interfacing avr with 74hc14n

[xinz]

74hc14n

I try to connecting 2 board.

the 1st board contain AVR, it's output is ttl signal clock.

the 2nd board, is the receiver of ttl signal, it use 74hc14n chip.
at the input stage of 74hc14n, i put 10k pull up, followed by 10k resistor to input pin of 74hc14n.

it don't work.

is it because I use different power source & ground on each board?

 

[matbob]

Hi xinz,
If you have two different power supplies for the two boards then you should tie the grounds of the two supplies together. Hope this would solve your problem.

 

[banjo]

As matbob said, the grounds of the two boards must be connected. The minimum cable between the two boards is at least two connections, clock signal and ground.
The other issue are the 10K resistors. I would definitely remove the 10K series resistor between to the input pin. The 10K pullup is also probably not needed if the AVR pin is configured to be a standard output driver. Assuming the AVR is running from at least 3V, then it should be able to drive the 74HC14 directly.
Double check your pinouts and connections.

 

[xinz]

Thanks matbob & banjo.

now the circuit work as I expected.

like your suggestion banjo, I remove the pull up and limiter resistor.
here's my other question:
If the signal clock come from opto-isolated parallel port, is your suggestion for those resistor still stand? or I should keep them on circuit. since I heard there are some laptop which only produce ttl signal up to 3.3v at logic high.

 

[neelandan]

If the output is directly from the collector of the transistor half of the optoisolator, the pull up is needed.

The rule for interfacing two pieces of equipment in the ttl days used to be that all outputs are open collector (ie, no pullup) and that all inputs must provide their own pull up.

Since the 74xx14 has schmitt trigger inputs, almost anything will work. You should still check the signal levels, because a change in the surroundings (lab table to lecture hall table) might cause it to stop working.