STM32的CAN波特率計(jì)算_轉(zhuǎn)
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bxCAN掛接在APB1總線上,采用總線時(shí)鐘,所以我們需要知道APB1的總線時(shí)鐘是多少. 我們先看看下圖,看看APB1總線時(shí)鐘:
本文引用地址：http://cafeforensic.com/article/201611/316398.htm
APB1時(shí)鐘取自AHB的分頻, 而AHB又取自系統(tǒng)時(shí)鐘的分頻, 系統(tǒng)時(shí)鐘可選HSI,HSE, PLLCLK, 這個(gè)在例程的RC設(shè)置里都有的,
然后再看看有了APB1的時(shí)鐘后,如何算CAN的總線速率, 先看下圖:
有了上邊的這個(gè)圖,基本就清楚了.
總線時(shí)鐘MHz (3+TS1+TS2)*(BRP+1)
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下面是我的計(jì)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(shí)鐘 72MHZ/2/4=9 Mhz
tq=1/36Mhz
波特率為 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 */
撥特率10K，公式：72MHZ/2/200/（1+9+8）=0.01，即10K，和SJA1000測(cè)試通過
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120歐姆電阻要加上?。?！
哦 確實(shí)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(shí) 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
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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(shí)鐘是RCC_APB1PeriphClock，你要注意CAN時(shí)鐘頻率
CAN波特率 = RCC_APB1PeriphClock/CAN_SJW+CAN_BS1+CAN_BS2/CAN_Prescaler;
如果CAN時(shí)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
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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數(shù)目 = 1 (固定SYNC_SEG) + 8 (BS1) + 7 (BS2) = 16 如果CAN時(shí)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(shí)鐘是 8 MHz : (8M / 5 ) / 16 = 100K 如果想得到 1M 的波特率， CAN時(shí)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(shí)鐘是 8 MHz : (8M / 1 ) / 8 = 1000K 另外盡可能的把采樣點(diǎn)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(duì)于 100K 的波特率（假定使用 8MHz 時(shí)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(duì)于 500K 的波特率（假定使用 8MHz 時(shí)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(duì)于 1000K 的波特率（假定使用 8MHz 時(shí)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(gè)人見解, 僅供參考。 |
上邊這個(gè)公式算出來的就是CAN的速率了.
評(píng)論