There is synchroscope and synchrophase as applied to aircraft. The former is to help the pilot ensure the props are at the same speed. The latter is to help get them in phase. It makes a big difference in vibration and sound levels when the props go into phase. Larger transport aircraft, like the DC6 and B29, had phasing for their four engines. Small light twins usually do not.

Anyway, based on what you are trying to do, maybe there is a different approach that might be easier. Starting with a Hall signal from each motor, why not simply compare phase? Say, the position of the falling edge from each. You could also pull an RPM measurement from the same signal, but that would not be what would drive your propellor indicator. John

 

That is what I wanted to do in the first place but couldn’t find enough info. A PLL almost looked like it would work but I’m not an analog guy and it wouldn’t come together for me.
I have a friend that repairs avionics. He helped some. I did find some really trick mechanisms that control RPM by varying the prop pitch instead of throttle.

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The frequencies range from approx. 1400 Hz at 700 RPM to 8000 Hz at 4000 RPM so if I count pulses for half a second I get and actual RPM number but that takes a 16 bit counter. I started using timer0, incrementing the high byte in an interrupt from Nigel’s tutorials but it fluctuated by up to 15 on every reading. Timer1 working in the background, without interruptions was perfectly steady and all I had to is calibrate the read time with delays. If timer1 will keep counting during the PWM interrupts, it will be perfect and I will just recalibrate.
To switch inputs, I used a 74LS03 open collector NAND gate controlled by RB7 as a data selector as per diagram below. With open collector, I could tie two outputs together and either gate can pull the output low without fighting each other. I will build another one using a 74LS157 data selector from that batch of chips Philba gave me.
Will timer1 keep counting during the PWM interrupts ?

Attached Images

Dual tach input.jpg (30.6 KB, 4 views)

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I was only analog until last year. Maybe that's showing my age. I used the LM1496, which gives a signal proportional to phase (product of the two input voltages). There are possibly many other chips for the same purpose. I have also used the XR-2211 and CD4046.

That raises an interesting issue. Knowing the props are out of sync/phase is usually not a problem when you have only two props. Keeping them in sync, particularly in rough weather is another issue. I assume you just want the indication, not the control. And by indication, I mean something that would show which prop was turning faster or slower. Right?

As for the code example I gave, in that project, I was faced with a problem of having variable delays that could affect the servo pulse width and position. I considered interrupts, but being new to digital, I wanted to avoid them. I solved the problem by carefully flow charting the logic and adding NOP instructions to make each loop the same regardless of path taken. That may be caveman, but it worked. John

 

Just the same RPM is good for a boat. Keeps the vibration down and sounds better. My problem with phase was that I could see detecting out of phase at the same frequency but not if they were 100 RPM different. It just wouldn’t click in my head.
LOL, I usually try the caveman approach first, then when all else fails, I try reading the instructions.

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If this project evolves, I might try making a synchronizer. Until recently the only one was Glendinning which used a tach cable to physically drive screws inside and pull the throttle cable on the slave engine. That and his shore cord reels are really neat contraptions. He has a wild imagination. Yanmar and some others don’t have a tach cable drive so they couldn’t use it. Sturdy makes one now that takes frequency input but they are $$$$$$$.$$. They also make DDEC. One thing holding me back from that is that if it ever failed, the liability could be terrible.

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The 16F628A has three timers, you could use Timers 0 & 1 as external counters and it's real easy if you add an external gate to start / stop the counters (as TMR0 will continue to run once started as it's not gated on the 16F628A) Plus you've got the very programmable Timer2 available for a jiffy clock.

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You can stop Timer 0 from counting by changing the T0CKI pin from an input to an output.

 

As long as you have a resistor or a capacitor on the pin I guess that would work. Neat.

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If the PWM carrier is slowed will that just effect the position refresh rate? Or will it also tell the servo to move slowly to the new position? If not, I guess I will just have to move it in very small increments to get it to move slower.

 

From what I know operationally, the PWM frame rate just affects refresh rate. The interfaces that allow slower operation for more realism, such as landing gear doors, do it by ramping the pulse width over several frames. John