RA4 of PIC 16F872 TTL ? or ST?

[scdoro]

Hi,
I am doing a frequency counter circuit wif PIC and need to input a signal to the RA4 of my PIC 16F872.
I have set RA4/TOCK1 as my external clock input for timer0
1) As stated in the data sheet, it say RA4/TOCK1 has a ST schmitt trigger input , may i know what does that time ??
2) Must the signal be pumped in be ST or TTL ?? ( meaning 0 to 5 V , no negative voltage or ? ??? )
3) If the input muz be a ttl, and my funtion generator don't have that function, what shld i do ?
4) If is ST input, what shld i do to input my signal ( -5 to 5 V) to the pin. direct connection can ? ? Is there anything i need to take note ?

Thanks a lot

 

[gidimiz]

Hi,

I will try to answer all your questions:

As stated in the data sheet, it say RA4/TOCK1 has a ST schmitt trigger input , may i know what does that time ??

Well when you have a ST input, it means that a logic 1 = 0.8 * Vdd, where Vdd is the voltage to your pic. So let say you have 5V to your pic, then for the input to see logic 1, you have to put 0.8 * 5 = 4V. For logic 0 you will have to put 0.2* Vdd or 0.2 * 5V = 1V. See section 17.2 in the data sheet for more information.

Must the signal be pumped in be ST or TTL ?? ( meaning 0 to 5 V , no negative voltage or

If you use the timer, then you can only use the ST ( internal configured and you cant change that ).

If the input muz be a ttl, and my funtion generator don't have that function, what shld i do ?

As long as your generator wil output this voltages as stated in the asnwer for question 1, you will be fine.

If is ST input, what shld i do to input my signal ( -5 to 5 V) to the pin. direct connection can ? ? Is there anything i need to take note ?

My previus answers coverd that i think.

You can also read application note at microchip AN545, will give you some more info on how to count the frequancy.

Good luck.