STM32 CAN 波特率、采样点设置
为了总结在STM32上设置CAN波特率和采样点，所以整理自己的理解如下：
一，CAN波特率
1.CAN位时序
2.公式推导
这里Tpclk理解成CAN时钟的周期（实际上CAN使用APB1时钟）
根据图中公式有：
(1) 正常的位时间（Tnbt） = （1+Tbs1 + Tbs2）*Tq
(2) Tq = (BRP+1)*Tpclk
设CAN时钟频率为Fpclk，则有
(3) Tq = (BRP+1)/Fpclk
由(1),(3)有：
(4) Tnbt =（1+Tbs1 + Tbs2）* (BRP+1) / Fpclk
由(4)知CAN输出波特率(Fcan = 1 / Tnbt )为：
Fcan = Fpclk / ( （1+Tbs1 + Tbs2）* (BRP[9:0]+1) )
3.程序配置
a.预备知识：
在stm32手册中BTR寄存器内容如下：
公式：Fcan = Fpclk / ( （1+Tbs1 + Tbs2）* (BRP[9:0]+1) )结合上表有：
Fcan = Fpclk / ( (1 + (TS1[3:0] + 1) + (TS2[2:0] + 1) ) * (BRP[9:0]+1))
在 stm32f10x.h中有：
/******************* Bit definition for CAN_BTR register ********************/
#define CAN_BTR_BRP ((uint32_t)0x000003FF) /*!< Baud Rate Prescaler */
#define CAN_BTR_TS1 ((uint32_t)0x000F0000) /*!< Time Segment 1 */
#define CAN_BTR_TS2 ((uint32_t)0x00700000) /*!< Time Segment 2 */
#define CAN_BTR_SJW ((uint32_t)0x03000000) /*!< Resynchronization Jump Width */
#define CAN_BTR_LBKM ((uint32_t)0x40000000) /*!< Loop Back Mode (Debug) */
#define CAN_BTR_SILM ((uint32_t)0x80000000) /*!< Silent Mode */
在stm32f10x_can.h中有：
#define CAN_BS1_1tq ((uint8_t)0x00) /*!< 1 time quantum */
#define CAN_BS1_2tq ((uint8_t)0x01) /*!< 2 time quantum */
...
#define CAN_BS2_1tq ((uint8_t)0x00) /*!< 1 time quantum */
#define CAN_BS2_2tq ((uint8_t)0x01) /*!< 2 time quantum */
...
由上可知表示值比实际值大1，Tbs1 = Tq*(TS1[3:0] + 1) 等价于 Tbs1 = Tq*CAN_BS1_xtq，CAN_BS1_xtq即为表示值
在stm32f10x_can.c中CAN_Init接口中设置BTR部分有：
/* Set the bit timing register */
CANx->BTR = ( uint32_t )(( uint32_t )CAN_InitStruct->CAN_Mode << 30 ) | \
(( uint32_t )CAN_InitStruct->CAN_SJW << 24 ) | \
(( uint32_t )CAN_InitStruct->CAN_BS1 << 16 ) | \
(( uint32_t )CAN_InitStruct->CAN_BS2 << 20 ) | \
(( uint32_t )CAN_InitStruct->CAN_Prescaler - 1 );
由上边代码可知BTR的BRP[9:0]的值为CAN_Prescaler - 1
从2中所推导的公式可知：
CAN波特率 = CAN时钟/( (1 + CAN_BS1 + CAN_BS2) * CAN_Prescaler)
举个例子：
/* CAN cell init */
CAN_InitStructure.CAN_TTCM=DISABLE;
CAN_InitStructure.CAN_ABOM=DISABLE;
CAN_InitStructure.CAN_AWUM=DISABLE;
CAN_InitStructure.CAN_NART=DISABLE;
CAN_InitStructure.CAN_RFLM=DISABLE;
CAN_InitStructure.CAN_TXFP=DISABLE;
CAN_InitStructure.CAN_Mode=CAN_Mode_LoopBack;
CAN_InitStructure.CAN_SJW=CAN_SJW_1tq;
CAN_InitStructure.CAN_BS1=CAN_BS1_8tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_7tq;
CAN_InitStructure.CAN_Prescaler=5;
CAN_Init(&CAN_InitStructure);100k
/* CAN cell init */CAN_Interrupt
CAN_InitStructure.CAN_TTCM=DISABLE;
CAN_InitStructure.CAN_ABOM=DISABLE;
CAN_InitStructure.CAN_AWUM=DISABLE;
CAN_InitStructure.CAN_NART=DISABLE;
CAN_InitStructure.CAN_RFLM=DISABLE;
CAN_InitStructure.CAN_TXFP=DISABLE;
CAN_InitStructure.CAN_Mode=CAN_Mode_LoopBack;
CAN_InitStructure.CAN_SJW=CAN_SJW_1tq;
CAN_InitStructure.CAN_BS1=CAN_BS1_8tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_7tq;
CAN_InitStructure.CAN_Prescaler=1;
CAN_Init(&CAN_InitStructure);//500k
如果CAN时钟为8M， CAN_BS1 = 8，CAN_BS2 = 7，CAN_Prescaler = 5
那么波特率就是=8M/(1+8+7)/5=100K
二，CAN采样点
有上面可知得到相同的波特率 CAN_BS1，CAN_BS2会有多种组合，而采样点由这2个参数确定，计算公式为：
sample = ( 1 + CAN_BS1) / (1 + CAN_BS1 + CAN_BS2)
参看这篇文章，采样点设置在80%到80.75%之间比较好。
参考资料：
http://lib.csdn.net/article/embeddeddevelopment/29438
http://www.61ic.com/Technology/embed/201103/31046.html
《ISO 11898-1-2003》
《STM32_RM_CH_V10_1》