# Re: How to use PWM?

```Glad I could help

On Thu, Jan 21, 2016 at 12:42 PM, Jake <k1llfre...@hotmail.co.uk> wrote:```
```
> 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 :)
>
> On 19/01/16 19:18, John Duncan wrote:
>
> 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.
>
> On Tue, Jan 19, 2016 at 1:53 PM, Jake <k1llfre...@hotmail.co.uk> wrote:
>
>> 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:
>>
>> 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>
>> 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.
>>>
>>
>>
>>
>> --
>> John Duncan
>>
>>
>>
>
>
> --
> John Duncan
>
>
>

--
John Duncan
```