摘要
DS2482是I2C到1-Wire的桥接器件。DS2482可以使任何具备I2C通信功能的主机产生正确时序和具有摆率控制的1-Wire波形。本应用笔记是DS2482 I2C至1-Wire线驱动器的用户指南,详细介绍了通用1-Wire主控制器操作的通信过程。
引言
1-Wire网络包括一个主机和一个或多个从机器件,1-Wire主机可以由微处理器的一个I/O引脚构成,手动产生定时脉冲。DS2482 I2C至1-Wire网络的桥接器件可以产生详细的1-Wire通信时序,无需工程师参与设计。图1所示为DS2482配置的简化框图。本文介绍了采用DS2482实现应用程序接口(API)有效方法,支持基本的和扩展的1-Wire操作。详细介绍了对应于1-Wire操作的I2C通信。这些操作为执行当前和未来1-Wire器件的所有功能打下了全面的基础,但基于DS250x系列EPROM的器件编程除外。以这种方式概要介绍1-Wire操作,适合不依赖1-Wire主机的1-Wire应用。
本文仅作为DS2482数据资料的补充,并不能替代数据资料。DS2482可提供两种配置,单通道1-Wire主控制器(DS2482-100)和带有低功耗休眠模式的1-Wire主控制器(DS2482-101),以及八通道1-Wire主控制器(DS2482-800)。
1-Wire接口
下面给出几个基本的1-Wire函数,称之为原函数,也就是为了执行所有1-Wire操作,应用中必须具备的函数。第一个函数(OWReset)是使网络上所有1-Wire从器件复位,为接收来自1-Wire主控制器的指令做好准备。第二个函数(OWWriteBit)完成1-Wire主控制器向从器件写入一位的操作,而第三个函数(OWReadBit)完成从1-Wire从器件中读取一位的操作。由于必须由1-Wire主控制器启动所有的1-Wire位通信,所以“读取”实际上是在“写入”一位后采样得到的结果。几乎所有其他1-Wire操作都可以由这三个操作构成。例如,向1-Wire网络写1个字节相当于8次写一位操作。
1-Wire搜索算法也可以利用相同的三个原始函数构成。通1-Wire的三个命令,DS2482可以执行搜索功能,大大降低了搜索操作所需的通信量。同样,单字节的1-Wire通信命令要比八次逐位操作效率更高。
表1所示是三个基本原函数(OWReset、OWWriteBit/OWReadBit和OWWriteByte/OWReadByte)以及其它三个非常有用的函数(OWBlock、OWSearch和msDelay),它们构成了一系列主要的基本1-Wire操作。这些操作名称将在下文中使用。
Operation | Description |
OWReset | Sends the 1-Wire reset stimulus and check for the presence pulse of 1-Wire slave devices. |
OWWriteBit/OWReadBit | Sends to or receives from the 1-Wire network a single bit of data. |
OWWriteByte/OWReadByte | Sends to or receives from the 1-Wire network a single byte of data. |
OWBlock | Sends to and receives from the 1-Wire network multiple bytes of data. |
OWSearch | Performs the 1-Wire Search Algorithm (see application note 187). |
msDelay | Delays at least the specified number of milliseconds. Used for timing strong pullup operations. |
许多1-Wire从器件可以工作在两种不同的通信速率下:标准速率和高速模式。所有器件都支持标准速率模式。高速速率大约是标准速率的10倍。DS2482同时支持这两种1-Wire速率。
1-Wire器件通常从1-Wire总线上获取部分或全部工作电源。不过有些器件在协议的特定操作中需要额外供电。例如,某个器件可能需要进行温度转换或执行SHA-1散列算法。这种操作的电源是通过使能1-Wire总线的强上拉提供的。这种供电方式下,不能进行正常通信。DS2482通过设置强上拉标志(SPU)位供电,这将在1-Wire的下一个字节/位通信完成后启动强上拉。DS2482-100和DS2482-101还具有一个外部引脚(PCTLZ),可控制大电流强上拉。
表2列出了用于1-Wire速率设定、供电和编程脉冲的扩展1-Wire操作。
Operation | Description |
OWSpeed | Sets the 1-Wire communication speed, either standard or overdrive. Note that this only changes the communication speed of the 1-Wire master; the 1-Wire slave device must be instructed to make the switch when going from normal to overdrive. The 1-Wire slave will always revert to standard speed when it encounters a standard-speed 1-Wire reset. |
OWLevel | Sets the 1-Wire power level (normal or power delivery). |
OWReadBitPower | Reads a single bit of data from the 1-Wire network and optionally applies power delivery immediately after the bit is complete. |
OWWriteBytePower | Sends a single byte of data to the 1-Wire network and applies power delivery immediately after the byte is complete. |
主机配置
DS2482的主机应具有一个I2C通信口,本文并没有讲述主机的配置。然而,主机必须提供标准I2C接口操作。需要注意的是,主机接口具有包含一些I2C接口操作的高级函数。所需操作请参考表3。
Operation | Description |
I2C_start | I2C start command. |
I2C_rep_start | I2C repeated start command. |
I2C_stop | I2C stop command. |
I2C_write | Writes a byte to the I2C bus. The byte to write is passed to the function. |
I2C_read | Reads a byte from the I2C bus. The byte read is returned from the function. |
配置DS2482
在尝试1-Wire操作之前,主机必须设置I2C至1-Wire线驱动器DS2482,并与其同步。要与DS2482通信,主机必须知道其从地址。图2所示为DS2482-100、DS2482-101和DS2482-800的从地址。
