Post by thread:[PICLIST] How many CMOS TTL chips can a PIC 84A dr

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? Is there a table somewhere that shows the current requirements for driving each chip family? TIA Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

[Sorry for the repost. Haven't sent a message to the list in years and didn't realize I needed a topic tag] 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? Is there a table somewhere that shows the current requirements for driving each chip family? TIA Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics