你好,

這其中的玄機就在於 CANCON 暫存器的 bit 1 .. bit 3 啦.
因為 PIC18F 為了要有效率的存取各組不同的 buffer (包括 RXB0 , RXB1 , TXB0..TXB2 ) , 就採取了將大家都可以被 Mapping 到 RXB0 的機制 , 此機制由 CANCON 的 bit 1..3 控制.
所以在程式的一開頭有先對 CANCON bit 1..3 做 & and | 的動作 , 就是要將指定的 TX Buffer Mapping 到 RXB0 的區間. 如此一來就可以較快的 Access TX Buffer 的資料了 ( RXB0 的位址位於 PIC18F 的 Access Buffer 區間 , 最有效率 )

希望您的問題能因此有所解決 :)

各位好
我在適用 can-bus 的部分
在看附上的 can18xx8.c 的程式裡面
我發現在 CanSendMessage 這個 function 裡面
都再利用一些 RXB0CON 的 buffer

我想問的，在傳出資料，應該要動的是 TXB0CON 而不是 RX

以下是程式碼

BOOL CANSendMessage(unsigned long id,
BYTE* Data,
BYTE DataLen,
enum CAN_TX_MSG_FLAGS MsgFlags)
{
BYTE i;
BYTE *ptr;

// Find the first empty transmitter.
if ( TXB0CON_TXREQ == 0 )
{
// TxBuffer0 is empty. Set WIN bits to point to TXB0
CANCON &= 0b11110001;
CANCON |= 0b00001000;
}
else if ( TXB1CON_TXREQ == 0 )
{
// TxBuffer1 is empty. Set WIN bits to point to TXB1
CANCON &= 0b11110001;
CANCON |= 0b00000110;
}
else if ( TXB2CON_TXREQ == 0 )
{
// TxBuffer2 is empty. Set WIN bits to point to TXB2
CANCON &= 0b11110001;
CANCON |= 0b00000100;
}
else
// None of the transmit buffers were empty.
return FALSE;

/*
* Now that WIN has remapped RXB0 to empty buffer, simply
* populate RXB0 buffer
*/

// Set transmit priority.
RXB0CON = MsgFlags & CAN_TX_PRIORITY_BITS;

// Populate Extended identifier information only if it is
// desired.
if ( !(MsgFlags & CAN_TX_FRAME_BIT) )
CANIDToRegs((BYTE*)&RXB0SIDH, id, CAN_CONFIG_XTD_MSG);
else
CANIDToRegs((BYTE*)&RXB0SIDH, id, CAN_CONFIG_STD_MSG);

RXB0DLC = DataLen;

if ( !(MsgFlags & CAN_TX_RTR_BIT) )
RXB0DLC |= 0b01000000;

// Populate data values.
ptr = (BYTE*)&RXB0D0;
for ( i = 0; i < DataLen; i++ )
ptr[i] = Data[i];

/*
* Mark this buffer as ready to start transmit.
* We are not using C bit field structure because RXB0 registers
* are remapped to one of the empty transmit buffers and their
* bit3 is not same as RXB0CON bit3. To avoid confusion, in-line
* assembly is used to directly set bit 3 of corresponding TXBnCON
* register.
*/
#if defined(MCHP_C18)
_asm
bsf RXB0CON, 3, 0
_endasm
#endif
#if defined(HITECH_C18)
asm("bsf _RXB0CON,3");
#endif

/*
* Restore CAN buffer mapping so that subsequent access to RXB0
* buffers are to the real RXB0 buffer.
*/
CANCON &= 0b11110001;

return TRUE;
}


我用紅色的部分標示我說的地方
謝謝