DS2482的I2C命令
下列符号说明来自于DS2482数据资料,以简写符号表示,用来说明器件的I2C通信过程。接下来,我们会重复这些通信过程,并为实现基本的和扩展的1-Wire操作提供附加注释和C语言例程。
Symbol | Description |
S | START Condition |
AD, 0 | Select DS2482 for Write Access |
AD, 1 | Select DS2482 for Read Access |
Sr | Repeated START Condition |
P | STOP Condition |
A | Acknowledged |
A\ | Not acknowledged |
(Idle) | Bus not busy |
<byte> | Transfer of one byte |
DRST | Command 'Device Reset', F0h |
WCFG | Command 'Write Configuration', D2h |
CHSL | Command 'Channel Select', C3h (DS2482-800 only) |
SRP | Command 'Set Read Pointer', E1h |
1WRS | Command '1-Wire Reset', B4h |
1WWB | Command '1-Wire Write Byte', A5h |
1WRB | Command '1-Wire Read Byte', 96h |
1WSB | Command '1-Wire Single Bit', 87h |
1WT | Command '1-Wire Triplet', 78h |
数据方向表示法
Master-to-Slave | Slave-to-Master |
本文中许多图中的数据方向表示方法指出了通信方向是主机到从机(灰色),还是从机到主机(白色)。
DS2482配置操作
以下操作用于设置、配置DS2482,有些操作需要调用1-Wire操作的子程序。
DS2482检测
例1所示C程序提供检测和配置流程,写入DS2482的默认值包括1-Wire速率(标准)、强上拉(关断)、在线脉冲屏蔽(关断)和有源上拉(通)。该状态保持为通用变量,如果器件需要复位到该默认状态可以进行恢复。针对不同应用可以选择不同的默认值。
// DS2482 state unsigned char I2C_address; int c1WS, cSPU, cPPM, cAPU; int short_detected; //-------------------------------------------------------------------------- // DS2428 Detect routine that sets the I2C address and then performs a // device reset followed by writing the configuration byte to default values: // 1-Wire speed (c1WS) = standard (0) // Strong pullup (cSPU) = off (0) // Presence pulse masking (cPPM) = off (0) // Active pullup (cAPU) = on (CONFIG_APU = 0x01) // // Returns: TRUE if device was detected and written // FALSE device not detected or failure to write configuration byte // int DS2482_detect(unsigned char addr) { // set global address I2C_address = addr; // reset the DS2482 ON selected address if (!DS2482_reset()) return FALSE; // default configuration c1WS = FALSE; cSPU = FALSE; cPPM = FALSE; cAPU = CONFIG_APU; // write the default configuration setup if (!DS2482_write_config(c1WS | cSPU | cPPM | cAPU)) return FALSE; return TRUE; }
DS2482器件复位
图3是DS2482器件复位I2C通信的流程。例2给出了DS2482复位命令的C程序,可实现器件状态机逻辑的完全复位,并终止所有正在进行的1-Wire通信。器件的复位命令代码是F0h。
//-------------------------------------------------------------------------- // Perform a device reset on the DS2482 // // Returns: TRUE if device was reset // FALSE device not detected or failure to perform reset // int DS2482_reset() { unsigned char status; // Device Reset // S AD,0 [A] DRST [A] Sr AD,1 [A] [SS] A\ P // [] indicates from slave // SS status byte to read to verify state I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_DRST, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); status = I2C_read(NACK); I2C_stop(); // check for failure due to incorrect read back of status return ((status & 0xF7) == 0x10); }
DS2482写配置
图4所示为DS2482写配置的I2C通信例程;例3所示为实现DS2482写配置命令时序的C程序。写配置命令代码为D2h。
//-------------------------------------------------------------------------- // Write the configuration register in the DS2482. The configuration // options are provided in the lower nibble of the provided config byte. // The uppper nibble in bitwise inverted when written to the DS2482. // // Returns: TRUE: config written and response correct // FALSE: response incorrect // int DS2482_write_config(unsigned char config) { unsigned char read_config; // Write configuration (Case A) // S AD,0 [A] WCFG [A] CF [A] Sr AD,1 [A] [CF] A\ P // [] indicates from slave // CF configuration byte to write I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_WCFG, EXPECT_ACK); I2C_write(config | (~config << 4), EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); read_config = I2C_read(NACK); I2C_stop(); // check for failure due to incorrect read back if (config != read_config) { // handle error // ... DS2482_reset(); return FALSE; } return TRUE; }
