C8051F340采样6路电压值问题

以下是Silicon Laboratories提供的example。即依次对P2.0、P2.1、P2.2、P2.3、P2.5、P2.6的电压值进行采样
，这样的话为什么在Port_Init()里把P2口配置为P2MDIN &= ~0x3F？不应该是P2MDIN &= ~0x6F吗？？板子上的P2.5
连接的是一电位器，采取P2MDIN &= ~0x3F的配置扭动电位器时P2.6（P2.6是悬空的）的值不会随电位器变化而变化
；而改为P2MDIN &= ~0x6F后扭动电位器时P2.6的值也会随电位器的变化而变化？？这是为什么呢？？麻烦各位给看
看这是怎么个情况！！

//-----------------------------------------------------------------------------
// F34x_ADC0_ExternalInput_Mux.c
//-----------------------------------------------------------------------------
// Copyright 2005 Silicon Laboratories, Inc.
// http://www.silabs.com
//
// This code example illustrates using the internal analog multiplexer to
// measure analog voltages on up to 8 different analog inputs.  Results are
// printed to a PC terminal program via the UART.
//
// The inputs are sequentially scanned, beginning with input 0 (P2.0), up
// to input number -1 based on the values in .
//
//
// ADC Settling Time Requirements, Sampling Rate:
// ----------------------------------------------
//
// The total sample time per input is comprised of an input setting time
// (Tsettle), followed by a conversion time (Tconvert):
//
// Tsample  = Tsettle + Tconvert
//
// Settling and conversion times may overlap, as the ADC holds the value once
// conversion begins.  This program takes advantage of this to increase the
// settling time above the minimum required.  In other words, when
// converting the value from analog input Ain(n), the input mux is switched
// over to the next input Ain(n+1) to begin settling.
//
// |---Settling Ain(n)---|=Conversion Ain(n)=|
//                      |---Settling Ain(n+1)---|=Conversion Ain(n+1)=|
//                                              |---Settling Ain(n+2)---|
// ISR:  Timer 2       ^                      ^                      ^
// ISR:  ADC0                               ^                      ^
//
// The ADC input voltage must be allowed adequate time to settle before the
// conversion is made.  This settling depends on the external source
// impedance, internal mux impedance, and internal capacitance.
// Settling time is given by:
//
//                | 2^n |
// Tsettle = ln | --- | * Rtotal * Csample
//                | SA  |
//
// In this application, assume a 100kohm potentiometer as the voltage divider.
// The expression evaluates to:
//
//                | 2^12 |
// Tsettle = ln | ---- | * 105e3 * 10e-12 = 10.2uS
//                | 0.25 |
//
// In addition, one must allow at least 1.5 us after changing analog MUX
// inputs or PGA settings.  The settling time in this example, then, is
// dictated by the large external source resistance.
//
// The conversion is 16 periods of the SAR clock.  At 2.5 MHz,
// this time is 16 * 400nS = 6.4 uS.
//
//
// Tsample, minimum  = Tsettle + Tconvert
//                = 10.2uS + 6.4uS
//                = 16.6 uS
//
// Timer2 is set to change the MUX input and start a conversion every 20 us.
//
// General:
// --------
//
// The system is clocked using the internal 12MHz oscillator. Results are
// printed to the UART from a loop with the rate set by a delay based on
// Timer0. This loop periodically reads the ADC value from a global array,
// .
//
// The ADC makes repeated measurements at 20 us intervals based on Timer2.
// The end of each ADC conversion initiates an interrupt which calls an
// averaging function. samples are averaged, then the Result
// values updated.
//
// For each power of 4 of , you gain 1 bit of effective resolution.
// For example, = 256 gain you 4 bits of resolution: 4^4 = 256.
//
// The ADC input multiplexer is set for a single-ended input.  The example
// sequentially scans through the inputs, starting at P2.0.
// inputs are read.  The amplifier is set for unity gain so a voltage range of
// 0 to Vref (2.43V) may be measured.  Although voltages up to Vdd may be
// applied without damaging the device, only the range 0 to Vref may be
// measured by the ADC.
//
// A 100 kohm potentiometer may be connected as a voltage divider between
// VREF and AGND as shown below:
//
// ---------
//       |
//       o| AGND ----|
//       o| VREF ----|<-|
//       o| P2.x    |  |
//       o| |       |
//       o|    --------
//       o|
//       o|
//       o|
//       |
// ---------
//
// How To Test:
//
// 1) Download code to a 'F34x device that is connected to a UART transceiver
// 2) Verify the TX and RX jumpers are populated on J3.
