🚀 go-pugleaf

In article <1992Nov17.180205.19383@dartvax.dartmouth.edu> bruce@coos.dartmouth.edu (Bruce K. Johnson) writes:
>Hello all.
>
>I have a question about making a clock for my circuit.  First, let me give
>you a background --- I'm making an A to D data aquisition box for the MAC's
>serial port.  I'm using an ADC0831 chip and want to time it as well as provide
>an external clock signal for the MAC's handshake line (the MAC can only run its
>serial interface at a top speed of 57900 BAUD internally, therfore, if I want
>to run say a 22 KHZ sample rate, I need a much faster ext clock...)

	The upper limit on the Mac serial port is 239 kbaud, NOT 57.9k.
You are perhaps speaking of the upper limit of some particular
piece of software?
	In fact, with external clocking the Mac can take up to 4 Mbaud
in synchronous-input mode.  Depending on the model, it might not
always keep up with the input data at that rate, of course.

>Anyway, I originally started off with an oscillator and a decade counter to
>get the two clock signals ...

>This costs too much.  My project is for use by students in a Physics class
>and must be relatively inexpensive for them to purchase.  I then turned to
>using a 556 timer to get two clock signals with one chip (a .70 cents chip
>compared to a 1.70 osc + other stuff).  The 556 is capable of providing
>a stable signal for the 22KHZ clock but not for the 220>KHZ clock.
>
>Is there another cheap alternative to making two clock signals or can I make
>the 556 (two 555 on one chip) produce a more stable signal at these high speeds?

	A variety of '555-equivalents are available.  What you
want is an ICM7556 or similar CMOS timer chip, if the straight
NE556 is too slow for your preferred clock rate.

	In fact, you can make astable or gated clock generators from 
CMOS Schmitt triggers (74HC14 or 74HC132), for even less money.

	John Whitmore