DS2482通道选择
图5所示为实现DS2482-800通道选择的I2C通信实例,有效通道为0至7。注意,该操作不适合DS2482-100或DS2482-101。例4给出了执行DS2482-800通道选择命令的C程序,通道选择命令码为C3h。选择通道后,选中通道可执行所有1-Wire操作。
//-------------------------------------------------------------------------- // Select the 1-Wire channel on a DS2482-800. // // Returns: TRUE if channel selected // FALSE device not detected or failure to perform select // int DS2482_channel_select(int channel) { unsigned char ch, ch_read, check; // Channel Select (Case A) // S AD,0 [A] CHSL [A] CC [A] Sr AD,1 [A] [RR] A\ P // [] indicates from slave // CC channel value // RR channel read back I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_CHSL, EXPECT_ACK); switch (channel) { default: case 0: ch = 0xF0; ch_read = 0xB8; break; case 1: ch = 0xE1; ch_read = 0xB1; break; case 2: ch = 0xD2; ch_read = 0xAA; break; case 3: ch = 0xC3; ch_read = 0xA3; break; case 4: ch = 0xB4; ch_read = 0x9C; break; case 5: ch = 0xA5; ch_read = 0x95; break; case 6: ch = 0x96; ch_read = 0x8E; break; case 7: ch = 0x87; ch_read = 0x87; break; }; I2C_write(ch, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); check = I2C_read(NACK); I2C_stop(); // check for failure due to incorrect read back of channel return (check == ch_read); }
DS2482的1-Wire操作
OWReset
1-Wire Reset命令(B4h)在1-Wire总线上执行1-Wire复位和1-Wire器件在线应答脉冲检测。通过状态寄存器中的在线应答脉冲检测(PPD)和短路检测(SD)字段,可以采样和报告1-Wire总线的状态。图6所示为1-Wire复位命令的I2C通信流程。例5为发送命令的C程序,将检查状态寄存器以确定在线应答脉冲状态。
//-------------------------------------------------------------------------- // Reset all of the devices on the 1-Wire Net and return the result. // // Returns: TRUE(1): presence pulse(s) detected, device(s) reset // FALSE(0): no presence pulses detected // int OWReset(void) { unsigned char status; int poll_count = 0; // 1-Wire reset (Case B) // S AD,0 [A] 1WRS [A] Sr AD,1 [A] [Status] A [Status] A\ P // \--------/ // Repeat until 1WB bit has changed to 0 // [] indicates from slave I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_1WRS, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); // loop checking 1WB bit for completion of 1-Wire operation // abort if poll limit reached status = I2C_read(ACK); do { status = I2C_read(status & STATUS_1WB); } while ((status & STATUS_1WB) && (poll_count++ < POLL_LIMIT)); I2C_stop(); // check for failure due to poll limit reached if (poll_count >= POLL_LIMIT) { // handle error // ... DS2482_reset(); return FALSE; } // check for short condition if (status & STATUS_SD) short_detected = TRUE; else short_detected = FALSE; // check for presence detect if (status & STATUS_PPD) return TRUE; else return FALSE; }
OWWriteBit/OWReadBit
1-Wire位命令(0x87)生成一个1-Wire位时隙。图7所示是1-Wire位命令的I2C通信C程序。图8时字节的位分配,V是发送位。例6给出了OWWriteBit、OWReadBit和OWTouchBit程序。
