f500_oscillator_rc.c

C8051F500开发编程：MCU全部资源应用实例

//-----------------------------------------------------------------------------
// F500_Oscillator_RC.c
//-----------------------------------------------------------------------------
// Copyright 2008 Silicon Laboratories, Inc.
// http://www.silabs.com
//
// Program Description:
//
// This program demonstrates how to configure the oscillator for use with an
// external RC Oscillator.
//
//
// How To Test:
//
// 1) Ensure that a capacitor (C6) and resistor (R3) are installed.
// 2) Specify the target frequency in the constant <RC_FREQUENCY>.
// 3) Download code to an 'F500 target board.
// 4) Measure the frequency output on P0.0.
//
// Choosing Component Values:
//
//       frequency (Hz) = 1 / (1.05 * R * C )
//
//       where R is in Ohms
//             C is in Farads
//
//       Note: Add 2pF stray capacitance to C.
//
// For a resistor value of 330K, and a capacitor value of 33pF, the
// expected RC_FREQUENCY is 82.5 kHz. The actual measured frequency
// on the target board was 82.7 kHz.
//
//
// Target:         C8051F500 (Side A of a C8051F500-TB)
// Tool chain:     Keil C51 8.0 / Keil EVAL C51
// Command Line:   None
//
// Release 1.0 / 03 MAR 2008 (GP)
//    -Initial Revision

//-----------------------------------------------------------------------------
// Includes
//-----------------------------------------------------------------------------

#include <compiler_defs.h>
#include <C8051F500_defs.h>            // SFR declarations

//-----------------------------------------------------------------------------
// External Oscillator RC Frequency Selection
//-----------------------------------------------------------------------------
//
// Please initialize the following constant to the target frequency
//
#define RC_FREQUENCY   91000           //  Target RC Frequency in Hz

// XFCN Setting Macro
#if  (RC_FREQUENCY <= 25000)
   #define XFCN 0
#elif(RC_FREQUENCY <= 50000)
   #define XFCN 1
#elif(RC_FREQUENCY <= 100000)
   #define XFCN 2
#elif(RC_FREQUENCY <= 200000)
   #define XFCN 3
#elif(RC_FREQUENCY <= 400000)
   #define XFCN 4
#elif(RC_FREQUENCY <= 800000)
   #define XFCN 5
#elif(RC_FREQUENCY <= 1600000)
   #define XFCN 6
#elif(RC_FREQUENCY <= 3200000)
   #define XFCN 7
#else
   #error "RC Frequency must be less than or equal to 3.2 MHz"
#endif

//-----------------------------------------------------------------------------
// Function Prototypes
//-----------------------------------------------------------------------------

void OSCILLATOR_Init (void);
void PORT_Init (void);

//-----------------------------------------------------------------------------
// main() Routine
//-----------------------------------------------------------------------------

void main (void)
{
   SFRPAGE = ACTIVE_PAGE;              // Set SFR Page for PCA0MD

   PCA0MD &= ~0x40;                    // Disable the watchdog timer

   PORT_Init ();                       // Initialize Port I/O
   OSCILLATOR_Init ();                 // Initialize Oscillator

   while (1);                          // Loop forever
}                                 

//-----------------------------------------------------------------------------
// Initialization Subroutines
//-----------------------------------------------------------------------------

//-----------------------------------------------------------------------------
// OSCILLATOR_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters   : None
//
// This function initializes the system clock to use the external oscillator
// in RC mode.
//
//-----------------------------------------------------------------------------
void OSCILLATOR_Init (void)
{
   U16 i;                              // Delay counter

   U8 SFRPAGE_save = SFRPAGE;
   SFRPAGE = ACTIVE_PAGE;

   RSTSRC = 0x06;                      // Enable missing clock detector and
                                       // VDD Monitor reset

   SFRPAGE = CONFIG_PAGE;

   OSCXCN = (0x40 | XFCN);             // Start external oscillator with
                                       // the appropriate XFCN setting
                                       // based on RC frequency

   for (i=0; i < 256; i++);            // Wait for RC osc. to start

   CLKSEL = 0x01;                      // Select external oscillator as system
                                       // clock source

   OSCICN = 0x00;                      // Disable the internal oscillator.

   SFRPAGE = SFRPAGE_save;
}

//-----------------------------------------------------------------------------
// PORT_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters   : None
//
// This function configures the crossbar and ports pins.
//
// The oscillator pins should be configured as analog inputs when using the
// external oscillator in crystal mode.
//
// P0.0   digital    push-pull     /SYSCLK
// P0.3   analog     don't care     XTAL2
//-----------------------------------------------------------------------------
void PORT_Init (void)
{
   U8 SFRPAGE_save = SFRPAGE;
   SFRPAGE = CONFIG_PAGE; 

   // Oscillator Pins
   P0MDIN &= ~0x08;                    // P0.3 is analog
   P0SKIP |= 0x08;                     // P0.3 skipped in the Crossbar

   // Buffered System Clock Output
   P0MDOUT |= 0x01;                    // P0.0 is push-pull

   // Crossbar Initialization
   XBR1    = 0x02;                     // Route /SYSCLK to P0.0
   XBR2    = 0x40;                     // Enable Crossbar and weak pull-ups

   SFRPAGE = SFRPAGE_save;
}

//-----------------------------------------------------------------------------
// End Of File
//-----------------------------------------------------------------------------