SPI接口的工作原理.docx

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1、SPI接口的工作原理发布：|作者：|来源：mahuaxiao |查看：1979次|用户关注:MAX7456随屏显示(OSD)发生器具有SPP兼容接口，本应用笔记介绍了 SPI接口的工作原理，文中还 包含在微控制器内逐位摹拟SPI接口的控制器C程序。MAX7456串行接口 MAX7456单通道单色随屏显 示(OSD)发生器预装了 256个字符和图形，并可通过SPI接口在线编程。通过SPI兼容串行接口可以 设置工作模式、显示存储器以及字符存储器。状态(STAT)寄存器、显示存储器数据输出(DMDO)寄存器 和字符存储器数据输出(CMDO)寄存器都可读，可以对其进行MAX7456随屏显示(OSD)发

2、生器具有SPF兼容接口，本应用笔记介绍了 SPI接口的工作原理，文中还 包含在微控制器内逐位摹拟SPI接口的控制器C程序。MAX7456串行接口MAX7456单通道单色随屏显示(OSD)发生器预装了 256个字符和图形，并可通过SPI接口在线编程。 通过SPI兼容串行接口可以设置工作模式、显示存储器以及字符存储器。状态(STAT)寄存器、显示存 储器数据输出(DMDO)寄存器和字符存储器数据输出(CMDO)寄存器都可读，可以对其进行写操作和读操 作。关于MAX7456寄存器及存储器结构的详细信息请参考数据资料和应用笔记4117, 使用MAX7456 存储器和评估板文件生成定制字符和图形。MAX

3、7456支持高达10MHz接口时钟(SCLK)。图1为写数据时序，图2是从器件读数据的时序。写寄存器时，拉低/CS可使能串行接口。在SCLK的上升沿从SDIN读取数据。当/CS变为高电平时， 数据锁存到输入寄存器。如果传输过程中/CS变高，程序终止(即数据不写入寄存器)。/CS变低之后， 器件等待从SDIN读入第一个字节，以确定正在执行的数据传输类型。读寄存器时，如上文所述，拉低/CS。地址在SCLK的上升沿锁入SDINo然后数据在SCLK的下降沿从 SD0UT输出。SPI命令长度为16位：最高8位(MSB)代表寄存器地址，最低8位(LSB)代表数据(图1和2)。这种 格式有两个例外：1自动递

4、增写模式，用于访问显示存储器，是一个8位操作(图3)。写数据前必须写入起始地址。 对显示存储器执行自动递增写命令时，8位地址由内部产生，串口只需8位数据，如图3所示。2.从显示存储器读字符数据时，若处于16位工作模式，应该是24位(8位地址+16位数据)。执行读操作时，只需要8位地址，如图2所示。-0A6A5A4A3A2A1A0D7D6D5D4D3D2D1D0 -SDINSCLK图1.写操作cs -nSDOUTD7D6D5D4D3D2D1DOXSCLK图2,读操作CS -ISDIN D7 D6 D5 D4 D3 D2 DI DO1A6A5A4A3A2A1A0SDIN图3.自动递增写操作C程序下

5、文给出的C程序已针对MAXQ2000微控制器进行了编译，用于MAX7456评估（EV）板。本文给出了完 整的程序例程。程序是自述文档，几乎没有附加说明。C程序可从以下文件获得：spi. c和MAX7456. h。以下程序使用了 SPI协议的标准定义，MAXQ2000处理器为SPI主机，MAX7456是SPI从器件。CS与MAX7456数据资料中的定义相同。SDIN对应于MOSI （主机出从器件入）。SDOUT对应于MOSI （主机入从器件出）。SCLK对应于CKo前缀SPI_用于全部程序。数据结构下文所示数据结构可直接或者逐位读写数据，用于独立访问SPI端口。C+和一些较新的C编译器支 持位字

