Hydra PWM issue

Archive
1

On advice from Gus I’ve started a new thread since the my previous post was on an old thread. My problem is that I don’t get a meaningful PWM signal on my Hydra with a time period much more than 2ms. Here’s the simple code:

 using Microsoft.SPOT.Presentation.Media;
using Microsoft.SPOT.Touch;

using Gadgeteer.Networking;
using GT = Gadgeteer;
using GTM = Gadgeteer.Modules;
using Gadgeteer.Modules.GHIElectronics;
using Gadgeteer.Interfaces;

namespace GadgeteerApp4
{
    public partial class Program
    { 
        void ProgramStarted()
        {

            GT.Socket socket7 = GT.Socket.GetSocket(7, true, null, null);
            PWMOutput myPWM = new PWMOutput(socket7, GT.Socket.Pin.Seven, false, null);
            myPWM.Active = true;

            uint l = 2000000; // this works
            //   l = 3000000  does not work

            uint h = 1000000;
            myPWM.SetPulse( l, h);
            
        }

    }
}

Exactly 2ms is fine, I get a nice square wave on my scope. 3ms is not, all I get is a signal pegged high. When I try 20ms ( what I need for a servo) I get signal which is about 1.7ms in length with the high length of 1ms ! Anyone have an idea what’s going on?

Here’s the firmware/SDK
TinyBooter Version: 4.2.3.1
Firmware Version: 4.2.3.1
SDK Version v0.0.8 , October 24, 2012

Thanks!

2

Sounds like a prescaler overflow, which would mean that 3ms is too slow for the hardware PWM.

3

WouterH, thanks for the reply. Doing a quick search it looks like someone has got a Spider to control a servo from its PWM
.Net Gadgeteer; Servo Connection | GOTO Loop

Does anyone know if this means that a Hydra can’t control a servo from a PWM pin but, a Spider can. Or is there’s something wrong with my Hydra or my setup? Just want to be sure before I go down the path of buying an I2C servo conroller or I suppose I could do PWM from a GPIO pin.

4

I do not see why not but we will test this Monday

5

Gus, did you get a chance to test this?

Thanks

6

As PWM stands on Hydra, the current smallest frequency that can be achieved is 381.4755474 Hz which translates to 2.6214 ms. This is also based on a 25 MHz frequency. It may be possible to change the frequency range which we will try to implement in a future release. You may be able to use the register class to directly manipulate the PWM control registers to get the desired frequency or perhaps control the PWM from RLPLite where again you can control the PWM registers.

An alternative for Hydra right now to control a servo is our Pulse InOut Module, where you can control up to 8 servos on one module; plus, because it can be daisy-chained (via DaisyLink), it can control even more than 8. Here is the link to the module: http://www.ghielectronics.com/catalog/product/392

7

Thanks for confirming. I will look into the Pulse InOut module or an I2C controller.

8

Poor guy paid $80 for a Hydra and now he needs to pay another $20 to get it to drive a servo? That’s just sad.

9

That was only a suggestion but we will see how we can tweak the drivers for servos.

Keep in mind that pulse in out module will provide a lot more than a single servo.

And welcome to the community :slight_smile:

10

@ Gus - Do you have any plans to open source the Pulse In/Out module firmware?

11

Yes