STM32的CAN波特率計算_轉(zhuǎn)

作者：時間：2016-11-13 來源：網(wǎng)絡(luò)

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STM32里的CAN 支持2.0A,2.0B, 帶有FIFO,中斷等, 這里主要提一下內(nèi)部的時鐘應(yīng)用.

bxCAN掛接在APB1總線上,采用總線時鐘,所以我們需要知道APB1的總線時鐘是多少. 我們先看看下圖,看看APB1總線時鐘:

本文引用地址：http://www.biyoush.com/article/201611/316398.htm

APB1時鐘取自AHB的分頻, 而AHB又取自系統(tǒng)時鐘的分頻, 系統(tǒng)時鐘可選HSI,HSE, PLLCLK, 這個在例程的RC設(shè)置里都有的,

然后再看看有了APB1的時鐘后,如何算CAN的總線速率, 先看下圖:

有了上邊的這個圖,基本就清楚了.

總線時鐘MHz (3+TS1+TS2)*(BRP+1)

===================================================

下面是我的計算：

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
關(guān)于分頻系數(shù) 查看 system_stm32f10x.c下面的
static void SetSysClockTo72(void) 函數(shù)
/* HCLK = SYSCLK */
/* PCLK2 = HCLK */
/* PCLK1 = HCLK/2 */

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

=========================================

-----------------------------------------------

====================================================

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 */
撥特率10K，公式：72MHZ/2/200/（1+9+8）=0.01，即10K，和SJA1000測試通過

================================================

120歐姆電阻要加上！??！

哦確實是

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);

總結(jié)一下
Fpclk=36M 時 can波特率為250k 的配置為

/* 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

======================================

的：將can總線波特率設(shè)置為250k
　　
　在官方的can例程上給出了100k 查詢和500k 中斷方式的例子分別設(shè)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時鐘是RCC_APB1PeriphClock，你要注意CAN時鐘頻率
CAN波特率 = RCC_APB1PeriphClock/CAN_SJW+CAN_BS1+CAN_BS2/CAN_Prescaler;
如果CAN時鐘為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數(shù)目 = 1 (固定SYNC_SEG) + 8 (BS1) + 7 (BS2) = 16

如果CAN時鐘是 8 MHz : (8M / 1 ) / 16 = 500K

其中：
1 為分頻系數(shù)
16 為每一位的Tq數(shù)目

為了設(shè)置為 100K, 把分頻系數(shù)改為5即可, BS1 BS2 不變

每一位的Tq數(shù)目 = 1 (固定) + 8 (BS1) + 7 (BS2) = 16
如果CAN時鐘是 8 MHz : (8M / 5 ) / 16 = 100K

如果想得到 1M 的波特率， CAN時鐘仍然是 8 MHz的情況下，分頻系數(shù)不變

應(yīng)該改變 BS1 BS2

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

每一位的Tq數(shù)目 = 1 (固定) + 5 (BS1) + 2 (BS2) = 8
如果CAN時鐘是 8 MHz : (8M / 1 ) / 8 = 1000K

另外盡可能的把采樣點設(shè)置為 CiA 推薦的值：

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

所以對于 100K 的波特率（假定使用 8MHz 時鐘）
可以修改該BS1 BS2 為：

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%

所以對于 500K 的波特率（假定使用 8MHz 時鐘）
可以修改該BS1 BS2 為：

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%

所以對于 1000K 的波特率（假定使用 8MHz 時鐘）
可以修改該BS1 BS2 為：

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%

個人見解, 僅供參考。

上邊這個公式算出來的就是CAN的速率了.

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STM32的CAN波特率計算_轉(zhuǎn)

得到500Kb/s的波特率

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