6、段联合/结构语句）。/* Port 5 Output Register */_no_init volatile i o union{ unsigned char P05; struct unsigned char bitO : 1; unsigned char bitl : 1; unsigned char bit2 : 1; unsigned char bit3 : 1; unsigned char bit4 : 1; unsigned char bit5 : 1; unsigned char bit6 : 1; unsigned char bit7 : 1; } ; PO

7、5_bit；）上述代码将一个单字节赋值给P05,这是微控制器输出端口的地址。然后将另一个字节赋值给相同的 可以逐位访问的存储器地址。因此，可用以下命令直接对该端口进行寻址:P05 = 0x10;或者用以下命令逐位读写:PO5_bit. bit4 = 1;如果该程序用于其它处理器，该结构需要重新编写。如果采用不支持位字段宽度的老式C编译器，可用位布尔运算设置及清除位:/* Portable bit-set and bit-clear macros. */#define BIT_SET(sfr, bitmask) sfr = (bitmask)#define BIT_CLR(sfr, bitmas

8、k) sfr &= (bitmask)ttdefine BITO OxOlftdefine BIT1 0x02#define BIT2 0x04#define BIT3 0x08#define BIT4 OxlOttdefine BIT5 0x20#define BIT6 0x40#define BIT7 0x80example: BIT_SET(P05, BITO) ; BIT_CLR (P05, BIT6);宏以下是一个简单的编程技巧，使程序更容易移植：用宏定义控制器引脚罗列，如下所示。#define SPI_CS P05_bit. bit4 / P05_bit. bit4 = a

9、ctive-low CS一chip selectttdefine SPI_MOSI P05_bit. bit5 / P05_bit. bit5 = MOST一master out slave in, / data to MAX7456#define SPI_MISO PI5_bit. bit7 / P05_bit. bit7 = MISO一master in slave out, / data from MAX7456#define SPI_CK P05_bit. bit6 / P05_bit. bit6 = SCK - SPI clock用以上宏和数据结构可以单独置位及复位每一个10 口，命

10、令如下:SPI_CS = 1;改变宏时相应引脚也将改变，将上述代码用于其它设计时，如果SPI 口引脚罗列不同，或者为了实现 更理想的PCB布局而对引脚进行重新罗列，上述程序非常实用。单字节写操作程序单字节写操作(图1)程序如下所示。如果可以保证在程序入口处的/CS和CK线状态正确，可以去掉前 两条命令。程序首先发送地址，然后发送数据。进行两次循环。采用单循环及16位数据存储可以简化程序。在 MAXQ2000微控制器中执行16位“int”所占用的时间比执行8位“char”长，因此需进行权衡考虑。/* * spiWriteReg * * Writes to an 8-bit register wi

11、th the SPI port*/void spiWriteReg(const unsigned char regAddr, const unsigned char regData) unsigned char SPICount; / Counter used to clock out the data unsigned char SPIData; / Define a data structure for the SPI data SPI_CS = 1; / Make sure we start with active-low CS high SPI_CK = 0; / and CK low

12、 SPIData =regAddr; / Preload the data to be sent with Address SPI_CS = 0; / Set active-low CS low to start the SPI cycle / Although SPIData could be implemented as an int， / resulting in one / loop, the routines run faster when two loops / are implemented with / SPIDataimplemented as two /zcharts, f

13、or (SPICount = 0; SPICount 8; SPICount+) / Prepare to clock out the Address byte if (SPIData & 0x80) / Check for a 1 SPI_M0SI = 1; / and set the MOSI line appropriately else SPIMOSI = 0; SPICK = 1; / Toggle the clock line SPICK = 0; SPIData = 1; / Rotate to get the next bit / and loop back to send t

14、he next bit / Repeat for the Data byte SPIData = regData; / Preload the data to be sent with Data for(SPICount = 0; SPICount 8; SPICount+) if (SPIData & 0x80) SPIJ10SI = 1; else SPIJIOSI =0; SPI_CK = 1; SPI_CK = 0; SPIData = 1; SPI_CS = 1; SPIJIOSI = 0;读字节操作程序读字节操作(图2)程序如下所示，与上述程序类似。首先发送地址，然后发送时钟从MI

