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  1. #1
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    Default 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    Hi there,

    I'd like to have my external 32.768kHz crystal as time base onTMR1 and, in the same time, run the internal oscillator at 4MHz.

    Thoretically, this simple code hereunder should blink my LED only when the TMR1IF overflow flag is set.

    But...

    This code works erratically:
    - sometimes the program starts, sometimes not
    - commenting the fuses will mostly make the program start almost always
    - physically removing the 32.768kHz crystal from my breadboard will change nothing => when the program works, the LED still blinks?!

    Code:
    ' ====== FUSES ====================================================================================
    ' PIC 16F690
    @ __Config _FCMEN_ON &_IESO_OFF &_CPD_OFF &_WDT_OFF &_BOR_OFF &_CP_OFF &_PWRTE_OFF &_MCLRE_OFF &_LP_OSC 
    
    ' ====== INTERRUPT SERVICE ROUTINE ================================================================
    GOTO INIT: ' Just to place ISR at top of program
    DISABLE
    ISR: 'if TMR1IF is set (TMR1 has overflown)
        toggle PORTB.6 'LED
        PIR1.0 = 0 ' clear TMR1IF overflow flag
        resume
    ENABLE
    
    ' ====== INITIALIZE ===============================================================================
    INIT:
    PAUSE 1000 'circuit settle time
    
    ' ====== DEFINES ==================================================================================
    DEFINE OSC 4
    
    ' ====== REGISTERS ================================================================================
    '             76543210
    OPTION_REG = %10000000 ' Pull-Ups disabled
    OSCCON     = %01100000 ' Internal RC set to 4Mhz
    ANSEL      = %00000000 ' Analog inputs Channels 0 to 7
    ANSELH     = %00000000 ' Analog inputs Channels 8 to 11
    INTCON     = %11000000 ' INTerrupts CONtrol: GIE is ON, PEIE is ON 
    T1CON      = %00111001 ' Timer1 OSC enabled, Timer1 enabled, presc.1:8
    PIE1       = %00000001 ' Enable TMR1IF overflow flag
    
    ' ====== PROGRAM ==================================================================================
    ON INTERRUPT GOTO ISR
    
    MAIN:
        goto MAIN:
    END
    Setting the FCMEN to OFF will never let the program run which lets me think the 32.768Khz crystal doesn't actually run at all. But le LED blinks?!

    What am I missing?
    Roger

  2. #2
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    TMR1CS:
    Timer1 Clock Source Select bit




    T1CON = %00111011 ' Timer1 OSC enabled, Timer1 enabled, presc.1:8

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  3. #3
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    plus config needs to enable internal osc
    Code:
     __config  _INTRC_OSC_NOCLKOUT & _WDT_ON & _PWRTE_OFF & _MCLRE_OFF & _CP_OFF & _CPD_OFF & _BOD_ON & _IESO_ON & _FCMEN_ON
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  4. #4
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    I've modified the code according to the answers.

    But with or without the 32,768kHz crystal, the LED will always blink.

    As per datasheet, I have enabled LP oscillator. Why should I set the internal oscillator instead please?

    Same question for T1CON.TMR1CS; why should I set this one?

    Code:
    ' ====== FUSES ====================================================================================
    ' PIC 16F690
     __config  _INTRC_OSC_NOCLKOUT & _WDT_ON & _PWRTE_OFF & _MCLRE_OFF & _CP_OFF & _CPD_OFF & _BOD_ON & _IESO_ON & _FCMEN_ON
    
    ' ====== INTERRUPT SERVICE ROUTINE ================================================================
    GOTO INIT: ' Just to place ISR at top of program
    DISABLE
    ISR: 'if TMR1IF is set (TMR1 has overflown)
        toggle PORTB.6 'LED
        PIR1.0 = 0 ' clear TMR1IF overflow flag
        resume
    ENABLE
    
    ' ====== INITIALIZE ===============================================================================
    INIT:
    PAUSE 1000 'circuit settle time
    