// 3) Connect a serial cable from the DB9 connector to a PC
// 4) On the PC, open HyperTerminal (or any other terminal program) and connect
// to the COM port at and 8-N-1
// 5) Connect a variable voltage source (between 0 and Vref) to the Port2 pins,
// or a potentiometer voltage divider as shown above.
// 6) HyperTerminal will print the voltages measured by the device if
// everything is working properly.  Note that some of the analog inputs are
// floating and will return nonzero values.
//
// FID:          34X000092
// Target:       C8051F34x
// Tool chain:    Keil C51 7.50 / Keil EVAL C51
// Command Line: None
//
// Release 1.0
// -Initial Revision (SM / TP)
// -19 OCT 2006
//

//-----------------------------------------------------------------------------
// Includes
//-----------------------------------------------------------------------------
#include                // SFR declarations
#include
//-----------------------------------------------------------------------------
// 16-bit SFR Definitions for 'F34x
//-----------------------------------------------------------------------------
sfr16 TMR2RL = 0xCA;                // Timer2 reload value
sfr16 TMR2 = 0xCC;                   // Timer2 counter
sfr16 ADC0 = 0xBD;                   // 10-bit ADC0 result
//-----------------------------------------------------------------------------
// Global CONSTANTS
//-----------------------------------------------------------------------------
#define SYSCLK       12000000       // SYSCLK frequency in Hz
#define BAUDRATE    115200          // Baud rate of UART in bps
#define ANALOG_INPUTS 6             // Number of AIN pins to measure,
                                    // skipping the UART0 pins
#define INT_DEC    256             // Integrate and decimate ratio
#define TIMER0_RELOAD_HIGH  0       // Timer0 High register
#define TIMER0_RELOAD_LOW 255       // Timer0 Low register
//-----------------------------------------------------------------------------
// Function PROTOTYPES
//-----------------------------------------------------------------------------
void Oscillator_Init (void);
void Port_Init (void);
void Timer2_Init(void);
void ADC0_Init(void);
void UART0_Init (void);
void Timer0_wait(int ms);
//-----------------------------------------------------------------------------
// Global Variables
//-----------------------------------------------------------------------------
long RESULT[ANALOG_INPUTS];          // ADC0 decimated value, one for each
                                    // analog input
// The values are the values to be written to the AMX0P
// register to select the P2. port pins.
// For the 'F340, the AMX0P settings correspond to the following port pins:
//
// AMX0P    Port Pin
// 0x00       P2.0
// 0x01       P2.1
// 0x02       P2.2
// 0x03       P2.3
// 0x04       P2.5
// 0x05       P2.6
//
unsigned char idata PIN_TABLE[ANALOG_INPUTS] = {0,1,2,3,5,6};
unsigned char idata PIN_MUX_TABLE[ANALOG_INPUTS] = {0,1,2,3,4,5};
unsigned char AMUX_INPUT = 0;       // Index of analog MUX inputs
//-----------------------------------------------------------------------------
// MAIN Routine
//-----------------------------------------------------------------------------
void main (void)
{
unsigned char i;
unsigned long measurement;
PCA0MD &= ~0x40;                   // WDTE = 0 (clear watchdog timer
                                    // enable)
Oscillator_Init ();                // Initialize system clock to
                                    // 12MHz
Port_Init ();                      // Initialize cros***ar and GPIO
Timer2_Init();                   // Init Timer2 to generate
                                    // overflows to trigger ADC
UART0_Init();                      // Initialize UART0 for printf's
ADC0_Init();                      // Initialize ADC0
EA = 1;                            // Enable global interrupts
while (1)
{
   EA = 0;                         // Disable interrupts
   printf("f");
   for(i = 0; i < ANALOG_INPUTS; i++)
   {
      // The 10-bit ADC value is averaged across INT_DEC measurements.
      // The result is then stored in RESULT, and is right-justified
      // The measured voltage applied to the port pins is then:
      //
      //                         Vref (mV)
      // measurement (mV) = --------------- * Result (bits)
      //                      (2^10)-1 (bits)
      measurement =  RESULT * 2430 / 1023;
      printf("P2.%bu voltage: %4ld mVn",PIN_TABLE,measurement);
   }
   EA = 1;                         // Re-enable interrupts
   Timer0_wait(25);                // Wait before displaying new values
}
}
//-----------------------------------------------------------------------------
// Initialization Subroutines
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
// SYSCLK_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters : None
//
// This routine initializes the system clock to use the internal 12MHz
// oscillator as its clock source.  Also enables missing clock detector reset.
