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Frequency conversion?


Moderators: Ajay, Junied , abbas1707, Arun Kumar V, pdi33, Shailesh NAYAK, ۞ TPS ۞, shyam, sashijoseph

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Sat Aug 09 2008, 10:14AM

Registered Member #8175

Joined: Sat May 31 2008, 04:27PM

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I'm using a DS89C450

I need to monitor a +7V input with variable frequency - is it possible to sample the frequency on a 8051 without additional hardware?

I need this to log revolutions of an engine. Output is as said +7V, when the engine is idling output is 26Hz (approx. 750rpm). So basically I need to translate the frequency of the input to a given value. Is it possible?

EDIT: Can I use the external interrrupt and a counter to solve this? eg a timer that resets a counter every one second, and then the external interrupt incrementing the counter? So the value of the counter equals the frequency when reset?

[ Edited Sat Aug 09 2008, 10:17AM ]

frequency conversion variable frequency revolutions timer

pdi33

Sat Aug 09 2008, 10:33AM

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hi TB,
u would need only two timers for the task. use the first timer as a counter and connect the input to the corresponding counter input pin( e.g. if u are using timer0 as the counter , connect the input to the T0 pin of the uC. The second timer can be used to create the fixed time interval and just read the counter value when the timer overflows (i,e when the second timer interrupt occurs.
As for the +7V, better check the datasheet of the uC whether this level ( which is greater than the usual +5V level) can be safely used as the input directly to the uC . Else, u can always use a voltage divider/5.1V zener to reduce the level to required +5V level.

P.S. using the internal counter doesnot interrupt ur program flow so it is recommended. Ur idea of using external iterrupt also holds good but would affect other tasks of ur uC , as ur frequency of input increases generating an interrupt on every input transition which might reduce the efficiency/timing of ur other coding.

[ Edited Sat Aug 09 2008, 10:38AM ]

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voltage divider two timers first timer time interval input pin

Sat Aug 09 2008, 11:03AM

Registered Member #8175

Joined: Sat May 31 2008, 04:27PM

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Ahh good point on the external interrupt - just didn't think that thought to an end

There is absolutely no need to run an interrupt handler every single time.

I'l look at that diode - I guess it's better than using a resistor?

resistor diode

pdi33

Sat Aug 09 2008, 12:16PM

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definitely TB, a zener is always preferred to a simple voltage divider because it would clamp the input to a max. voltage to 5V even if ur signal voltage accidentally rises a few volts above above 7V during measurement.

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simple voltage divider signal voltage max voltage zener volts

Arun Kumar V

Sat Aug 09 2008, 08:31PM

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Hi TB,

As our friend pdi33 has put it rightly, you are basically building a freq.counter/meter.

i would suggest to go with 1 sec sampling period, which gives accuracy of 1 Hz.

also what is the max freq./rpm of the engine ?

you can also use a normal NPN transistor like BC547 for connecting the input voltage,
feed the +7V to the base of transitor via a suitable value resistor (4k7/10k) and the collector is connected to the micro's pin( with a pullup) and emitter to Gnd

one more thing ( i think you already know this), you have to common the Gnd of the Micro circuit with the Gnd of engine circuit

all the best !

Arun

npn transistor sampling period bc547 micro circuit suitable value

sashijoseph

Sun Aug 10 2008, 04:24AM

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If it's an inductive pickup being used to derive the rpm pulses,a Schmitt trigger before the micro would be helpful,else there could be lots of false triggers for the counter.

Tb,what are you using for the input signal .. magnet-coil,Hall effect sensor,...?

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magnet coil inductive pickup hall effect sensor schmitt trigger input signal

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pdi33

Arun Kumar V

Sun Aug 10 2008, 07:04AM

Registered Member #426

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Hi TB,

are a attempting a tachometer, to know the RPM of engine ?

Arun

tachometer rpm tb

pdi33

Sun Aug 10 2008, 10:53AM

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@ TB,
as shashi bhai so rightly pointed out ( a very critical point in RPM measurement which i forgot to mention),a schmitt trigger would be highly useful especially when ur signal is not purely digital.

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schmitt trigger bhai critical point rpm measurement