frequency generator output causing problem

[PRAVEEN K]

Hi,

I designed a simple 3 channel frequency measuring unit with PIT(82C54) and a microcontroller. It measures the frequency from 3 channels simultaneoulsy and display it in an LCD. I am using the read-back function of PIT to indicate the null count. And I have given pull down resistors(10k) at each input(CLK0,CLK1,CLK2) of PIT. So if no input is connected the unit will display null count for the corresponding channel.

The problem is that if connected frequency generator at one channel and let the other 2 channels open, the frequency in the 1st channel is exactly reflected in the next two. But if input is shorted to ground it shows null count.

And to my surprise it happens only with function generator. I done the same thing with a constant crystal oscillator, no reflection was shown in the open channels/input terminals. What may be the problem?

Thanks
Praveen

 

[FvM]

- 10 K isn't actually a low impedance pulldown, but should be sufficient normally
- You possibly managed to drive the timer chip with a voltage outside the allowed voltage range. In this case, you can achieve anything, including permanent damage of the device, and surely crosstalk to an unconnected channel.

 

[PRAVEEN K]

The output voltage of the frequency generator is kept 5V, square wave mode. The crosstalk happens only with function generator output and not with crystal oscillator output which is of the same voltage level.

I tried with 20K pulldown resistors, but there is no change.

Regards
Praveen