Ideal crystal frequencies

[Paul E. Schoen]

As a tangent from the post on low frequency crystals, there were several
frequencies that were recommended.

When I was designing 4 MHz Z80 CPU circuits, I used a 3.6864 MHz crystal,
which divided down nicely to standard baud rates of 115200 to 300, and also
could generate 100 Hz for 0.01 second resolution timers, and multiples of
50 or 60 Hz for power line frequency synthesis or sampling. The frequencies
are multiples of 2, 3, and 5.

For my newer 20 MHz PIC applications, I use 14.7456 MHz which is just 4
times higher and also can be used for precise timing of baud rates and such
as listed above.

Of course, other applications may have different needs, but I think these
frequencies are ideal for pretty much everything I need to do. I might also
consider 18.432 MHz which divides down to an exact 1.0 kHz for mSec timing.

All of these are standard values, and are available from Jameco for $0.31
each. At that price, it would not be worthwhile to take apart a TV, but
modem boards have crystals that are of similar multiples, and almost
everyone has some blown modems. The crystal in the one I found in my junk
pile is 22.1184 MHz.

Paul

 

[PeteS]

As a tangent from the post on low frequency crystals, there were several
frequencies that were recommended.

When I was designing 4 MHz Z80 CPU circuits, I used a 3.6864 MHz crystal,
which divided down nicely to standard baud rates of 115200 to 300, and also
could generate 100 Hz for 0.01 second resolution timers, and multiples of
50 or 60 Hz for power line frequency synthesis or sampling. The frequencies
are multiples of 2, 3, and 5.

For my newer 20 MHz PIC applications, I use 14.7456 MHz which is just 4
times higher and also can be used for precise timing of baud rates and such
as listed above.

Of course, other applications may have different needs, but I think these
frequencies are ideal for pretty much everything I need to do. I might also
consider 18.432 MHz which divides down to an exact 1.0 kHz for mSec timing.

All of these are standard values, and are available from Jameco for $0.31
each. At that price, it would not be worthwhile to take apart a TV, but
modem boards have crystals that are of similar multiples, and almost
everyone has some blown modems. The crystal in the one I found in my junk
pile is 22.1184 MHz.

Paul

Interestingly, the PXA series from Intel (ARM cores) specify a 3.6864MHz
crystal (for the very reason you cite, no doubt) which is then multipled
as necessary, and gives a memory frequency of 99.x MHz (not quite 100).

Quite amusing when doing timing analysis on the memory bus.

Cheers

PeteS