Newbie question about 500khz & 555

[Joe Plane]

Rich Grise <spamdump@aol.com> wrote in message news:<jr4lb.5525$S52.4518@newsread4.news.pas.earthlink.net>...

Why do you need a whole nother oscillator? What do you have clocking
the 89C51? Just divide that down, or for that matter, assign an output
pin and bit-bang it.

Good Luck!
Rich

Ahh forgive the newbie jargon 'bring me up to speed' question. Do you
mean by bit banging assigning a pin on the 8051 to be the 'clock' and
then manually setting it to 0 and 1 at the appropriate frequency (or
just about).

Many thanks,

 

[Spehro Pefhany]

On 30 Oct 2003 20:58:24 -0800, the renowned jcpearce2005@yahoo.com
(Joe Plane) wrote:

The ADC808 will run at a wide range of clock frequencies. You could
consider driving a divider chain off the uP clock. If you are running
it at (say) 12MHz, you could use any 4-bit HCMOS counter to get a
750kHz clock. You drive it off the XTAL2 output with very short trace
length and reduce the load cap on that pin by 5pF or so (assuming you
are using the internal oscillator).

Best regards,
Spehro Pefhany

Uhh, a little too fast for my proverbial walking legs.

A 'divider chain?' Is there a place I can read up on this?

Just a counter. Eg. 74HC4040 or 74HC161. Look up the data sheets.

And when you say using a 4 bit HCMOS counter do you mean using one of
the internal uP counters and then oscillating one of it's IO pins at a
cycle to generate 750Khz?

No, an external counter chip.

More generally is it a typical practice to use a uP pin as a clock for
another device and 'manually' set and unset this pin in the software?

Really hardware dependent. Some micros have peripherals on board that
can support that sort of thing, even at that relatively high clock
speed, but it consumes those resources and they can't be used for
anything else. You'd have to look carefully at the micro data sheet
and see if you can do it.

My suggestion is more general and doesn't make any assumptions or use
any microcontroller resources, not even a single pin, but it requires
external hardware (one chip).

Best regards,
Spehro Pefhany
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