Bypass Cap when two IC pins are close on board

Go To Last Post
4 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I am building a machine that i will be using K type thermistors with Max6675 chips and attiny45's

The idea is making a board with both chips using a one wire communication scheme. I am using 1/8 inch stereo jacks that will provide power and communication.

I am only sending one byte from a PC program to a master control board, that board will communicate with the thermistors in a daisy chain. Basically it sends one byte to the first slave, that slave looks at the byte, if it is greater than 0 it subtracts 1 and sends the number to the next IC in the chain. The IC (tiny45) that gets the 0 then sends the 2 bytes it reads from the max6675 back up the chain to the master control board that communicates with the PC.

This allows one to put thermistors at any point on the machine with short thermistor wires and allows up to 255 thermistors to be read by one control board.

The question, I attached a board drawing, the yellow line is the two positive pins, tiny45 and the max6675 and two holes for a .01 cap. Initially I forgot about the caps so it is a tight fit. I am also using a 220 uf cap on the board. the lower right holes are for pullup resistors and an led to show communication status.

Should i redesign the board to fit a another cap closer to the avr and how would you test if one cap is sufficient when two pins are so close when both IC's should have a bypass cap?

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I think one 0.1u Cap will be good enough.
The GND line seems broad and the VCC connections
relatively short.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I agree with Ossi however, you could add a capacitor "ugly style" on the bottom of the board with the shortest possible leads.

Quote:
It is better to be there and not needed than needed and not there!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I do have some caps with very long leads and i have plenty of room on the top as the thermocouple socket is vertical. I will add 2 holes and put a cap on top of the AVR across the VCC and GND pins.

Still have to mount the max but i am going to test the chip software comm first.

Attachment(s):