15、SO读回数据。with the SPI port. * Data is returned.spiReadReg * * Reads an 8-bit register*/unsigned char spiReadReg (const unsigned char regAddr) unsigned char SPICount; / Counter used to clock out the data unsigned char SPIData; SPICS = 1; / Make sure we start with active-low CS high SPICK = 0; / and CK

16、low SPIData = regAddr; / Preload the data to be sent with Address and Data SPI_CS = 0; / Set active-low CS low to start the SPI cycle for (SPICount = 0; SPICount 8; SPICount+) / Prepare to clock out the Address and Data if (SPIData & 0x80) SPI_M0SI = 1; else SPI_M0SI =0; SPI_CK = 1; SPI_CK= 0; SPIDa

17、ta = 1; / and loop back to send the next bit SPI_M0SI =0; / Reset the MOSI data line SPIData = 0; for (SPICount 二 0; SPICount 8; SPICount+) / Prepare to clock in the data to be read SPIData =1; / Rotate the data SPICK = 1; / Raise the clock to clock the data out of the MAX7456 SPIData += SPI_MIS0; /

18、 Read the data bit SPI_CK = 0; / Drop the clock ready for the next bit / and loop back SPICS = 1; / Raise CS return (unsigned char)SPIData); / Finally return the read data)自动递增模式下的写字节操作程序自动递增模式下的写字节操作（图3）程序如下所示，与和上述单字节写程序类似。首先发送地址，然 后发送时钟从MISO读回数据。/* * spiWriteRegAutolncr * * Writes to an 8-bit regi

19、ster with the SPI port using the MAX7456, s autoincrement mode*/void spiWriteRegAutolncr(const unsigned char regData) unsigned char SPICount; / Counter used to clock out the data unsigned char SPIData; / Define a data structure for the SPI data. SPI_CS = 1; / Make sure we start with active-low CS hi

20、gh SPI_CK = 0; / and CK low SPIData = regData; / Preload the data to be sent with Address and Data SPI CS = 0; / Set active-low CS low to start the SPI cycle for (SPICount 二 0; SPICount 8; SPICount+) / Prepare to clock out the Address and Data if (SPIData & 0x80) SPI_M0SI = 1; else SPI_M0SI = 0; SPI

21、_CK = 1; SPI_CK = 0; SPIData = 1; / and loop back to send the next bit SPI_MOSI = 0; / Reset the MOSI data line自动递增模式下写显示存储器的程序自动递增模式下写显示存储器的程序如下，程序使用称为data的全局变量数组。定义如下:extern volatile unsigned char dataDATA_BUF_LENGTH;DATA_BUF_LENGTH = 968调用程序时，data口包含显示存储器内容，格式如下: data0 = ignored (contains a comma

22、nd byte used by the EV kit GUI software)datal = character byte ldata2= attribute byte ldata3= character byte 2data4= attribute byte 8tc.自动递增模式通过写OxFF结束，所以该模式下不能向显示寄存器写OxFF。如果需要写OxFF,可以采 用单字节写指令。/* * spiWriteCM * * Writes to the Displayvs.Memory (960 bytes) from data extern. * 960 = 16 rows X 30 colu

23、mns X 2 planes char attr screen-position-indexed memory/On entry:*/void spiWriteCM() global datal. 960 / contains char+attr bytes / (optionally terminated by OxFF data) / First, write datal, 3, 5,. Character plane; / MAX7456 WriteReg(0x05, 0x41) / Character Memory Address High; / 0x02:Attribute byte

24、s; / 0x01:character memory address msb volatile unsigned int Index = 0x0001; / Index for lookup into / datal. 960 spiWriteReg(DM_ADDRH WRITE, 0x00); / initialise the Display Memory high-byte spiWriteReg(DM_ADDRL_WRITE, 0x00); / and the low-byte spiWriteReg(DMJ10DEJVRITE , 0x41); / MAX7456 WriteReg(0

