MCU_STM32的位带操作 -- bit banding

超、凢脫俗 2023-06-20 06:19 20阅读 0赞

## 位带操作 ##

STM32芯片除了通用的寄存器访问，还有一个比较有意思的位带操作。

这个位带的意思，就是每个比特(bit)位膨胀成一个32位的字(word)，当访问这些字的时候就达到了访问“位”的目的，这就是位带操作！

位带操作一个典型的特点，就是把1个位映射到32位，在程序处理中方便操作具体位，典型的如RCC寄存器，需要逐位操作的地方比较多，直接寄存器操作的话，程序的可读性不强；

位带操作的缺点也很明显，就是在时间上开销也要稍大一些，因为不可避免要对位带映射进行计算。

Cortex-M4的手册中有这样一个例子：

参考地址：[http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0337h/Behcjiic.html][http_infocenter.arm.com_help_index.jsp_topic_com.arm.doc.ddi0337h_Behcjiic.html]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Rhbm14MjE5_size_16_color_FFFFFF_t_70][]

原文的映射解释是这样的，

The alias word at 0x23FFFFE0 maps to bit \[0\] of the bit-band byte at 0x200FFFFF: 0x23FFFFE0 = 0x22000000 + (0xFFFFF\**32) + 0\**4.

The alias word at 0x23FFFFFC maps to bit \[7\] of the bit-band byte at 0x200FFFFF: 0x23FFFFFC = 0x22000000 + (0xFFFFF\**32) + 7\**4.

The alias word at 0x22000000 maps to bit \[0\] of the bit-band byte at : 0x22000000 = 0x22000000 + (0\**32) + 0\**4.

The alias word at 0x2200001C maps to bit \[7\] of the bit-band byte at 0x20000000: 0x2200001C = 0x22000000 + (0\**32) + 7\**4.

解释一下，即：

0x200FFFFF的bit\[0\] 位带字节映射计算: = 0x22000000 + (0xFFFFF\*32) + 0\*4 = 0x23FFFFE0  
0x200FFFFF的bit\[7\] 位带字节映射计算: = 0x22000000 + (0xFFFFF\*32) + 7\*4 = 0x23FFFFFC  
0x20000000的bit\[0\] 位带字节映射计算: = 0x22000000 + (0x0\*32) + 0\*4 = 0x22000000  
0x20000000的bit\[7\] 位带字节映射计算: = 0x22000000 + (0x0\*32) + 7\*4 = 0x2200001C

## 关于程序 ##

我找了个常用的具体的例子，比如这个函数，在STM32F407中是对RCC\_CR中的第19位CSSON进行操作，程序如下，一目了然，

void HAL\_RCC\_EnableCSS(void) \{ \*(\_\_IO uint32\_t \*) RCC\_CR\_CSSON\_BB = (uint32\_t)ENABLE; \}

理解：

首先有关RCC\_CR\_CSSON\_BB的定义是这样的，

\#define PERIPH\_BB\_BASE 0x42000000UL  
\#define AHB1PERIPH\_BASE (PERIPH\_BASE + 0x00020000UL)  
\#define RCC\_BASE (AHB1PERIPH\_BASE + 0x3800UL)  
\#define RCC\_OFFSET (RCC\_BASE - PERIPH\_BASE) //计算结果也就是 = 0x00020000UL + 0x3800UL  
\#define RCC\_CR\_OFFSET (RCC\_OFFSET + 0x00U)  
\#define RCC\_CSSON\_BIT\_NUMBER 0x13U  
\#define RCC\_CR\_CSSON\_BB (PERIPH\_BB\_BASE+(RCC\_CR\_OFFSET \* 32U)+(RCC\_CSSON\_BIT\_NUMBER \* 4U))

### 这些数据去哪里找 ###

这里，PERIPH\_BB\_BASE的映射地址是0x42000000，前面例子中的SRAM映射地址是0x22000000，具体可参考STM32的编程参考手册PM0214，如下图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Rhbm14MjE5_size_16_color_FFFFFF_t_70 1][]

其次，在STM32F405xx/STM32F407xx的datasheet表10中，有说明RCC寄存器地址为0x40023800-0x40023BFF，

所以，RCC\_CR\_CSSON\_BB 的计算就是0x42000000UL + 0x00023800\*32 + 0x13\*4

## 参考资料 ##

STM32F405/407 datasheet下载地址：

[https://www.st.com/resource/en/datasheet/stm32f405og.pdf][https_www.st.com_resource_en_datasheet_stm32f405og.pdf]

STM32F4编程手册（STM32 Cortex®-M4 MCUs and MPUs programming manual）下载地址：

[https://www.st.com/content/ccc/resource/technical/document/programming\_manual/6c/3a/cb/e7/e4/ea/44/9b/DM00046982.pdf/files/DM00046982.pdf/jcr:content/translations/en.DM00046982.pdf][https_www.st.com_content_ccc_resource_technical_document_programming_manual_6c_3a_cb_e7_e4_ea_44_9b_DM00046982.pdf_files_DM00046982.pdf_jcr_content_translations_en.DM00046982.pdf]

Cortex-M3 Technical Reference Manual （Cortex-M3的手册）下载地址：

[http://infocenter.arm.com/help/topic/com.arm.doc.ddi0337e/DDI0337E\_cortex\_m3\_r1p1\_trm.pdf][http_infocenter.arm.com_help_topic_com.arm.doc.ddi0337e_DDI0337E_cortex_m3_r1p1_trm.pdf]

Cortex-M4 Technical Reference Manual （Cortex-M4的手册）下载地址：

[http://infocenter.arm.com/help/topic/com.arm.doc.ddi0439b/DDI0439B\_cortex\_m4\_r0p0\_trm.pdf][http_infocenter.arm.com_help_topic_com.arm.doc.ddi0439b_DDI0439B_cortex_m4_r0p0_trm.pdf]

[http_infocenter.arm.com_help_index.jsp_topic_com.arm.doc.ddi0337h_Behcjiic.html]: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0337h/Behcjiic.html
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[https_www.st.com_resource_en_datasheet_stm32f405og.pdf]: https://www.st.com/resource/en/datasheet/stm32f405og.pdf
[https_www.st.com_content_ccc_resource_technical_document_programming_manual_6c_3a_cb_e7_e4_ea_44_9b_DM00046982.pdf_files_DM00046982.pdf_jcr_content_translations_en.DM00046982.pdf]: https://www.st.com/content/ccc/resource/technical/document/programming_manual/6c/3a/cb/e7/e4/ea/44/9b/DM00046982.pdf/files/DM00046982.pdf/jcr:content/translations/en.DM00046982.pdf
[http_infocenter.arm.com_help_topic_com.arm.doc.ddi0337e_DDI0337E_cortex_m3_r1p1_trm.pdf]: http://infocenter.arm.com/help/topic/com.arm.doc.ddi0337e/DDI0337E_cortex_m3_r1p1_trm.pdf
[http_infocenter.arm.com_help_topic_com.arm.doc.ddi0439b_DDI0439B_cortex_m4_r0p0_trm.pdf]: http://infocenter.arm.com/help/topic/com.arm.doc.ddi0439b/DDI0439B_cortex_m4_r0p0_trm.pdf