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Frequency conversion?


Moderators: Ajay Bhargav, Arun Kumar V, pdi33, Shailesh NAYAK, ۞ TPS ۞, shyam, sashijoseph, ExperimenterUK, DavesGarage, majoka

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Sat Aug 09 2008, 01:14PM

Registered Member #8175

Joined: Sat May 31 2008, 07:27PM
Location (Home Town): Denmark
Posts: 52

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, 01:17PM ]

Tags: frequency conversion, variable frequency, revolutions, timer

pdi33

Sat Aug 09 2008, 01:33PM

Registered Member #1329

Joined: Mon Jun 04 2007, 12:28PM
Location (Home Town): mumbai/sirsi
Posts: 857

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, 01:38PM ]

Tags: voltage divider, two timers, first timer, time interval, input pin

Sat Aug 09 2008, 02:03PM

Registered Member #8175

Joined: Sat May 31 2008, 07:27PM
Location (Home Town): Denmark
Posts: 52

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?

Tags: resistor, diode

pdi33

Sat Aug 09 2008, 03:16PM

Registered Member #1329

Joined: Mon Jun 04 2007, 12:28PM
Location (Home Town): mumbai/sirsi
Posts: 857

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.

Tags: simple voltage divider, signal voltage, max voltage, zener, volts

Arun Kumar V

Sat Aug 09 2008, 11:31PM

Registered Member #426

Joined: Mon Jan 29 2007, 02:50AM
Location (Home Town): Hyderabad-India
Posts: 761

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

Tags: npn transistor, sampling period, bc547, micro circuit, suitable value

sashijoseph

Sun Aug 10 2008, 07:24AM

Registered Member #5870

Joined: Mon Feb 04 2008, 09:26PM
Location (Home Town): Jamshedpur
Posts: 767

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,...?

pdi33 like this.

Tags: magnet coil, inductive pickup, hall effect sensor, schmitt trigger, input signal

Arun Kumar V

Sun Aug 10 2008, 10:04AM

Registered Member #426

Joined: Mon Jan 29 2007, 02:50AM
Location (Home Town): Hyderabad-India
Posts: 761

Hi TB,

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

Arun

Tags: tachometer, rpm, tb

pdi33

Sun Aug 10 2008, 01:53PM

Registered Member #1329

Joined: Mon Jun 04 2007, 12:28PM
Location (Home Town): mumbai/sirsi
Posts: 857

@ 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.

Tags: schmitt trigger, bhai, critical point, rpm, measurement

rug.burn

Sat Jan 17 2009, 12:24AM

Registered Member #14112

Joined: Fri Jan 16 2009, 11:07PM
Location (Home Town): Brea, CA
Posts: 1

I'm working on a tachometer- and what occurs to me is that there are two ways to do a tach with a high speed mcu:
1. Count the pulses from the engine (I assume an ignition engine since this is what my tach is for...) over some known time period. Then you'd multiply this number by some constant to get the rpm (or Hz)
2. Use the engine ignition pulses as defining the time period, then count the 5mHz (F_osc = 20mHz)counter pulses in this time period. Divide F_osc by this number, multiply by 60 to get rpm. It may seem like a lot of math, but using integer and binary division by just shifting things right to divide, left to multiply, I think it's do-able... The good thing is the much faster response time- you don't have to wait a second or so to get a result , and it's a hell of a lot more accurate, even with all the integer math... What do you guys think?

The second thing I was wondering is the best way, with a pic to get the enging pulses in. I'm planning on using an inductive input, a wire wrapped around the spark plug wire, the other end to engine ground, thru some small coax, and this will be fed into the PICs ECCP input, after conditioning by some means...

For this conditioning, I was thinking about first checking the signal with a scope, to find its peak, then put the signal thru some resistor, and maybe a small capacitor to keep the spike within the limits able to be handled by the PIC. Of couirse, a little hysterisis network would also make the PIC read the signel more reliably. Any thoughts as to how this wopuld work?


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