//-------------------------------------------------------------------------- // Send 1 bit of communication to the 1-Wire Net. // The parameter 'sendbit' least significant bit is used. // // 'sendbit' - 1 bit to send (least significant byte) // void OWWriteBit(unsigned char sendbit) { OWTouchBit(sendbit); } //-------------------------------------------------------------------------- // Reads 1 bit of communication from the 1-Wire Net and returns the // result // // Returns: 1 bit read from 1-Wire Net // unsigned char OWReadBit(void) { return OWTouchBit(0x01); } //-------------------------------------------------------------------------- // Send 1 bit of communication to the 1-Wire Net and return the // result 1 bit read from the 1-Wire Net. The parameter 'sendbit' // least significant bit is used and the least significant bit // of the result is the return bit. // // 'sendbit' - the least significant bit is the bit to send // // Returns: 0: 0 bit read from sendbit // 1: 1 bit read from sendbit // unsigned char OWTouchBit(unsigned char sendbit) { unsigned char status; int poll_count = 0; // 1-Wire bit (Case B) // S AD,0 [A] 1WSB [A] BB [A] Sr AD,1 [A] [Status] A [Status] A\ P // \--------/ // Repeat until 1WB bit has changed to 0 // [] indicates from slave // BB indicates byte containing bit value in msbit I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_1WSB, EXPECT_ACK); I2C_write(sendbit ? 0x80 : 0x00, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); // loop checking 1WB bit for completion of 1-Wire operation // abort if poll limit reached status = I2C_read(ACK); do { status = I2C_read(status & STATUS_1WB); } while ((status & STATUS_1WB) && (poll_count++ < POLL_LIMIT)); I2C_stop(); // check for failure due to poll limit reached if (poll_count >= POLL_LIMIT) { // handle error // ... DS2482_reset(); return 0; } // return bit state if (status & STATUS_SBR) return 1; else return 0; }
OWWriteByte
1-Wire写字节命令(A5h)向1-Wire总线写入单个数据字节。在DS2482执行该命令之前,必须结束1-Wire的工作状态。图9所示为通过I2C写1-Wire字节的情况。例7中的程序在从该操作返回前检查1-Wire是否完成有效的操作。
//-------------------------------------------------------------------------- // Send 8 bits of communication to the 1-Wire Net and verify that the // 8 bits read from the 1-Wire Net are the same (write operation). // The parameter 'sendbyte' least significant 8 bits are used. // // 'sendbyte' - 8 bits to send (least significant byte) // // Returns: TRUE: bytes written and echo was the same // FALSE: echo was not the same // void OWWriteByte(unsigned char sendbyte) { unsigned char status; int poll_count = 0; // 1-Wire Write Byte (Case B) // S AD,0 [A] 1WWB [A] DD [A] Sr AD,1 [A] [Status] A [Status] A\ P // \--------/ // Repeat until 1WB bit has changed to 0 // [] indicates from slave // DD data to write I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_1WWB, EXPECT_ACK); I2C_write(sendbyte, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); // loop checking 1WB bit for completion of 1-Wire operation // abort if poll limit reached status = I2C_read(ACK); do { status = I2C_read(status & STATUS_1WB); } while ((status & STATUS_1WB) && (poll_count++ < POLL_LIMIT)); I2C_stop(); // check for failure due to poll limit reached if (poll_count >= POLL_LIMIT) { // handle error // ... DS2482_reset(); } }
OWReadByte
1-Wire读字节命令(96h)从1-Wire网络读取单个数据字节。在DS2482执行该命令之前,必须等1-Wire的工作状态结束。图10所示为I2C通信流程,1-Wire读字节命令程序如例8。发送读字节命令之前,必须检查1-Wire的工作状态是否结束。
