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From: Bob Monsen on 8 Feb 2010 00:54


"Phil Hobbs" <pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
news:4B6BA5B4.2000806(a)electrooptical.net...
> Okay, so I'm trying to resurrect my long-deceased PIC skills. (I do a lot
> of C++ programming, off and on, but my last significant firmware was about
> 2000--the Footprints sensor, running on a PIC17C456.)
>
> From last time, I remember that MCU programming is all about getting the
> configuration bits right--once that's done, it's pretty simple. Sooo, I'm
> trying something fairly simple to start with, namely controlling an RC
> airplane servo.
>
> The servo needs 0.8-2.4 ms pulses, at a rep rate between 50 and 100 Hz.
> I'm using a PIC 18F46k20 demo board with a nice little OLED display,
> running at a clock frequency of 64 MHz (16 MHz instruction clock). The
> PWM is done with Timer 2 and capture/PWM module ECCP 1.
>
> I started with demo program, which ran fine. Next I ported that to
> Hi-Tech C, which I remember very fondly from back when--it was pretty
> nearly bug-free, in stark contrast to Microchip's C17, which stank on ice.
> So far, so good.
>
> I got the PWM working OK, but with a 64 MHz clock, the slowest rate the
> PWM can go is 4 kHz (10-bit resolution and 16x prescaler), which is too
> quick for the servos. Sooo, I used the TMR2 interrupt handler to
> implement a divide-by-64 counter. For N+M/1024 cycles, it turns ON the
> RD5 output when the down-count reaches N, and then switches to the PWM for
> cycle 0 only. That way, I could have N complete cycles and M/1024 partial
> cycles, yielding 15 bits resolution at a 61 Hz rep rate. Plenty good for
> an RC servo.
>
> However....
>
> The 15 bits are made up of the number of full cycles (5 bits), bits 4 and
> 5 of the CCP1CON register, plus an 8-bit value in CCP1L. According to the
> datasheet, any time I'm using x256 division in TMR2, I should be getting
> 10 bits worth of resolution, but it's acting exactly as though I
> were getting only 8--changing the programmed delay produces 4 times more
> PWM duty cycle change than it should, and the two CCP1CON bits don't seem
> to do anything, though I can read and write them correctly.
>

The PR register and TMR2 are in cycles, whereas the CCP1CON and CCP1L are in
oscillations (ie, 4x cycles). Sigh. Read the datasheet carefully. They have
additional hidden bits for the oscillations that are set by the two bits in
the CCP1CON.

> The setup values are:
> T2CKPS0 = 0;
> T2CKPS1 = 1; //prescale by 16
> PR2 = 0xFF; // divide by 256
> DC1B1:DC1B0:CCPR1L = many values
>
> This is almost certainly pilot error, but the reason doesn't exactly leap
> out of the 450-page datasheet.
>
> C'mon, all you PIC guys--what gives?
>
> Thanks
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot net
> http://electrooptical.net

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