# Re: How to use PWM?

Thanks John, I had to wiggle the % around. I think I ended up with 10 and 3, but it spins the same speed both ways. Thanks :)
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On 19/01/16 19:18, John Duncan wrote:
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You have a cycle that is 21.7ms (t) in length, with 1.7ms on and 20ms off. The frequency is 1/t or 46Hz. The duty cycle is the amount of time on divided by the length of the cycle, 1.7/21.7 or 8%. I'm rounding to the nearest integers.

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On Tue, Jan 19, 2016 at 1:53 PM, Jake <k1llfre...@hotmail.co.uk <mailto:k1llfre...@hotmail.co.uk>> wrote:
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I can't test those numbers as I don't have a power supply at home.
I'm curious how your work out hte duty cycle as a percentage?
Thanks.

On 19/01/16 18:32, John Duncan wrote:
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```    Just a guess, you probably need to consider the whole 21.7ms or
21.3ms to be your period (46Hz or 47Hz) and then your duty cycle
will be 8% or 6% respectively.

On Tue, Jan 19, 2016 at 10:32 AM, J B <k1llfre...@hotmail.co.uk
<mailto:k1llfre...@hotmail.co.uk>> wrote:

I'm trying to use the PWM module to control a parallax servo
I purchased. I'm trying to get it to spin both ways as I need
it for a project, I have tried to spin the servo both
clockwise and anti clockwise but I do not know how to set a
20ms gap between each pulse, I need 1.7ms to go anti
clockwise with a 20ms gap and 1.3ms with a 20ms gap to spin
clockwise. I've converted the ms into frequency but it
doesn't change the direction. When I plug the data cable into
any pin on the entire board the servo spins clockwise, even
when plugged into PWM0 and (pwm-stop 0) being called.

On another point is there a dedicated ground pin? I plugged
the PWM0 port into an oscilloscope and it was littered with
static, according to my electrical engineering tutor there is
a bad ground and I need to earth it.

Another another point, my LCD display still doesn't work and
I don't know what to do about it.

The attachment is a datasheet for the servo I bought.
Thanks.

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-- John Duncan
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--
John Duncan
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