🚀 go-pugleaf

>In article <1993Jan21.183404.1534@mnemosyne.cs.du.edu> rduta@nyx.cs.du.edu
>(Radu) writes:
>>Can someone please tell me some more info about crystals.  I need to make
>>a electronic switch to select between two crystlas (2MHz, and 8MHz -about),I
>>tried using a 4016 (IC switch), and it didn't work.  Can someople please
>>give me another solution.  I'm desperate.

A quick hack to do it is to use the following Oscillator/Multiplexor
circuit that I used to switch clock frequency on my pocket computer back
in high school days...

Use 74HC00 as                      +------------------------------+  __
the 4 nand               4.7M      |                  4.7M        +-|  \o
gates.            +----/\/\/\/\----+           +----/\/\/\/\----+---|__/|
                  |        _       |           |       _        |       |
Xtal1             +------|| ||-----+           +-----|| ||------+       |
Xtal2             |        -  Xtal1|           |       -   Xtal2|   Clock
are the 2         |       __       |           |       __       |    out
crystals          +------|  \o-----+           +------|  \o-----+
2, 8 MHz          |   +--|__/      |           |   +--|__/      |
            20pF ===  |  nand     ===  20pF   ===  |           ===
                  |   |  gate      |           |   |            |
             __   V   |            V Gnd       V   |            V
Select ---+-|  \o-----+                            |
          +-|__/                                   |
          +----------------------------------------+

(Some of you might want to add a series resistor to limit the current
through the crystal and change the capacitor or whatever.) The circuit
has the lowest components count because it combines the oscillator with
the multiplexor circuit and it does work nicely and it might take
a little bit of time for the oscillator circuit to start after you pull
the select line high/low to select the clock source.  If you don't want
that delay, you might want to build 2 oscillator circuits and feed the
outputs to a 2-1 mulitplexor.  These days I would use a GAL16V8 to
implement the whole thing.  The CMOS inputs works nicely for a simple
oscillator.

In article <C1Cx1r.MIv@ccu.umanitoba.ca>, shad04@ccu.umanitoba.ca (Dan Fandrich)
says:
>
>The 4016 can have a fairly high internal resistance (2k).  Why not use the

More likely the high capacitance is going to cause trouble.

>8 MHz crystal and selectively divide the output by 4 or use a PLL?

Could use the 8MHz and divide by 4 to get 2 MHz.  Less components than
a PLL.  (Since your PLL needs a /4 circuit and a PLL chip)

>>>> Dan

K. C. Lee
Elec. Eng. Grad. Student