//-------------------------------------------------------------------------- // Send 8 bits of read communication to the 1-Wire Net and return the // result 8 bits read from the 1-Wire Net. // // Returns: 8 bits read from 1-Wire Net // unsigned char OWReadByte(void) { unsigned char data, status; int poll_count = 0; // 1-Wire Read Bytes (Case C) // S AD,0 [A] 1WRB [A] Sr AD,1 [A] [Status] A [Status] A // \--------/ // Repeat until 1WB bit has changed to 0 // Sr AD,0 [A] SRP [A] E1 [A] Sr AD,1 [A] DD A\ P // // [] indicates from slave // DD data read I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_1WRB, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); // loop checking 1WB bit for completion of 1-Wire operation // abort if poll limit reached status = I2C_read(ACK); do { status = I2C_read(status & STATUS_1WB); } while ((status & STATUS_1WB) && (poll_count++ < POLL_LIMIT)); // check for failure due to poll limit reached if (poll_count >= POLL_LIMIT) { // handle error // ... DS2482_reset(); return 0; } I2C_rep_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_SRP, EXPECT_ACK); I2C_write(0xE1, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); data = I2C_read(NACK); I2C_stop(); return data; }
OWBlock
OWBlock用于执行一组1-Wire字节操作,对于1-Wire网络的数据块传输非常有用。例9所示为OWBlock例程。
//-------------------------------------------------------------------------- // The 'OWBlock' transfers a block of data to and from the // 1-Wire Net. The result is returned in the same buffer. // // 'tran_buf' - pointer to a block of unsigned // chars of length 'tran_len' that will be sent // to the 1-Wire Net // 'tran_len' - length in bytes to transfer // void OWBlock(unsigned char *tran_buf, int tran_len) { int i; for (i = 0; i < tran_len; i++) tran_buf[i] = OWTouchByte(tran_buf[i]); } //-------------------------------------------------------------------------- // Send 8 bits of communication to the 1-Wire Net and return the // result 8 bits read from the 1-Wire Net. The parameter 'sendbyte' // least significant 8 bits are used and the least significant 8 bits // of the result are the return byte. // // 'sendbyte' - 8 bits to send (least significant byte) // // Returns: 8 bits read from sendbyte // unsigned char OWTouchByte(unsigned char sendbyte) { if (sendbyte == 0xFF) return OWReadByte(); else { OWWriteByte(sendbyte); return sendbyte; } }
OWSearch/1-Wire的三合一命令
1-Wire搜索命令用于搜索1-Wire网络中每个器件唯一的64位注册码,这个唯一的注册码通常在数据资料中用ROM码表示,因为它存储在只读存储器。搜索从1-Wire复位开始,随后是搜索命令。所有1-Wire器件应答的搜索命令是F0h。搜索命令之后,1-Wire主机将进行二叉树搜索,找到一个器件。二叉树搜索通过以下操作判断64位的每一位:首先读一位,读取该位的补码,然后写入一位以确定留在搜索中的器件。这三个单独的位操作时序称为三合一操作。DS2482带有一个简短命令,利用该三合一命令可以更高效地执行1-Wire搜索。
三合一命令(78h)可在1-Wire总线上产生3个时隙,其中包括两个读时隙和一个写时隙。状态寄存器中的方向字节(DIR)决定了写时隙的类型(图11)。例10介绍了完整的1-Wire搜索过程,采用1-Wire三合一命令。调用OWFirst,然后重复调用OWNext,可以查找到1-Wire网络的所有器件。有关1-Wire搜索算法的详细介绍请参考应用笔记187,“1-Wire搜索算法”。
// Search state unsigned char ROM_NO[8]; int LastDiscrepancy; int LastFamilyDiscrepancy; int LastDeviceFlag; unsigned char crc8; //-------------------------------------------------------------------------- // Find the 'first' devices on the 1-Wire network // Return TRUE : device found, ROM number in ROM_NO buffer // FALSE : no device present // int OWFirst() { // reset the search state LastDiscrepancy = 0; LastDeviceFlag = FALSE; LastFamilyDiscrepancy = 0; return OWSearch(); } //-------------------------------------------------------------------------- // Find