PWM the most efficient way

Hi, I'm in need of help.. lets see if I'll manage to explain the
problem..

I'm planning to build a mechanical arm that will be moved by 4 DC motors
(6-50W, depends..) and 1 stepper, whole thing will be controlled by a
microcontroller (AVR). The DC motors will be controlled by separate
H-bridges together with somekind of PWM control.

Now, I've seen alot of pages using either a microcontroller to generate
the PWM sequence or a separate circuit where the shape of signal is
regulated with a potentiometer, like one circuit here:
http://www.nomad.ee/micros/pwm555.html

None of these solutions seems to suit.. First would take up much of the
uC's time and one would not be able to move several motors at the same
time. While, the second solution seems to be not suitable for an uC,
because, there, I would need to generate a variable voltage to regulate
the PWM output.. kinda tedious (would require DA converter..)..

I'm wondering if it's possible to make such a PWM circuit, in which I
input a digital value (say a byte through one pin or through several
pins) which is remembered and executed?

Say,
I input 0001 and PWM circuit gives smallest pulses
I input 0010 and PWM circuit increases the pulse width
and so on, where 1111 would be the maximal pulse width..

Or, maby theres a nicer solution that includes automatic ramping of the
puls width?

Also, would large DC motors be operated without problems from common
4-transistor or 4-MOSFET H-bridges, as long as I use high enough values
of the components? I've only built H-bridges for <3W motors.

Thanks in advance..


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