t61x_oscillator_capacitor.c

芯科原厂所有c8051fxx程序的例子。

//-----------------------------------------------------------------------------
// T61x_Oscillator_Capacitor.c
//-----------------------------------------------------------------------------
// Copyright 2007 Silicon Laboratories, Inc.
// http://www.silabs.com
//
// Program Description:
//
// This program demonstrates how to configure the oscillator for use with an
// external capacitor.
//
//
// How To Test:
//
// 1) Ensure that a capacitor is installed between EXTCLK (P0.3) and GND.
// 2) Specify the target frequency in the constant <C_FREQUENCY>. 
// 3) Download code to an 'T61x target board.
// 4) Measure the frequency output on P0.0.
//
// Choosing Component Values:
//  
//       frequency (Hz) is proportional to [ KF(XFCN) / (C * VDD) ] 
//
//       where KF(XFCN) is a factor based on XFCN. 
//
// The actual frequency is best determined by measurement. In this example,
// a 10pF capacitor was used with VDD = 3.3V. Component values are best
// chosen by trial and error.
//
// The resulting frequencies achievable using 10pF and 3.3V VDD are:
//
//       frequency   |   XFCN
//      ----------------------
//           8 kHz   |    0
//          23 kHz   |    1
//          70 kHz   |    2
//         130 kHz   |    3
//         603 kHz   |    4
//         1.7 MHz   |    5
//         7.0 MHz   |    6
//          21 MHz   |    7
//
//
// FID:            31X000016
// Target:         C8051T61x
// Tool chain:     Keil C51 7.50 / Keil EVAL C51
// Command Line:   None
//
// Release 1.0
//    -Initial Revision (SM)
//    -18 AUG 2007
//

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

#include <compiler_defs.h>
#include <c8051T610_defs.h>            // SFR declarations

//-----------------------------------------------------------------------------
// External Oscillator RC Frequency Selection
//-----------------------------------------------------------------------------
//
// Please initialize the following constant to the target frequency
//
#define C_FREQUENCY   200000            // Target C Frequency in Hz

// XFCN Setting Macro
#define XFCN             3             // XFCN/KF setting.

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

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

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

void main (void)
{
   PCA0MD &= ~0x40;                    // WDTE = 0 (clear watchdog timer
                                       // enable)

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

   while (1) {};                       // loop forever

}                                      // end of main()

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

//-----------------------------------------------------------------------------
// OSCILLATOR_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters   : None
//
// This function initializes the system clock to use the external oscillator
// in C mode.
//
//-----------------------------------------------------------------------------
void OSCILLATOR_Init (void)
{

   int i;                              // delay counter

   OSCXCN = (0x50 | XFCN);             // start external oscillator with
                                       // the appropriate XFCN setting
                                       // in C Mode

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

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

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

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

}

//-----------------------------------------------------------------------------
// 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    EXTCLK
//-----------------------------------------------------------------------------
void PORT_Init (void)
{

    // 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
   XBR0    = 0x08;                     // Route /SYSCLK to first available pin
   XBR1    = 0x40;                     // Enable Crossbar and weak pull-ups

}


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