is not a comparator input anyway) and pin 12 input, but now close to 85C.
I think this does not work, because by the time this sensor reaches 85C other parts will be cooked.
Another question is "why make that complicated circuit to start the fan only at about 80C"?
MSP430 manual suggests doing "precision slope analog-to-digital conversions".
There is actually a ceramic capacitor 2C31 which potentially could be used for that purpose.
Playing with random pulses in the LTspice I could make it cross 550mV threshold at about 35C:
I used 10uF capacitor for simulation, but I later measured it in-circuit 100nF. (by DE5000 at 100kHz)
It is also possible to make it work with the 100nF cap:
But it looks weird. Pulse width 0.0002ms suggests system clock at least 5Mhz, but this MC has only 4.096MHz crystal.
Also I ccould not see any change in behavior when setting pin 13 to either 2.5V or 0V. It does not seem to affect anything.
Ok. I powered the device that I am repairing and measured:
- Pin 11: 816mV
- Pin 12: 685mV
- Pin 13: 570mV
which is very close to the static simulation above of 900mV, 755mV, 624mV.
I did no see any activity on these pins.
Could somebody help me understand how this sensor circuit is supposed to work?