the 'next' devices on the 1-Wire network // Return TRUE : device found, ROM number in ROM_NO buffer // FALSE : device not found, end of search // int OWNext() { // leave the search state alone return OWSearch(); } //-------------------------------------------------------------------------- // The 'OWSearch' function does a general search. This function // continues from the previous search state. The search state // can be reset by using the 'OWFirst' function. // This function contains one parameter 'alarm_only'. // When 'alarm_only' is TRUE (1) the find alarm command // 0xEC is sent instead of the normal search command 0xF0. // Using the find alarm command 0xEC will limit the search to only // 1-Wire devices that are in an 'alarm' state. // // Returns: TRUE (1) : when a 1-Wire device was found and its // Serial Number placed in the global ROM // FALSE (0): when no new device was found. Either the // last search was the last device or there // are no devices on the 1-Wire Net. // int OWSearch() { int id_bit_number; int last_zero, rom_byte_number, search_result; int id_bit, cmp_id_bit; unsigned char rom_byte_mask, search_direction, status; // initialize for search id_bit_number = 1; last_zero = 0; rom_byte_number = 0; rom_byte_mask = 1; search_result = FALSE; crc8 = 0; // if the last call was not the last one if (!LastDeviceFlag) { // 1-Wire reset if (!OWReset()) { // reset the search LastDiscrepancy = 0; LastDeviceFlag = FALSE; LastFamilyDiscrepancy = 0; return FALSE; } // issue the search command OWWriteByte(0xF0); // loop to do the search do { // if this discrepancy if before the Last Discrepancy // on a previous next then pick the same as last time if (id_bit_number < LastDiscrepancy) { if ((ROM_NO[rom_byte_number] & rom_byte_mask) > 0) search_direction = 1; else search_direction = 0; } else { // if equal to last pick 1, if not then pick 0 if (id_bit_number == LastDiscrepancy) search_direction = 1; else search_direction = 0; } // Perform a triple operation on the DS2482 which will perform // 2 read bits and 1 write bit status = DS2482_search_triplet(search_direction); // check bit results in status byte id_bit = ((status & STATUS_SBR) == STATUS_SBR); cmp_id_bit = ((status & STATUS_TSB) == STATUS_TSB); search_direction = ((status & STATUS_DIR) == STATUS_DIR) ? (byte)1 : (byte)0; // check for no devices on 1-Wire if ((id_bit) && (cmp_id_bit)) break; else { if ((!id_bit) && (!cmp_id_bit) && (search_direction == 0)) { last_zero = id_bit_number; // check for Last discrepancy in family if (last_zero < 9) LastFamilyDiscrepancy = last_zero; } // set or clear the bit in the ROM byte rom_byte_number // with mask rom_byte_mask if (search_direction == 1) ROM_NO[rom_byte_number] |= rom_byte_mask; else ROM_NO[rom_byte_number] &= (byte)~rom_byte_mask; // increment the byte counter id_bit_number // and shift the mask rom_byte_mask id_bit_number++; rom_byte_mask <<= 1; // if the mask is 0 then go to new SerialNum byte rom_byte_number // and reset mask if (rom_byte_mask == 0) { calc_crc8(ROM_NO[rom_byte_number]); // accumulate the CRC rom_byte_number++; rom_byte_mask = 1; } } } while(rom_byte_number < 8); // loop until through all ROM bytes 0-7 // if the search was successful then if (!