Help me with Clock of PIC16F877A microcontroller

[electronics_guy]

Hi everyone, I need some help in understanding the clock of PIC16F877A microcontroller. I am currently working on the ADC module of PIC16f877A. I read the data sheet but I couldn't understand it clearly. I have some questions:
I am using a 4 MHz crystal on my development board, so what I understood from the data sheet is I need to make the clock select bits ADCS2:ADCS1:ADCS0 as 001(coz the max device frequency can be 5 MHz and since I am using 4 MHz I select 001). Is my selection right?

Plz help



Thanks in advance.

 

[Kurenai_ryu]

I am using a 4 MHz crystal on my development board, so what I understood from the data sheet is I need to make the clock select bits ADCS2:ADCS1:ADCS0 as 001(coz the max device frequency can be 5 MHz and since I am using 4 MHz I select 001). Is my selection right?

Right! that way you have a 8 TOSC divider which gives you a TAD cycle of 2us (as the datasheet states, you need more than 1.6us TAD cycle...)

 

[electronics_guy]

Thanx a lot Ryu. I also have another doubt... regarding PCFG<3:0> bits.... i.e If i select PCFG<3:0> as 011x then all the pins of PORTA and PORTE will be configured as digital pins... What does that mean? Can anyone help me understand?

 

[wp100]

Thanx a lot Ryu. I also have another doubt... regarding PCFG<3:0> bits.... i.e If i select PCFG<3:0> as 011x then all the pins of PORTA and PORTE will be configured as digital pins... What does that mean? Can anyone help me understand?

Hi,

The 877A has 8 analogue channels RA0-5 RE0-2.

When the chip is powered on , all these lines are set by default to Analogue mode. ie they are set as INputs expecting to see an analogue signal.

Much of the time you do not want 8 analogue pins, so you modify there funtion to Digital Input/Output by changing those 4 bits of ADCON1.
eg
banksel ADCON1 ; make all analogue ports Digital i/o
movlw 0x06
movwf ADCON1

 

[electronics_guy]

@wp100 thanks a lot. So my question is why should I change or what happens when I change them to digital pins? I don't know if I am asking very silly question but, thats eating me a lot....
Thanks

 

[wp100]

@wp100 thanks a lot. So my question is why should I change or what happens when I change them to digital pins? I don't know if I am asking very silly question but, thats eating me a lot....
Thanks

Hi,

The Analogue ports are input only, expect to receive a signal anywhere between 0 and +5v and require the micro to use its ADC hardware which is quiet slow.

Ports that are set to Digital can be Inputs or Outputs, are very fast and require a simple high +5v or low 0v signal to denote the two possible states.
They can be anything from simple Switch inputs to Relay outputs or things like data transmission input/output at high speed

You want to follow a good tutorial like this one which will teach you all the basics, it covers both Assember and C languages
**broken link removed**

 

[electronics_guy]

Hi,

The Analogue ports are input only, expect to receive a signal anywhere between 0 and +5v and require the micro to use its ADC hardware which is quiet slow.

Ports that are set to Digital can be Inputs or Outputs, are very fast and require a simple high +5v or low 0v signal to denote the two possible states.
They can be anything from simple Switch inputs to Relay outputs or things like data transmission input/output at high speed

You want to follow a good tutorial like this one which will teach you all the basics, it covers both Assember and C languages
**broken link removed**

Thanks a lot. That helped.