classb_freq.c

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/**
 * \file
 *
 * \brief CPU frequency test.           
 * 
 * \par Application note:
 *      AVR1610: Guide to IEC60730 Class B compliance with XMEGA
 *
 * \par Documentation
 *      For comprehensive code documentation, supported compilers, compiler 
 *      settings and supported devices see readme.html
 *
 * \author
 *      Atmel Corporation: http://www.atmel.com \n
 *      Support email: avr@atmel.com
 * 
 * Copyright (C) 2012 Atmel Corporation. All rights reserved.
 *
 * \page License
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation
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 *
 * 3. The name of Atmel may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 *
 * 4. This software may only be redistributed and used in connection with an
 * Atmel AVR product.
 *
 * THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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#include "avr_compiler.h"
#include "classb_freq.h"

//! \addtogroup cpu_freq
//@{

//! \internal \brief This counts the number of TC overflows. 
//! This variable is used to keep the MSW (bits 31 to 16) of the TC count.
volatile uint16_t classb_tc_ovf_cnt = 0;

//@}

//! \brief This sets up the TC that is used in the frequency test.
//!     This configures the period, prescaler and overflow interrupt. 
void classb_freq_setup_timer() 
{
    // Set up period as configured in the header file. 
        CLASSB_TEST_TC.PER = CLASSB_TC_PER;
        
        // Set up overflow interrupt.
    CLASSB_TEST_TC.INTCTRLA = TC_OVFINTLVL_LO_gc;    
    
    // Start the TC with the prescaling factor configured in the header file. 
    CLASSB_TEST_TC.CTRLA = CLASSB_TC_PRESCALER_gc;
        
        // Reset count
    CLASSB_TEST_TC.CNT = 0;
}

//! \brief This is called back from the RTC interrupt.
//!
//! This function calculates the difference between the TC count and the 
//! predefined reference. The error handler would be called if the relative 
//! difference should be higher than the tolerance.
void classb_freq_callback() 
{
        // Read the count in the TC and include the overflow counter as MSB
        volatile uint32_t tccount = CLASSB_TEST_TC.CNT;         
        tccount |= ((uint32_t)classb_tc_ovf_cnt) << 16;
          
        // Compute the absolute difference between reference and counter.
        tccount = (tccount > CLASSB_TC_COUNT_REF)?(tccount-CLASSB_TC_COUNT_REF):(CLASSB_TC_COUNT_REF-tccount);
        
        // If the difference was larger than expected, the error should be handled 
        if (tccount > CLASSB_MAX_DIF)
                CLASSB_ERROR_HANDLER_FREQ();

        // Reset the overflow counter.
        classb_tc_ovf_cnt = 0;
        
        // Reset TC
        CLASSB_TEST_TC.CNT = 0;
}

//! \addtogroup func_freq
//@{
//! \brief TC overflow interrupt. 
//! This counts the number of TC overflows and the error handler is called if 
//! the configurable maximum is exceeded. The purpose of the latter is to detect 
//! RTC failure: the TC is reset in the RTC interrupt. If the RTC interrupt 
//! is too slow, the number of TC overflows will exceed the limit. Note that
//! if the TC frequency increased abruptly, the error would probably be detected
//! here.
ISR(CLASSB_TEST_TC_OVF_vect) 
{       
        // Increase the overflow counter. If the TC had more overflows than expected, 
        // the error should be handled.
        if ( classb_tc_ovf_cnt++ >= CLASSB_COUNT_OVF_MAX )
                CLASSB_ERROR_HANDLER_FREQ();
}
//@}