At 07:52 AM 8/6/01 -0700, you wrote:
>I have a relay mux that I am actuating using a PIC16F84A, a bunch of
>74HC595 serial to parallel shift registers and ULN2803 drivers.
>
>Given that I have the SCLK, RCLK, CLR lines all tied in parallel on the
>74HC595's, how many 74HC595's can a PIC drive?

As HC inputs have almost no leakage current at the inputs (<1uA over
temperature), the answer is a VERY large number (hundreds, thousands
perhaps depending on the temperature range) if speed is not a factor.

-HC inputs look like a capacitor
-You want them to switch HOW fast?
-your microcontroller will source/sink HOW much current?
-how long will it take for the inputs to charge at that current?
 (use dv = t*I/C for a first approximation)

Now, the HC595 data sheet tells us that the (non-ST) clock input
wants no more than 500nsec rise time at 4.5V. So.. making
some approximations and guesses..

Cmax = 0.02 * 0.5E-6/2.5 = 4,000pF

Input capacitance is 10pF, add 5pF per input for stray, and we
get 268 inputs. The leakage current of 268uA won't be a factor
either, so that's probably the limit unless I missed something.

Best regards,
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