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PWM frequency divider

T

Tom Bruhns

Jan 1, 1970
0
Need simple circuit to divide the frequency of a 90 Hz PWM signal by 4
while retaining the variable duty cycle.

Why? What do you want to do if the duty cycles go as, say, 10%, 20%,
30%, 40%, 10%, 20%, 30%, 40%, (repeat ad infinitum)? What sort of
resolution do you want?

One way: divide the leading edge (assumed to be evenly spaced, with
the trailing modulated) by 4. Integrate the "on" time for 4 input
cycles. "Deintegrate" that value during the next output cycle to
determine the "on" time for that cycle. Have two integrators that
swap back and forth between the two tasks of integrating and
"deintegrating". Easy enough to do in a 6-pin microprocessor, if the
instruction cycle time of the uP is short enough for your required
resolution. But there's a 4/90 second delay with this scheme.
 
T

Tony Williams

Jan 1, 1970
0
Need simple circuit to divide the frequency of a 90 Hz PWM signal
by 4 while retaining the variable duty cycle.

I've been thinking about something similar recently.

You need a 2x D-types for a divide by four, 2x counters,
2x R-S flipflops, some gating, and a clock (not less than
10Khz for about 1% resolution on the output 22.5KHz PWM).

<---T---><---T---><---T---><---T--->
__ __ __ __ _
90Hz | | | | | | | | |
_____|T1|_____|T2|_____|T3|_____|T4|_____| 90Hz
________ ________ | _
/2 | | | | | |
_____| |________| |__|_____| /2
_________________ _
/4 | | | |
____/|\ |___________|____/|\ /4
Set Set
__ __ __ __ | _
| | | | | | |
_____|T1 T2 T3 T4|_________________|_____| 22.5Hz

|_____________________________| _
| | |
_____| Counter1 gating signal |_____|

Gate Counter1 to accumulate 10KHz clock pulses
for the period T1+T2+T3+T4. At the end of T4,
transfer the total count to Counter2, and Zero
it The end of T4 is signalled by the 90Hz, /2,
and /4 waveforms all being down.

Use the Set output from /4 to set an R-S flipflop,
whose output is the 22.5Hz.

Also use the R-S output to Start the countdown of
Counter2, clocked by the 10KHz, (and to reStart the
Counter1 T1+T2+T3+T4 accumulation).

When Counter2 reaches Zero, Reset the o/p R-S flipflop.

The output duty cycle is then (T1+T2+T3+T4)/4*T.

The sequence can either be done in hardware or a uP.
 
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