基于STM32的CAN总线的数据采集卡设计(3)

2019-04-02 14:31

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，要加上 120欧姆电阻。

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);

Fpclk=36M 时can波特率250k的配置为

/* CAN cell init */

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); 100k 查询和500k 中断方式的例子分别设置如下： CAN_Polling： /* CAN cell init */

/* 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时钟是RCC_APB1PeriphClock，需注意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 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8MHz : (8M/5)/16 = 100K

如果想得到1M 的波特率，CAN时钟仍然是 8MHz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%

上边这个公式算出来的就是CAN的速率了

3硬件电路设计

数据采集卡整体硬件程序框图如图3.1所示，由信息调理电路，STM32芯片（内含CAN总线控制器），CAN总线收发单元组成

图3.1

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