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F_CPU=20M,ATMEGA168
/* ----------------------- Defines ------------------------------------------*/
#define MB_TIMER_PRESCALER ( 1024UL )
#define MB_TIMER_TICKS ( F_CPU / MB_TIMER_PRESCALER )
#define MB_50US_TICKS ( 20000UL ) //这个20000怎么来的？
/* ----------------------- Static variables ---------------------------------*/
static USHORT usTimerOCRADelta;
static USHORT usTimerOCRBDelta;
/* ----------------------- Start implementation -----------------------------*/
BOOL
xMBPortTimersInit( USHORT usTim1Timerout50us ) //
{
/* Calculate overflow counter an OCR values for Timer1. */
usTimerOCRADelta =
( MB_TIMER_TICKS * usTim1Timerout50us ) / ( MB_50US_TICKS );
TCCR1A = 0x00;
TCCR1B = 0x00;
TCCR1C = 0x00;
vMBPortTimersDisable( );
return TRUE;
}
/* ----------------------- Start implementation -----------------------------*/
eMBErrorCode
eMBRTUInit( UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{
eMBErrorCode eStatus = MB_ENOERR;
ULONG usTimerT35_50us;
( void )ucSlaveAddress;
ENTER_CRITICAL_SECTION( );
/* Modbus RTU uses 8 Databits. */
if( xMBPortSerialInit( ucPort, ulBaudRate, 8, eParity ) != TRUE )
{
eStatus = MB_EPORTERR;
}
else
{
/* If baudrate > 19200 then we should use the fixed timer values
* t35 = 1750us. Otherwise t35 must be 3.5 times the character time.
*/
if( ulBaudRate > 19200 )
{
usTimerT35_50us = 35; /* 1800us. */
}
else
{
/* The timer reload value for a character is given by:
*
* ChTimeValue = Ticks_per_1s / ( Baudrate / 11 )
* = 11 * Ticks_per_1s / Baudrate
* = 220000 / Baudrate
* The reload for t3.5 is 1.5 times this value and similary
* for t3.5.
*/
usTimerT35_50us = ( 7UL * 220000UL ) / ( 2UL * ulBaudRate );
}
if( xMBPortTimersInit( ( USHORT ) usTimerT35_50us ) != TRUE )
{
eStatus = MB_EPORTERR;
}
}
EXIT_CRITICAL_SECTION( );
return eStatus;
}
这个MB_50US_TICKS 20000怎么来的？ |
阿莫论坛20周年了！感谢大家的支持与爱护！！
曾经有一段真挚的爱情摆在我的面前，我没有珍惜，现在想起来，还好我没有珍惜……
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发表于 2008-4-23 15:14:04
