STM32 傳輸速率計算

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The baud rate for the receiver and transmitter (Rx and Tx) are both set to the same value

as programmed in the Mantissa and Fraction values of USARTDIV.

從可以看出，該寄存器高 16 位無效，最低 4 位為小數部分，其餘部分為整數部分。

這樣的設計可以使傳輸速率更加精確。關于波特率的產生，有這麼一段話來解釋：

分數傳輸速率的產生:  接收器和發送器（RX和TX）都是設定成為 USARTDIV 整數和小數寄存器中配置的值。 

This give the following equation for baud rate:

Tx/Rx baud = CK_APB1 / (8 x (2 - OVER8) x USARTDIV)

Where USARTDIV is an unsigned fixed point number that is coded on the USART_BRR register.

• When OVER8=0, the fractional part is coded on 4 bits and programmed by the DIV_fraction[3:0] bits in the USART_BRR register
• Tx/Rx baud = CK_APB1 / (16 x USARTDIV)
• When OVER8=1, the fractional part is coded on 3 bits and programmed by the DIV_fraction[2:0] bits in the USART_BRR register,
  and bit DIV_fraction[3] must be kept cleared.
• Tx/Rx baud = CK_APB1 / (8  x USARTDIV)

The baud counters are updated to the new value in the baud registers after a write operation to USART_BRR.

Hence the baud rate register value should not be changed during communication.

 

The USARTs are on the APB1 bus, Figure 12 in the reference manual is the clock tree,
which shows how the APB1 clock is derived from the PLL clock.
The maximum APB1 clock is 32MHz.

OVER8=1 is required for higher speeds, giving: baud = 32000000 / (USARTDIV x 8).

So USARTDIV = 32000000 / (baud x 8)

For 4Mbps therefore, USARTDIV=1 (see table 138 S.No.12 for details).

For 2Mps, USARTDIV=2.

To achieve 3Mbps you will have to reduce the APB1 clock to 24MHz and set USARTDIV=1 (see table 131).

But note that the clock rate changes for all other APB1 peripherals too.

The simplest way to correctly program the USART baud rate is via the STM32L1xx standard peripheral library.

Also to determine the correct peripheral clock settings (and more), and generate initialisation code,

you can use STM‘s MicroXplorer tool.

Fpclk = 12 MHz, Baud = 460800 Bps : OVER8 = 0

DIV = 12000000 / ( 16 * 460800 ) = 1.6276

DIV_Mantissa = 1

DIV_Fraction = 0.6276 * 16 = 10

USARTDIV = ( 1 << 4 ) | 10 = 0x001 A = 1.625

12000000 / ( 16 * 1.625 ) = 12000000 / 26 = 461538.5 Bps

 

Fpclk = 12 MHz, Baud = 460800 Bps : OVER8 = 1

DIV = 12000000 / ( 8 * 460800 ) = 3.255

DIV_Mantissa = 3

DIV_Fraction = 0.255 * 8 = 2.04

USARTDIV = ( 3 << 4 ) | 2 = 0x003 2 = 3.25

12000000 / ( 8 * 3.25 ) = 12000000 / 26 = 461538.5 Bps

 

stm32傳輸速率設定，在115200時候，實際是115384，會有0.15%的誤差， 不過還是可以接受的。

其實stm32的uart還是蠻簡單的，初始化4個寄存器完事，就是傳輸速率方面需要算算。 原文：

stm32 中文資料中有如下計算公式 ：

Tx / Rx 傳輸速率 ＝ fPCLKx/（16*USARTDIV）；

這裡的fPCLKx(x=1、2)是給外設的時鐘(PCLK1用於USART2、3、4、5，PCLK2用於USART1)

USARTDIV是一個無符號的定點數。這12位的值設定在USART_BRR寄存器。 

如果使用USART1,系統時鐘為72MHZ,USART1使用PCLK2時鐘，也定義為72MHz.

定義傳輸速率=9600，fPCLK2=72MHz,則： 計算USARTDIV=72MHz/9600/16=468.75

取整數468=0x1D4. 小數0.75*16=12=0x0C. 所以寫入寄存器USART_BRR中的值為：USART_BRR=0x1D4C. 

如果使用USART2,USART2使用PCLK1時鐘,PCLK1時鐘為36MHz.

