STM32 CAN 波特率的计算

STM32里的CAN 支持2.0A,2.0B, 带有FIFO,中断等, 这里主要提一下内部的时钟应用.

bxCAN挂接在APB1总线上,采用总线时钟,所以我们需要知道APB1的总线时钟是多少. 我们先看看下图,看看APB1总线时钟:

APB1时钟取自AHB的分频, 而AHB又取自系统时钟的分频, 系统时钟可选HSI,HSE, PLLCLK, 这个在例程的RC设置里都有的,

然后再看看有了APB1的时钟后,如何算CAN的总线速率, 先看下图:

有了上边的这个图,基本就清楚了.

                       总线时钟MHz   (3+TS1+TS2)*(BRP+1)

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下面是我的计算：

CAN_InitStructure.CAN_SJW = CAN_SJW_1tq;
CAN_InitStructure.CAN_BS1 = CAN_BS1_3tq; 

注意//#define CAN_BS1_3tq                 ((uint8_t)0x02) /*!< 3 time quantum */
CAN_InitStructure.CAN_BS2 = CAN_BS2_5tq;
CAN_InitStructure.CAN_Prescaler = 4;//2
nominal bit time（3+5+1）tq=9tq
关于分频系数 查看 system_stm32f10x.c下面的
static void SetSysClockTo72(void) 函数
/* HCLK = SYSCLK */
/* PCLK2 = HCLK */
/* PCLK1 = HCLK/2 */

所以can时钟 72MHZ/2/4=9 Mhz
tq=1/36Mhz
波特率为 1/nominal bit time= 9/9=1MHZ 

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void CAN_Configuration(void) 
{ 
CAN_InitTypeDef         CAN_InitStructure; 
CAN_FilterInitTypeDef   CAN_FilterInitStructure; 

/* CAN register init */ 
CAN_DeInit(); 
CAN_StructInit(&CAN_InitStructure); 

/* 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_Normal; 
CAN_InitStructure.CAN_SJW=CAN_SJW_1tq; 
CAN_InitStructure.CAN_BS1=CAN_BS1_9tq; 
CAN_InitStructure.CAN_BS2=CAN_BS2_8tq; 
CAN_InitStructure.CAN_Prescaler=200; 
CAN_Init(&CAN_InitStructure); 

/* CAN filter init */ 
CAN_FilterInitStructure.CAN_FilterNumber=0; 
CAN_FilterInitStructure.CAN_FilterMode=CAN_FilterMode_IdMask; 
CAN_FilterInitStructure.CAN_FilterScale=CAN_FilterScale_16bit; 
CAN_FilterInitStructure.CAN_FilterIdHigh=0x0000; 
CAN_FilterInitStructure.CAN_FilterIdLow=0x0000; 
CAN_FilterInitStructure.CAN_FilterMaskIdHigh=0x0000; 
CAN_FilterInitStructure.CAN_FilterMaskIdLow=0x0000; 
CAN_FilterInitStructure.CAN_FilterFIFOAssignment=0; 
CAN_FilterInitStructure.CAN_FilterActivation=ENABLE; 
CAN_FilterInit(&CAN_FilterInitStructure); 
} 
注意//#define CAN_BS1_3tq                 ((uint8_t)0x02) /*!< 3 time quantum */
拨特率10K，公式：72MHZ/2/200/（1+9+8）=0.01，即10K，和SJA1000测试通过   

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120欧姆电阻要加上！！！ 

哦   确实是 

      CAN->BTR = (u32)((u32)CAN_InitStruct->CAN_Mode << 30) | ((u32)CAN_InitStruct->CAN_SJW << 24) | 
                ((u32)CAN_InitStruct->CAN_BS1 << 16) | ((u32)CAN_InitStruct->CAN_BS2 << 20) | 
                ((u32)CAN_InitStruct->CAN_Prescaler - 1); 

   总结一下 
   Fpclk=36M 时 can波特率为250k   的配置为 
   
/* 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=9; 
   CAN_Init(&CAN_InitStructure); 250k 

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的：将can总线波特率设置为250k 
   　　 
　在官方的can例程上 给出了100k 查询 和500k 中断方式的例子 分别设置如下： 
   CAN_Polling： 
   /* 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 

can时钟是RCC_APB1PeriphClock，你要注意CAN时钟频率 
CAN波特率 = RCC_APB1PeriphClock/CAN_SJW+CAN_BS1+CAN_BS2/CAN_Prescaler; 
如果CAN时钟为8M， CAN_SJW = 1，CAN_BS1 = 8，CAN_BS2 = 7，CAN_Prescaler = 2 
那么波特率就是=8M/(1+8+7)/2=250K 

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得到500Kb/s的波特率

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;

每一位的Tq数目 = 1 (固定SYNC_SEG) +   8 (BS1) + 7 (BS2) = 16

如果CAN时钟是 8 MHz : (8M / 1 ) / 16 = 500K

其中：
1 为分频系数
16 为每一位的Tq数目

为了设置为 100K, 把分频系数改为5即可, BS1 BS2 不变

每一位的Tq数目 = 1 (固定) +   8 (BS1) + 7 (BS2) = 16
如果CAN时钟是 8 MHz : (8M / 5 ) / 16 = 100K

如果想得到 1M 的波特率， CAN时钟仍然是 8 MHz的情况下， 分频系数不变

应该改变 BS1 BS2

CAN_InitStructure.CAN_BS1=CAN_BS1_5tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_2tq;

每一位的Tq数目 = 1 (固定) +   5 (BS1) + 2 (BS2) = 8
如果CAN时钟是 8 MHz : (8M / 1 ) / 8 = 1000K




另外尽可能的把采样点设置为 CiA 推荐的值：

75%      when 波特率 > 800K
80%      when 波特率 > 500K
87.5%    when 波特率 <= 500K

所以对于 100K 的波特率（假定使用 8MHz 时钟） 
可以修改该BS1 BS2 为：

CAN_InitStructure.CAN_Prescaler=5;
CAN_InitStructure.CAN_BS1=CAN_BS1_13tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_2tq;

(1+13) / (1+13+2) = 87.5%

所以对于 500K 的波特率（假定使用 8MHz 时钟） 
可以修改该BS1 BS2 为：

CAN_InitStructure.CAN_Prescaler=1;
CAN_InitStructure.CAN_BS1=CAN_BS1_13tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_2tq;

(1+13) / (1+13+2) = 87.5%

所以对于 1000K 的波特率（假定使用 8MHz 时钟） 
可以修改该BS1 BS2 为：

CAN_InitStructure.CAN_Prescaler=1;
CAN_InitStructure.CAN_BS1=CAN_BS1_5tq;
CAN_InitStructure.CAN_BS2=CAN_BS2_2tq;

(1+5) / (1+5+2) = 75%

个人见解, 仅供参考。 

上边这个公式算出来的就是CAN的速率了