Thanks again for everyone help.  I think I got many of
you confused.  I am sorry about that.  What I meant
was I want to drive two L298 ICs, so that's 4
full-bridges in total.

Mike Young wrote: =93I missed the preceding discussion,
so don't know what was suggested. You can drive each
half using just one PWM per bridge =85=94

James: Yes, but I am using TWO L298 ICs and each IC
has two full-bridges.  So the total is =934 half=94, which
means I will need 4 PWM channels, right?

Julio Jumpa wrote: =93you set an interruption every
8us=85=94

James:  This would work, but the PIC will have to
spend a lot of time servicing the interrupts.  It
might not have enough time to update the LCD, read
encoder, do some math, serial comm, etc.  I am
thinking of a dedicated PIC just to drive the two L298
ICs=85

--- Mike Young <mikewhy@sbcglobal.net> wrote:

> From: "Gerhard Fiedler" <lists@connectionbrazil.com>
>=20
>=20
> > James James wrote:
> >
> >> Thank you everyone for your reply.  I just want
> to clarify that the L298
> >> is a DUAL Full-Bridge driver. Driving 2 of them
> means I need 4 PWM
> >> channels.
>=20
> I missed the preceding discussion, so don't know
> what was suggested. You can=20
> drive each half using just one PWM per bridge.
> Modulate the enable line with=20
> PWM, while holding steady levels on the two In pins.
> The In pins select=20
> direction, while Enable modulates speed. That's two
> digital output lines per=20
> bridge, plus one PWM line.
>=20
> I find it odd that the Fig. 6 truth table is set up
> that way. Enable would=20
> be more sensibly useful as a current sense limiter,
> except modulating the In=20
> lines would cause braking rather than free-wheel
> coasting during the off=20
> periods. Ick.
>=20
> >
> > I'm not sure, and please excuse me if I'm wrong,
> but I don't think you
> > understood the suggestions about how to reduce the
> number of PWM channels
> > required.
> >
> > The pairs of inputs for each channel are not
> driven independently. In
> > normal PWM operation, one is the inverse of the
> other. Look at figure 6 in
> >
>
http://www.st.com/stonline/products/literature/ds/1773.pdf,
> for example.=20
> > It
> > is easier to provide this type of encoding with
> hard-wired logic (or a
> > CPLD) than with the PIC's CCP modules. It would
> have one PWM input, an
> > enable and possibly a break input, and two outputs
> for the driver inputs,
> > and in between a few gates.
>=20
> I wouldn't bother putting gates in there for that.
> You still need a third=20
> state for braking, so it needs a second digital line
> for full control=20
> anyway.
>=20
> > To me, it seems that providing the required
> signals using the PIC's CCP
> > modules is not exactly easy: they are designed to
> set their outputs all
> > high at the same time. (The, or maybe some, ECCP
> modules have a polarity
> > selection, but not the CCP modules.)
>=20
> ECCP doesn't add much for Fig. 6 control.
> Full-bridge mode can drive the=20
> L298 In lines for direction control, but you would
> need an OR or XOR gate to=20
> modulate the B-D lines on the Enable pin. The only
> real gain is using the=20
> auto-shutdown comparators as a current limiter.
> Alas, I don't believe any of=20
> any mid-range or 18F come with dual ECCPs, at most
> one each of the enhanced=20
> and regular.
>=20
> To conclude, I think all you need is two
> independent, plain, ordinary CCPs.=20
> Each PWM line modulates the Enable line on one
> channel for speed control.=20
> You'll control the In lines directly for direction
> and braking.
>=20
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20



=09
	=09
__________________________________=20
Yahoo! Mail - PC Magazine Editors' Choice 2005=20
http://mail.yahoo.com
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist