cyrstal and capacitors setup

[jbchrist]

Hello,

I have been looking for some information on how & why they setup the crystal the way they do. Particularly, how do they choose the size of capacitor?

I have a 8 Mhz crystal and I am going to use it on a PIC 16F688 I don't know what size of capacitor to get. How do you calculate it and why?

Also I have seen some resistors thrown into the circuit, and am not sure why.

I would greatly appreciate it if you could point me in the right direction.

John

 

[eng1]

You can find detailed information in AN943, AN949 and AN826.
https://ww1.microchip.com/downloads/en/AppNotes/00943A.pdf
https://ww1.microchip.com/downloads/en/AppNotes/00949a.pdf
https://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf


The function of the high value resistor is described in those app notes iirc, but in practice you don't use it.


I'm just reading the datasheet of the 16f688...

 

[Nigel Goodwin]

Yes, the datasheets suggest suitable values, but they aren't critical - I generally use anything from 10pF to 47pF, depending what's to hand.

 

[eng1]

Surprisingly I can't fins the 'usual' table with suggested values for that pic
As Nigel said, they're not critical. I would choose 15 or 22 pF.

That PIC has an 8MHz internal oscillator, that allows you to save a crystal and two capacitors and it's suitable for most applications.

 

[jbchrist]

well the real reason I am looking into using a crystal is I am trying to communicate with my pc, and can't seem to do it. All I want to do is fill up the hyperterminal window a whole bunch of "r".

A friend suggested that perhaps the internal oscillator is working good enough.

I am using the a PIC16F688 which has a EUART. I am writing my program in Proton+ PIC Basis. The attached file is a block diagram of the setup.

When I start it up on the Serial LCD I get exactly what I asked for an "r" every second. On the PC I get "?" every second.

Here is my code is Proton+

'------------[Hardware setup]---------------------------- 
Device 16F688
Config INTRC_OSC_NOCLKOUT,_     'Internal clock no clock out
        WDT_OFF,_               'Watchdog Timer OFF
        PWRTE_ON,_              'Power-up Timer ON
        MCLRE_OFF,_             'Memory clear pin OFF 
        CP_OFF,_                'Code Protection OFF
        CPD_OFF,_               'Data Code Protection OFF
        BOD_OFF,_               'Brown Out Detection OFF
        IESO_OFF                'Internal External Switch Over OFF 
XTAL = 8    
ALL_DIGITAL = true
'HSERIAL_BAUD = 9600                ' Set baud rate to 9600
HSERIAL_BAUD = 4800                ' Set baud rate to 9600
HSERIAL_TXSTA = %00100000       ' Enable transmit and asynchronous mode 

DelayMS 1000 ' Wait for the PICmicro to stabilise 
'-------[MAIN PROGRAM LOOP STARTS HERE]----------------------------------

While 1 = 1 ' Create an infinite loop 
      HRSOut "r"
      DelayMS 1000
Wend

 

[eng1]

I can't help you with BASIC, but I find a discrepancy between
Config INTRC_OSC_NOCLKOUT 'Internal clock no clock out
and
XTAL = 8

If you select the internal oscillator, the default frequency is 4 MHz. You can change it in the OSCCON register.

 

[Nigel Goodwin]

Every PIC I've ever used works fine at 9600 baud with an internal oscillator, are you using a MAX232?, you MUST invert the data when using the hardware USART - as a MAX232 does.

 

[jbchrist]

Yes, I am using a MAX232cpe (to be specific) with 5 1uF capacitors.

 

[Peter_wadley]

I dont know proton at all but..

Looking at your code..

'------------[Hardware setup]---------------------------- 
Device 16F688
Config INTRC_OSC_NOCLKOUT,_     'Internal clock no clock out
        WDT_OFF,_               'Watchdog Timer OFF
        PWRTE_ON,_              'Power-up Timer ON
        MCLRE_OFF,_             'Memory clear pin OFF 
        CP_OFF,_                'Code Protection OFF
        CPD_OFF,_               'Data Code Protection OFF
        BOD_OFF,_               'Brown Out Detection OFF
        IESO_OFF                'Internal External Switch Over OFF 
XTAL = 8    
ALL_DIGITAL = true
'HSERIAL_BAUD = 9600                ' Set baud rate to 9600
[B]HSERIAL_BAUD = 4800                ' Set baud rate to 9600[/B]
HSERIAL_TXSTA = %00100000       ' Enable transmit and asynchronous mode 

DelayMS 1000 ' Wait for the PICmicro to stabilise 
'-------[MAIN PROGRAM LOOP STARTS HERE]----------------------------------

While 1 = 1 ' Create an infinite loop 
      HRSOut "r"
      DelayMS 1000
Wend

HSERIAL_BAUD = 4800 ' Set baud rate to 9600

Have you entered the wrong Value here? 4800 when your comment says 9600..

Just curious.

 

[eblc1388]

It is a direct result of copy & paste. People(or at least I myself) do that all the time while debugging. If you look up one line, you'll know why the comment does not match actual action.

Yeah, I know its bad, but the offending line will be deleted later if the code still doesn't work.