定義傳輸速率=9600，fPCLK1=36MHz,則： 計算USARTDIV=36MHz/9600/16=234.375

取整數234=0xEA.小數0.375*16=6=0x06.所以寫入寄存器USART_BRR中的值為：USART_BRR=0xEA6.

24000000 / 19200 = 1250 = 0x4E2 --> 78 + 2/16 = 78.12524000000 / ( 16 * 19200 ) = 78.12524000000 / 921600 = 26 = 0x1A --> 1 + 10/16 = 1.62524000000 / ( 16 * 921600 ) = 1.625

    uint32_t tmpreg = 0x00, apbclock = 0x00;
    uint64_t integerdivider = 0x00;
    uint32_t fractionaldivider = 0x00;

  /* Determine the integer part */  if ((USARTx->CR1 & USART_CR1_OVER8) != 0)  {    /* Integer part computing in case Oversampling mode is 8 Samples */    integerdivider = ((25 * apbclock) / (2 * (USART_InitStruct->USART_BaudRate)));    

    // ( 2^^32 - 1 ) / 25 = 4294967295 / 25 = 171798691.84 = 171.8 MHZ
    // STM32F429 MAX 180MHz
  }  else /* if ((USARTx->CR1 & USART_CR1_OVER8) == 0) */  {    /* Integer part computing in case Oversampling mode is 16 Samples */    integerdivider = ((25 * apbclock) / (4 * (USART_InitStruct->USART_BaudRate)));      }  tmpreg = (integerdivider / 100) << 4;  /* Determine the fractional part */  fractionaldivider = integerdivider - (100 * (tmpreg >> 4));  /* Implement the fractional part in the register */  if ((USARTx->CR1 & USART_CR1_OVER8) != 0)  {    tmpreg |= ((((fractionaldivider * 8) + 50) / 100)) & ((uint8_t)0x07);  }  else /* if ((USARTx->CR1 & USART_CR1_OVER8) == 0) */  {    tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);  }    /* Write to USART BRR register */  USARTx->BRR = (uint16_t)tmpreg;

(1)將算出的USARTDIV擴大100倍保留整數部分。

(2)百位以上的送入BRR[15:4],百位以下的換算成16進位值送入[3:0]

也即對於USARTDIV小數部分，只有小數點後兩位保留，兩位以後的全部捨去，

比如理論計算得到 USARTDIV = 234.28125 被處理為 USARTDIV = 234.28，這樣就損失了精度，

造成了最終送入BRR的實際值並不是理論值的四捨五入(5以上的有可能也被捨去)

如想要得到完全符合四捨五入原則的精確結果，則USART至少應該放大 100000 倍，

保留小數點後5位，5位以後的部分全部捨去也仍然落在正確的區間內。新程式如下：

     baudsource = apbclock * 100000 / ( 16 * BaudRate );     interger = baudsource / 100000;     fractional = ( baudsource % 100000 + 3125 ) / 6250;     USART1->BRR = ( interger << 4 ) + fractional;

以一個例子說明：  如設定傳輸速率為Baud Rate = 19207 bps

72000000 / ( 16 * 19207 ) = 234.28958192325714583224865934295...

 (1) BRR[3:0]理論值：16 * 0.28958192325714583224865934295... = 4.6333107721143333159785494871661... 

按四捨五入的原則取 BRR[3:0] = 0x05

(2)官方庫：( 28 * 16 + 50 ) / 100 = 498 / 100 = 4, BRR[3:0] = 0x04 與理論值差1

(3) 新程式：( 28958 + 3125 ) / 6250 = 32173 / 6250 = 5, BRR[3:0] = 0x05，與理論值一致。 

STM32的串口速率：APB速度/傳輸速率 = USARTDIV [ OVER8 = 0 ]

uint32_t brr( uint32_t clock, uint32_t baud ){  uint64_t clock_x_100000 = ( clock * 10000 );  return ( ( clock_x_100000 / baud ) + 5000 ) / 10000;}

72000000 * 10000 / 19207 = 37486333

37486333 + 5000 = 37491333 3

7491333 / 10000 = 3749 = 0x0EA5 --> 234.3125

72000000 / ( 16 * 234.3125 ) = 19205 

 

STM32 傳輸速率計算