logic level for PIC16F877A

[sheing3003]

What's the minimum voltage required for PIC16F877A to recognize a logic 1? Sorry, but I couldn't find it on the datasheet.
Thanks in advance

 

[hjames]

It's on page 178 of the data sheet under DC characteristics: look for Vih minimum (Voltage input high minimum). Threshold depends on the pin, and what mode it is in.

 

[sheing3003]

thanks! i got it~~ do you know what's the maximum clock for it? I'm thinking to use 40Mhz will it be possible?

 

[Nigel Goodwin]

thanks! i got it~~ do you know what's the maximum clock for it? I'm thinking to use 40Mhz will it be possible?

The maximum advised is 20MHz, as I'm sure you know, you will probably also struggle finding a 40MHz crystal that isn't an overtone one?.

Why do you need it so fast anyway?, usually 99.x% of a PIC's time is spent just wasting time. If you really do have a specific need for speed?, try the 18F series which run at 40MHz (using a PLL circuit).

 

[sheing3003]

The maximum advised is 20MHz, as I'm sure you know, you will probably also struggle finding a 40MHz crystal that isn't an overtone one?.

Why do you need it so fast anyway?, usually 99.x% of a PIC's time is spent just wasting time. If you really do have a specific need for speed?, try the 18F series which run at 40MHz (using a PLL circuit).

well i tried using a 40mhz one and it's not working...
i need it to be that fast bcoz my cmos image sensor can only go as low as 8Mhz, so the PIC are probably not fast enough?
==============================

i tried it with 40MHz and mmm it worked.... hope that won't damage the PIC though....
but still having problem reading the data from the sensor...><

 

[Alex Ng]

i tried it with 40MHz and mmm it worked.... hope that won't damage the PIC though....
but still having problem reading the data from the sensor...><

What type of sensor do u use?

 

[sheing3003]

I'm using a CMOS Image Sensor
Kodak KAC-9630

 

[sheing3003]

i have another problem hope someone can help me out:

do{
val=input(PIN_C1);
}while(!val); //wait for hsync to go high

I'm connecting this hsync signal from my image sensor, and what it suppose to do is wait for it to go high, I read the signal on an oscilloscope and it does go high to 3.3V. But my PIC can't seem to be reading this, any suggestions?
Thanks in advance

 

[kchriste]

You could use a MOSFET as a level shifter. Or you could use one of the many 3.3v --> 5V level shifter chips.
Also note that some of the inputs on the 877 are Schmitt trigger and some are TTL level. Try using an input that is TTL level such as RB1

 

[Pommie]

i have another problem hope someone can help me out:

do{
val=input(PIN_C1);
}while(!val); //wait for hsync to go high

I'm connecting this hsync signal from my image sensor, and what it suppose to do is wait for it to go high, I read the signal on an oscilloscope and it does go high to 3.3V. But my PIC can't seem to be reading this, any suggestions?
Thanks in advance

The easiest thing to do would be to run the pic at 3.3V or 4V if no LF version is available. Watch out for the Brown out level.

Mike.

 

[sheing3003]

The easiest thing to do would be to run the pic at 3.3V or 4V if no LF version is available. Watch out for the Brown out level.

Mike.

i tried running it on 3.3...but its not working....like I run it on 5V and its working then i set the voltage down to 3.3 then it wont work...

 

[sheing3003]

You could use a MOSFET as a level shifter. Or you could use one of the many 3.3v --> 5V level shifter chips.
Also note that some of the inputs on the 877 are Schmitt trigger and some are TTL level. Try using an input that is TTL level such as RB1

but wouldn't 3.3V be recognize by the PIC as logic 1? I thought the I minimum for it is 2V...so 3.3V should be enough shouldn't it? I'll give RB1 a try...hope that works
Thanks for helping out

 

[kchriste]

but wouldn't 3.3V be recognize by the PIC as logic 1? I thought the I minimum for it is 2V...so 3.3V should be enough shouldn't it?

A TTL input's minimum high voltage is 2V for 5V Vdd supply.
If you use a pin with Schmitt Trigger buffer it is 0.8 of VDD or 4V with 5V Vdd supply.
See page 178 of data sheet:
https://www.electro-tech-online.com/custompdfs/2006/07/39582b.pdf