25、x04, 0x41) “Display Memory Mode; / 0x40:Perform 8-bit operation;0x01:AutoIncrement Do / Loop to write the character data if (dataIndex = OxFF) /Check for the break character break; / and finish if foundspiWriteRegAutoIncr(dataIndex); / Write the character Index += 2; / Increment the index to the nex

26、t character, / skipping over the attribute while (Index 0x03Cl); / 0x03Cl =961 / and loop back to send the next character spiWriteRegAutoIncr(OxFF); / Write the “escape character to end AutoIncrement / mode spiWriteReg(DM ADDRH WRITE, 0x02); / Second, write data2, 4, 6,. / Attribute plane; MAX7456 /

27、 WriteReg (0x05, 0x41) / Character Memory Address High; / 0x02:Attribute bytes; 0x01:character memory address / msb spiWriteReg(DM_ADDRL_WRITE, 0x00); spiWriteReg(DM_MODE_WRITE, 0x41); / MAX7456WriteReg(0x04, 0x41) “Character Memory / Mode”; 0x40:Perforin 8-bit operation; 0x01:Auto- / Increment Inde

28、x = 0x0002; do if (dataIndex二二 OxFF) break;spiWriteRegAutoIncr (dataIndex); Index += 2; while (Index 0x03Cl); spiWriteRegAutoIncr(OxFF) ;写字符存储器程序向字符存储器写一个字符的程序如下，每一个字符占用18行，每行12像素，共216像素。由于每 一个字节定义4个像素，因此定义每一个字符需要54字节。字符数据位于程序入口处的data（ 与上述写显示存储器的程序类似）。写字符存储器时需要进行一些附加说明，存储器为非易失，因此，写存储器大约需要12ms,由MAX74

29、56 执行。惟独完整的54字节字符才可以写入字符存储器。该器件包含一个54字节映射存储器。首先把需要写入的字符数据写入映射存储器，然后器件将该数 据装载到NVM字符存储器。用来写字符存储器的寄存器有以下几种：1 .字符存储器模式=0x08o向寄存器写0xA0,使器件把映射存储器的内容装载到NVM字符存储 器。2 .字符存储器地址高位=0x09o包括了即将写入字符的地址。3 .字符存储器地址低位二OxOAo4 .字符存储器数据输入=OxOBo5 . Status = OxAO,读取该寄存器以决定何时可以写入字符存储器。在程序入口处，datal包括即将写入字符的地址，data 2. 54包括字符数

30、据。向NVM字符存储器写字符时，首先写字符地址。然后将每一个字节写入映射存储器。写映射存储器时 没有自动递增模式，所以每次写操作必须写入映射存储器地址。向字符存储器模式寄存器写OxAO, 可以把映射存储器的内容装载到NVM字符存储器。然后器件将状态寄存器第5位置高，表明不能写 入字符存储器。完成后，器件将该位复位至低。数据从映射存储器移向字符存储器时不能写映射存 储器。为了避免浮现显示器闪烁，在写字符存储器之前程序禁止了 OSDo /* * spiWriteFM * * Writes to the Character Memory (54 bytes) from “data extern*/v

31、oid spiWriteFMO unsigned char Index; spiWriteReg(VIDEO_MODE_O_WRITE, spiReadReg (VIDEO_MODE_O_READ) & 0xF7); / Clear bit 0x08 to DISABLE the OSD displayspiWriteReg(FM_ADDRH_WRITE, datal); / Write the address of the character to be written / MAX7456 glyph tile definition / length = 0x36 = 54 bytes /

32、MAX7456 64-byte Shadow RAM accessed / through FM_DATA_. . FM_ADDR. . contains a single / character/glyph-tile shape for (Index = 0x00; Index 0x36; Index+) spiWriteReg(FM_ADDRL_WRITE, Index); / Write the address within the shadow RAM spiWriteReg(FM_DATA_IN_WRITE, dataIndex + 2); / Write the data to t

