Lpc2000

Hi,

I presume you're talking about using some form of external DAC as 
part of your suggestion? If not, can you explain the statement "Use 
this to latch the input to the DAC".

A significant problem with the on-board DAC is that there is no latch 
signal that can be applied, making some degree of jitter inevitable 
on any software-driven setup.

We had to abandon the idea of using the on-board DAC for our software 
modem for this very reason: even a few clocks of jitter at 60 Mhz is 
enough to generate significant noise on the output.

I suspect anyone else (the original questioner on this topic?) trying 
to drive the DAC at high speeds will hit this problem before they hit 
the maximum o/p speed problem (unless having a noisy o/p isn't a 
concern). This is unfortunate, as it means that the DAC can't really 
be used for even relatively simple signal generation without a noisy 
o/p.

A very simple addition to the DAC, namely a latch signal, would be a 
great help here. 

Of course, a DMA-fed FIFO arrangement would be even better.....

Brendan

P.S. as an aside, we did come up with a jitter-free software-fed DAC, 
but it consumed significant MIPS, and just wasn't worth the effort.

--- In lpc2000@yahoogroups.com, David Hawkins <dwh@o...> wrote:
>
> 
> >>Does anyone know what is the maximum speed that I could send
> >>data to the D/A at fixed frequency?
> 
> You can eliminate the software jitter by introducing a set of
> latches.
> 
> Use the hardware timer or PWM hardware to generate a fixed
> frequency square-wave (or edge at least) on an output pin.
> Use this to latch the input to the DAC.
> 
> In your software, triggered from the same timer, write the
> next valid sample to the input of the latch, eg. on the
> output pins of the micro, or to another latch that
> preceeds the DAC latch.
> 
> The software jitter then only affects the data written
> to the first latch, data written to the second latch
> (or DAC internal latch) is done by the timer I/O pin,
> so is unaffected by the software jitter.
> 
> If you use software to fill a FIFO, then the DAC can be
> driven by the FIFO draining, and software can fill it
> up on an 'almost empty' interrupt. You'll get higher
> performance from the DAC with this scheme.
> 
> Cheers
> Dave
>