text: //*****************************************************************************
  //
  //! @file system_apollo3p.c
  //!
  //! @brief Ambiq Micro Apollo3 Blue Plus MCU specific functions.
  //
  //*****************************************************************************
  
  //*****************************************************************************
  //
  // Copyright (c) 2021, Ambiq Micro, Inc.
  // All rights reserved.
  //
  // 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 and/or other materials provided with the distribution.
  //
  // 3. Neither the name of the copyright holder nor the names of its
  // contributors may be used to endorse or promote products derived from this
  // software without specific prior written permission.
  //
  // Third party software included in this distribution is subject to the
  // additional license terms as defined in the /docs/licenses directory.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  // ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
  // LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  // CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  // SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  // INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  // CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  // ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  // POSSIBILITY OF SUCH DAMAGE.
  //
  //*****************************************************************************
  
  #include <stdint.h>
  #include "system_apollo3.h"
  #include "apollo3.h"
  
  //*****************************************************************************
  //
  // Defines
  //
  //*****************************************************************************
  
  //
  // Clocks
  //
  #define __HSI             (6000000UL)
  #define __XTAL            (32768UL)         // Crystal Oscillator frequency
  #define __SYS_OSC_CLK     (48000000)        // Main oscillator frequency
  #define __SYSTEM_CLOCK    (1*__SYS_OSC_CLK)
  
  //
  // Initialize SystemCoreClock with the system core clock frequency value
  // achieved after system intitialization.
  // This means system core clock frequency after call to SystemInit()
  //
  uint32_t SystemCoreClock = __SYSTEM_CLOCK;  // System Clock Frequency (Core Clock)
  
  //*****************************************************************************
  //
  //! @brief Set the global clock frequncy.
  //!
  //! This function sets the global clock frequency.
  //!
  //! @return None.
  //
  //*****************************************************************************
  void
  SystemCoreClockUpdate(void)
  {
      //
      // Calculate the system frequency based upon the current register settings.
      // This function can be used to retrieve the system core clock frequeny
      // after user changed register sittings.
      //
      SystemCoreClock = __SYS_OSC_CLK / (CLKGEN->CCTRL_b.CORESEL + 1);
  }
  
  //*****************************************************************************
  //
  //! @brief Initialize the system.
  //!
  //! This function sets up the microcontroller system.
  //!
  //! @return None.
  //
  //*****************************************************************************
  void
  SystemInit(void)
  {
      //
      // Initialize the system
      // Do not use global variables because this function is called before
      // reaching pre-main. RW section maybe overwritten afterwards.
      //
      SystemCoreClock = __SYSTEM_CLOCK;
  
      CLKGEN->CLKKEY = 0x47;              // Enable write to CCTRL
      CLKGEN->CCTRL_b.CORESEL = 0;        // Div by 1 for 48MHz
      CLKGEN->CLKKEY = 0;                 // Disable write to CCTRL
  }
  
  