    ' ====== DEFINES ==================================================================================
    DEFINE OSC 4
    
    ' ====== REGISTERS ================================================================================
    '             76543210
    OPTION_REG = %10000000 ' Pull-Ups disabled
    OSCCON     = %01100000 ' Internal RC set to 4Mhz
    ANSEL      = %00000000 ' Analog inputs Channels 0 to 7
    ANSELH     = %00000000 ' Analog inputs Channels 8 to 11
    INTCON     = %11000000 ' INTerrupts CONtrol: GIE is ON, PEIE is ON 
    T1CON      = %00111011 ' Timer1 OSC enabled, Timer1 enabled, presc.1:8
    PIE1       = %00000001 ' Enable TMR1IF overflow flag
    
    ' ====== PROGRAM ==================================================================================
    ON INTERRUPT GOTO ISR
    
    MAIN:
        goto MAIN:
    END
    Roger

  5. #5
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    As per datasheet, I have enabled LP oscillator. Why should I set the internal oscillator instead please?
    if I understand correctly your intent is to use the intosc @4mhz for cpu clock and the ext osc @32khz for timer1 clk source

    so to use the HFINTOSC @4mhz config= _INTRC_OSC_NOCLKOUT [no clk out assumed]
    osccon= $60
    Same question for T1CON.TMR1CS; why should I set this one?
    to use the ext osc for timer1

    you must enable the ext osc
    bit 3 T1OSCEN: LP Oscillator Enable Control bit
    and
    select ext osc as timer clock source
    bit 1 TMR1CS: Timer1 Clock Source Select bit

    so t1con= %00111011 with 8:1 prescale and timer on
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  6. #6
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    the data sheet says that T1OSCEN should be enable and the user should delay for a suitable time before timer is enabled

    I don't have a 16f690 handy to test with
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  7. #7
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    Thanks a lot Richard,

    The code hereunder works

    BTW, I had to change the config settings (fuses) since you might have copied them for another PIC and generated errors while compiling. Please let me know if I should take care to change some of them.

    For the test, I removed the 32,768kHz crystal while running () and the LED stops blinking which is correct.

    Code:
    ' ====== FUSES ====================================================================================
    ' PIC 16F690
    @ __config _INTRC_OSC_NOCLKOUT &_WDT_OFF &_PWRTE_OFF &_MCLRE_OFF &_CP_OFF &_CPD_OFF &_BOR_OFF &_IESO_OFF &_FCMEN_OFF
    
    ' ====== INTERRUPT SERVICE ROUTINE ================================================================
    GOTO INIT: ' Just to place ISR at top of program
    DISABLE
    ISR: 'if TMR1IF is set (TMR1 has overflown)
        toggle PORTB.6 'LED
        PIR1.0 = 0 ' clear TMR1IF overflow flag
        TMR1H  = 0
        TMR1L  = 0
        resume
    ENABLE
    
    ' ====== INITIALIZE ===============================================================================
    INIT:
    PIR1.0 = 0 ' clear TMR1IF overflow flag
    TMR1H  = 0
    TMR1L  = 0
    
    ' ====== DEFINES ==================================================================================
    DEFINE OSC 4
    
    ' ====== REGISTERS ================================================================================
    '             76543210
    OPTION_REG = %10000000 ' Pull-Ups disabled
    OSCCON     = %01100000 ' Internal RC set to 4Mhz
    ANSEL      = %00000000 ' Analog inputs Channels 0 to 7
    ANSELH     = %00000000 ' Analog inputs Channels 8 to 11
    INTCON     = %11000000 ' INTerrupts CONtrol: GIE is ON, PEIE is ON 
    T1CON      = %00111011 ' Timer1 OSC enabled, Timer1 enabled, presc.1:8
    PIE1       = %00000001 ' Enable TMR1IF overflow flag
    
    ' ====== PROGRAM ==================================================================================
    PAUSE 1000 'circuit settle time
    
    ON INTERRUPT GOTO ISR
    
    MAIN:
        goto MAIN:
    END
    Roger

  8. #8
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    BTW, I had to change the config settings (fuses) since you might have copied them for another PIC and generated errors while compiling. Please let me know if I should take care to change some of them.
    config was generated by meCONFIG for pbp3
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  9. #9
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    Default Re: 32.768kHz external crystal for TMR1 time base and internal 4MHz oscillator

    Quote Originally Posted by richard View Post
    config was generated by meCONFIG for pbp3
    I have 2.6 so this may be why there is a change
    Roger

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