//
void Oscillator_Init (void)
{
OSCICN = 0x83;                   // Configure internal oscillator for
                                    // its highest frequency
RSTSRC = 0x04;                   // Enable missing clock detector
}
//-----------------------------------------------------------------------------
// Port_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters : None
//
// Configure the Cros***ar and GPIO ports.
//
// P0.4 - UART TX (push-pull)
// P0.5 - UART RX
//
// P1.5 - analog (VREF)
//
// P2.0 - analog input (ADC0)
// P2.1 - analog input (ADC0)
// P2.2 - analog input (ADC0)
// P2.3 - analog input (ADC0)
// P2.5 - analog input (ADC0)
// P2.6 - analog input (ADC0)
//
//-----------------------------------------------------------------------------
void Port_Init (void)
{
P2SKIP = 0x3F;                   // Skip all analog pins
XBR0 = 0x01;                      // UART0 TX and RX pins enabled
XBR1 = 0x40;                      // Enable cros***ar and weak pull-ups
P0MDOUT |= 0x10;                   // Enable TX0 as a push-pull output
P1MDIN &= ~0x20;                   // P1.5 in analog mode
P2MDIN &= ~0x3F;                   // Set desired pins as analog inputs
}
//-----------------------------------------------------------------------------
// Timer2_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters : None
//
// Configure Timer2 to 16-bit auto-reload and generate an interrupt at 10 us
// intervals.  Timer2 overflows automatically triggers ADC0 conversion.
//
//-----------------------------------------------------------------------------
void Timer2_Init (void)
{
TMR2CN = 0x00;                   // Stop Timer2; Clear TF2;
                                    // use SYSCLK as timebase, 16-bit
                                    // auto-reload
CKCON |= 0x10;                   // Select SYSCLK for timer 2 source
TMR2RL = 65535 - (SYSCLK / 10000);  // Init reload value for 10 us
TMR2 = 0xffff;                   // Set to reload immediately
ET2 = 1;                         // Enable Timer2 interrupts
TR2 = 1;                         // Start Timer2
}
//-----------------------------------------------------------------------------
// ADC0_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters : None
//
// Configures ADC0 to make single-ended analog measurements on Port 2 according
// to the values of and .
//
//-----------------------------------------------------------------------------
void ADC0_Init (void)
{
ADC0CN = 0x02;                   // ADC0 disabled, normal tracking,
                                    // conversion triggered on TMR2 overflow
REF0CN = 0x03;                   // Enable internal VREF
AMX0P = PIN_MUX_TABLE[0];          // ADC0 initial positive input = P2.0
AMX0N = 0x1F;                      // ADC0 negative input = GND
                                    // i.e., single ended mode
ADC0CF = ((SYSCLK/3000000)-1)<<3; // Set SAR clock to 3MHz
ADC0CF |= 0x00;                   // Right-justify results
EIE1 |= 0x08;                      // Enable ADC0 EOC interrupt
AD0EN = 1;                         // Enable ADC0
}
//-----------------------------------------------------------------------------
// UART0_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters : None
//
// Configure the UART0 using Timer1, for and 8-N-1.
//
//-----------------------------------------------------------------------------
void UART0_Init (void)
{
SCON0 = 0x10;                      // SCON0: 8-bit variable bit rate
                                    //       level of STOP bit is ignored
                                    //       RX enabled
                                    //       ninth bits are zeros
                                    //       clear RI0 and TI0 bits
if (SYSCLK/BAUDRATE/2/256 < 1) {
   TH1 = -(SYSCLK/BAUDRATE/2);
   CKCON |=  0x08;                // T1M = 1; SCA1:0 = xx
} else if (SYSCLK/BAUDRATE/2/256 < 4) {
   TH1 = -(SYSCLK/BAUDRATE/2/4);
   CKCON &= ~0x0B;                // T1M = 0; SCA1:0 = 01
   CKCON |=  0x01;
} else if (SYSCLK/BAUDRATE/2/256 < 12) {
   TH1 = -(SYSCLK/BAUDRATE/2/12);
   CKCON &= ~0x0B;                // T1M = 0; SCA1:0 = 00
} else if (SYSCLK/BAUDRATE/2/256 < 48) {
   TH1 = -(SYSCLK/BAUDRATE/2/48);
   CKCON &= ~0x0B;                // T1M = 0; SCA1:0 = 10
   CKCON |=  0x02;
} else {
   while (1);                      // Error.  Unsupported baud rate
}
TL1 = TH1;                         // Init Timer1
TMOD &= ~0xF0;                   // TMOD: timer 1 in 8-bit autoreload
TMOD |=  0x20;
TR1 = 1;                         // START Timer1
TI0 = 1;                         // Indicate TX0 ready
}
//-----------------------------------------------------------------------------
// Interrupt Service Routines
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
// Timer2_ISR
//-----------------------------------------------------------------------------
//
// This routine changes to the next Analog MUX input whenever Timer2 overflows
// for the next ADC sample.  This allows the ADC to begin setting on the new
// input while converting the old input.