33、he shadow RAM spiWriteReg(FMMODEWRITE, OxAO); / MAX7456 Font Memory Mode” write OxAO triggers / copy from 64-byte Shadow RAM to NV array, while(spiReadReg(STATUS_READ) & 0x20) != 0x00); / Wait while NV Memory status is BUSY / MAX7456 OxAO status bit 0x20: NV Memory Status / Busy/ReadyMAX7456头文件下面列出了

34、 MAX7456的头文件，以下代码决定了器件的寄存器映射。/* * spiWriteRegAutoIncr * * Writes to an 8-bit register with the SPI port by using the MAX7456s autoincrement mode*/ / MAX7456 VIDEOMODE 0 registerttdefine VIDEO MODE 0 WRITE OxOOttdefine VIDEO MODE 0 READ 0x80#define VIDEO MODE 0 40 PAL 0x40#define VIDE0_M0DE_0_20_NoAu

35、toSync 0x20#define VIDE0_M0DE_0_10_SyncInt OxlOftdefineVIDE0_M0DE_0_08_En0SD 0x08#define VIDE0_M0DE_0_04_UpdateVsync 0x04#defineVIDEO_MODE_O_O2_Reset 0x02#define VIDEO_MODE_O_01_EnVideo 0x01 / VIDEO MODE 0 bitmapttdefine NTSC OxOOftdefine PAL 0x40#define AUTO_SYNC OxOOttdefine EXT_SYNC 0x20#defineIN

36、T_SYNC 0x30#define OSD_EN 0x08#define VERT_SYNC_IMM OxOOftdefine VERT_SYNC_VSYNCregisterttdefine registerttdefine registerttdefine registerttdefine registerttdefine registerttdefine0x04#define SW_RESET 0x02#define BUF_EN OxOOftdefine BUF_DI 0x01 / MAX7456 VIDEO_MODE_1VIDEO MODE.WRITE OxOlttdefine VI

37、DEO MODE_1 READ 0x81 / MAX7456 DMJ10DEDM_MODE_WRITE 0x04#define DM_MODE_READ 0x84 / MAX7456 DM_ADDRHDM_ADDRH_WRITE 0x05#define DM_ADDRH_READ 0x85 / MAX7456 DM_ADDRLDM_ADDRL_WRITE 0x06#define DM_ADDRL_READ 0x87 / MAX7456 DM_CODE_INDM_CODE_IN_WRITE 0x07#define DM_CODE_IN_READ 0x87 / MAX7456 DM_CODE_OU

38、TDM_CODE_OUT_READ OxBO / MAX7456 FM_MODE registerftdefine FM_MODE_WRITE0x08#define0x09#defineOxOAftdefineOxOBftdefineFM_MODE_READ 0x88 / MAX7456 FM_ADDRH registerttdefine FM_ADDRHJVRITEFM_ADDRH READ 0x89 / MAX7456 FM_ADDRL registerttdefine FM_ADDRL_WRITEFM_ADDRL_READ 0x8A / MAX7456 FM_DATA_IN regist

39、erttdefine FM_DATA_IN_WRITEFM_DATA_IN_READ 0x8B / MAX7456 FM_DATA_OUT registerttdefine FM_DATA_OUT_READOxCO / MAX7456 STATUS registerttdefine STATUS_READ OxAOttdefine STATUS_40_RESET_BUSY0x40#define STATUS_20_NVRAM BUSY 0x20#define STATUS_ 04_L0SS OF_SYNC 0x04#define STATUS_02_PAL_DETECTED 0x02#defi

40、ne STATUS_01_NTSC_DETECTED 0x01 / MAX7456 requires clearing OSD Black Level / register bit 0x10 after resetttdefine OSDBLWR 0x6C#define OSDBL_RD OxECftdefine 0SDBL_10_DisableAutoBlackLevel 0x10结论和性能MAX7456评估板采用工作在20MHz时钟的MAXQ2000微控制器，该微控制器包含内部硬件SPI控制器。 因此，MAX7456的SPI端口可以全速工作。上述软件SPI程序工作速度低于硬件控制器。无非针对客 户缺少硬件SPI端口的工作环境，程序已优化至最简。SPI 是 Motorola, Inc.的商标。