There is family of asynchronous D/A converter algorithm's
that are useful when peak execution cycles are at a premium
and the average is still less than the maximum available
execution cycles.

The random reference algorithm described a few days ago
is the only one I know that is both asynchronous and doesn't
need an accurate time reference.

There are a couple different ways that delta mod can be
used. A few days ago Nikolai Golovchenko clearly described
a technique that can be extended to be both asynchronous and
have extended precision. Nikolai's algothrim makes a
simple assumption that each sample error is integrated
over a normalized signal unit of time.

If the error is integrated accounting for time then
each output sample can both be asynchronous and the
errors can be compensated.

This idea can be extended and used in two ways.

First the simplest D/A converter is a single bit
(loosely delta mod) This reduces extended precision
to error management. More precision at increased
processing requirements.

Secondly, to have a stable D/A most of the delta
MOD D/A's make an assumption the output is periodic. By
including time in the error calculations we can eliminate
the need for tight periodic outputs. There is usually a
simple way that this can be done. When we are integrating
errors with normalized unit time we are actually multiplying
by 1. If we have a one bit D/A then the value that we are
integrating is 1 by the amount of elapsed time. The elapsed
time can be had by sampling a free running timer and
calculating the delta time with a subtract.

The net result is an asynronous software D/A using
very simple code.

Walter Banks

Nikolai Golovchenko wrote:
> An 8 bit DAC working at higher frequency. At each sample,
> calculate the error between 12 bit input and 8 bit output
> (output is taken as 8 higher bits of input). Error should
> be calculated in relative units, 1=full scale. (It is
> simply the lower bits of input). At the next sample, add
> this error to the input. This will compensate for reduced
> resolution in the long run. Works as some kind of PWM,
> but doesn't need high oversampling, since you need to
> add only 4 bits of resolution.

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