//
void Timer2_ISR (void) interrupt 5
{
TF2H = 0;                         // Clear Timer2 interrupt flag
// Set up the AMUX for the next ADC input
// ADC0 positive input = P2.
// ADC0 negative input = GND
// i.e., single ended mode
if (AMUX_INPUT == (ANALOG_INPUTS - 1))
{
   AMX0P = PIN_MUX_TABLE[0];
}
else
{
   AMX0P = PIN_MUX_TABLE[AMUX_INPUT+1];
}
}
//-----------------------------------------------------------------------------
// ADC0_ISR
//-----------------------------------------------------------------------------
//
// This ISR averages samples for each analog MUX input then prints
// the results to the terminal.  The ISR is called after each ADC conversion,
// which is triggered by Timer2.
//
//-----------------------------------------------------------------------------
void ADC0_ISR (void) interrupt 10
{
static unsigned int_dec = INT_DEC;  // Integrate/decimate counter
                                    // A new result is posted when
                                    // int_dec is 0
// Integrate accumulator for the ADC samples from input pins
static long accumulator[ANALOG_INPUTS] = 0x00000000;
unsigned char i;                   // Loop counter

AD0INT = 0;                      // Clear ADC conversion complete
                                    // overflow

accumulator[AMUX_INPUT] += ADC0; // Read the ADC value and add it to the
                                    // running total
// Reset sample counter and if the final input was
// just read
if(AMUX_INPUT == (ANALOG_INPUTS - 1))
{
   int_dec--;                      // Update decimation counter
                                    // when the last of the analog inputs
                                    // is sampled
   if (int_dec == 0)             // If zero, then post the averaged
   {                               // results
      int_dec = INT_DEC;          // Reset counter
      // Copy each averaged ADC0 value into the RESULT array
      for(i = 0; i < ANALOG_INPUTS; i++)
      {
         // Copy averaged values into RESULT
         RESULT = accumulator / int_dec;
         // Reset accumulators
         accumulator = 0x00000000;
      }
   }
   AMUX_INPUT = 0;                // Reset input index back to P2.0
}
// Otherwise, increment the AMUX channel counter
else
{
   AMUX_INPUT++;                   // Step to the next analog mux input
}
}
//-----------------------------------------------------------------------------
// Support Subroutines
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
// Timer0_Init
//-----------------------------------------------------------------------------
//
// Return Value : None
// Parameters :
// 1) int ms - number of milliseconds to wait
//                      range is positive range of an int: 0 to 32767
//
// This function configures the Timer0 as a 16-bit timer, interrupt enabled.
// Using the internal osc. at 12MHz with a prescaler of 1:8 and reloading the
// T0 registers with TIMER0_RELOAD_HIGH/LOW, it will wait for
// milliseconds.
// Note: The Timer0 uses a 1:12 prescaler
//-----------------------------------------------------------------------------
void Timer0_wait(int ms)
{
TH0 = TIMER0_RELOAD_HIGH;          // Init Timer0 High register
TL0 = TIMER0_RELOAD_LOW ;          // Init Timer0 Low register
TMOD |= 0x01;                      // Timer0 in 16-bit mode
CKCON &= 0xFC;                   // Timer0 uses a 1:12 prescaler
TR0  = 1;                         // Timer0 ON
while(ms)
{
   TF0 = 0;                      // Clear flag to initialize
   while(!TF0);                   // Wait until timer overflows
   ms--;                         // Decrement ms
}
TR0 = 0;                         // Timer0 OFF
}
//-----------------------------------------------------------------------------
// End Of File
//-----------------------------------------------------------------------------

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