UART 接受数据对FIFO处理技巧

1. 理由8bit数的溢出效果和FIFO长度2^n
   #define ILEN 8 // size of serial receiving buffer
   unsigned char istart; // receiving buffer start index
   unsigned char iend; // receiving buffer end index
   char idata inbuf[ILEN]; // storage for receiving buffer
2. 写入处理
   //----- Received data interrupt. -----
   if (RI) {
   c = SBUF; // read character
   RI = 0; // clear interrupt request flag
   if (istart + ILEN != iend) {
   inbuf[iend++ & (ILEN-1)] = c; // put character into buffer
   }
   }
   istart 和 iend 之间的差就是接收到的数据长度，所以可以使用 if (istart + ILEN != iend)来判定FIFO是不是填满。
3. 【iend++ & (ILEN-1)】可以巧妙的使用 取低n位来处理FIFO的正确位置。
4. 取数据时，判断iend和istart是不是相等就可以决定FIFO是不是为空
   while (iend == istart) {
   ; // wait until there are characters
   }
   ES = 0; // disable serial interrupts during buffer update
   c = inbuf[istart++ & (ILEN-1)];
   ES = 1;

附 keil help中UART的串口FIFO处理程序：

#include
#include

#define XTAL 11059200 // CPU Oscillator Frequency
#define baudrate 9600 // 9600 bps communication baudrate

#define OLEN 8 // size of serial transmission buffer
unsigned char ostart; // transmission buffer start index
unsigned char oend; // transmission buffer end index
char idata outbuf[OLEN]; // storage for transmission buffer

#define ILEN 8 // size of serial receiving buffer
unsigned char istart; // receiving buffer start index
unsigned char iend; // receiving buffer end index
char idata inbuf[ILEN]; // storage for receiving buffer

bit sendfull; // flag: marks transmit buffer full
bit sendactive; // flag: marks transmitter active

/*--------------------------------------------------------------------------------

• Serial Interrupt Service Routine
  ------------------------------------------------------------------------------/
  static void com_isr (void) interrupt 4 using 1 {
  char c;

//----- Received data interrupt. -----
if (RI) {
c = SBUF; // read character
RI = 0; // clear interrupt request flag
if (istart + ILEN != iend) {
inbuf[iend++ & (ILEN-1)] = c; // put character into buffer
}
}

//----- Transmitted data interrupt. -----
if (TI != 0) {
TI = 0; // clear interrupt request flag
if (ostart != oend) { // if characters in buffer and
SBUF = outbuf[ostart++ & (OLEN-1)]; // transmit character
sendfull = 0; // clear ‘sendfull’ flag
}
else { // if all characters transmitted
sendactive = 0; // clear ‘sendactive’
}
}
}

/*--------------------------------------------------------------------------------

• Function to initialize the serial port and the UART baudrate.
  ------------------------------------------------------------------------------/
  void com_initialize (void) {
  istart = 0; // empty transmit buffers
  iend = 0;
  ostart = 0; // empty transmit buffers
  oend = 0;
  sendactive = 0; // transmitter is not active
  sendfull = 0; // clear ‘sendfull’ flag

                                          // Configure timer 1 as a baud rate generator
  

PCON |= 0x80; // 0x80=SMOD: set serial baudrate doubler
TMOD |= 0x20; // put timer 1 into MODE 2

TH1 = (unsigned char) (256 - (XTAL / (16L * 12L * baudrate)));

TR1 = 1; // start timer 1

SCON = 0x50; // serial port MODE 1, enable serial receiver
ES = 1; // enable serial interrupts
}

/*--------------------------------------------------------------------------------

• putbuf: write a character to SBUF or transmission buffer
  ------------------------------------------------------------------------------/
  void putbuf (char c) {
  if (!sendfull) { // transmit only if buffer not full
  if (!sendactive) { // if transmitter not active:
  sendactive = 1; // transfer first character direct
  SBUF = c; // to SBUF to start transmission
  }
  else {
  ES = 0; // disable serial interrupts during buffer update
  outbuf[oend++ & (OLEN-1)] = c; // put char to transmission buffer
  if (((oend ^ ostart) & (OLEN-1)) == 0) {
  sendfull = 1;
  } // set flag if buffer is full
  ES = 1; // enable serial interrupts again
  }
  }
  }

/*--------------------------------------------------------------------------------

• Replacement routine for the standard library putchar routine.
• The printf function uses putchar to output a character.
  ------------------------------------------------------------------------------/
  char putchar (char c) {
  if (c == ‘\n’) { // expand new line character:
  while (sendfull); // wait until there is space in buffer
  putbuf (0x0D); // send CR before LF for
  }
  while (sendfull); // wait until there is space in buffer
  putbuf ©; // place character into buffer
  return ©;
  }

/*--------------------------------------------------------------------------------

• Replacement routine for the standard library _getkey routine.
• The getchar and gets functions uses _getkey to read a character.
  ------------------------------------------------------------------------------/
  char _getkey (void) {
  char c;
  while (iend == istart) {
  ; // wait until there are characters
  }
  ES = 0; // disable serial interrupts during buffer update
  c = inbuf[istart++ & (ILEN-1)];
  ES = 1; // enable serial interrupts again
  return ©;
  }

/*--------------------------------------------------------------------------------

• Main C function that start the interrupt-driven serial I/O.
  ------------------------------------------------------------------------------/
  void main (void) {
  EA = 1; // enable global interrupts
  com_initialize (); // initialize interrupt driven serial I/O
  while (1) {
  char c;
  c = getchar ();
  printf ("\nYou typed the character %c.\n", c);
  }
  }