((id_bit_number < 65) || (crc8 != 0))) { // search successful so set LastDiscrepancy,LastDeviceFlag // search_result LastDiscrepancy = last_zero; // check for last device if (LastDiscrepancy == 0) LastDeviceFlag = TRUE; search_result = TRUE; } } // if no device found then reset counters so next // 'search' will be like a first if (!search_result || (ROM_NO[0] == 0)) { LastDiscrepancy = 0; LastDeviceFlag = FALSE; LastFamilyDiscrepancy = 0; search_result = FALSE; } return search_result; } //-------------------------------------------------------------------------- // Use the DS2482 help command '1-Wire triplet' to perform one bit of a //1-Wire search. //This command does two read bits and one write bit. The write bit // is either the default direction (all device have same bit) or in case of // a discrepancy, the 'search_direction' parameter is used. // // Returns – The DS2482 status byte result from the triplet command // unsigned char DS2482_search_triplet(int search_direction) { unsigned char status; int poll_count = 0; // 1-Wire Triplet (Case B) // S AD,0 [A] 1WT [A] SS [A] Sr AD,1 [A] [Status] A [Status] A\ P // \--------/ // Repeat until 1WB bit has changed to 0 // [] indicates from slave // SS indicates byte containing search direction bit value in msbit I2C_start(); I2C_write(I2C_address | I2C_WRITE, EXPECT_ACK); I2C_write(CMD_1WT, EXPECT_ACK); I2C_write(search_direction ? 0x80 : 0x00, EXPECT_ACK); I2C_rep_start(); I2C_write(I2C_address | I2C_READ, EXPECT_ACK); // loop checking 1WB bit for completion of 1-Wire operation // abort if poll limit reached status = I2C_read(ACK); do { status = I2C_read(status & STATUS_1WB); } while ((status & STATUS_1WB) && (poll_count++ < POLL_LIMIT)); I2C_stop(); // check for failure due to poll limit reached if (poll_count >= POLL_LIMIT) { // handle error // ... DS2482_reset(); return 0; } // return status byte return status; }
扩展的1-Wire工作模式
OWSpeed
例11给出了如何用DS2482改变1-Wire总线速率的例程,所有1-Wire器件默认工作在标准通信速率,有些器件可以通过Overdrive-Match-ROM或Overdrive-Skip-ROM命令转移到高速工作模式。一旦器件工作在高速模式,所有1-Wire通信将采用高速时序。标准速率的1-Wire复位命令将使所有器件恢复到标准速率。
//-------------------------------------------------------------------------- // Set the 1-Wire Net communication speed. // // 'new_speed' - new speed defined as // MODE_STANDARD 0x00 // MODE_OVERDRIVE 0x01 // // Returns: current 1-Wire Net speed // int OWSpeed(int new_speed) { // set the speed if (new_speed == MODE_OVERDRIVE) c1WS = CONFIG_1WS; else c1WS = FALSE; // write the new config DS2482_write_config(c1WS | cSPU | cPPM | cAPU); return new_speed; }
OWLevel
例12给出了如何用DS2482改变1-Wire总线电平的程序,DS2482可以在执行一次位或字节通信后使能强上拉。随后,OWLevel程序将1-Wire网络返回到标准上拉,利用OWWriteBytePower或OWReadBitPower操作使能强上拉。
//-------------------------------------------------------------------------- // Set the 1-Wire Net line level pullup to normal. The DS2482 only // allows enabling strong pullup on a bit or byte event. Consequently this // function only allows the MODE_STANDARD argument. To enable strong pullup // use OWWriteBytePower or OWReadBitPower. // // 'new_level' - new level defined as // MODE_STANDARD 0x00 // // Returns: current 1-Wire Net level // int OWLevel(int new_level) { // function only will turn back to non-strong pullup if (new_level != MODE_STANDARD) return MODE_STRONG; // clear the strong pullup bit in the global config state cSPU = FALSE; // write the new config DS2482_write_config(c1WS | cSPU | cPPM | cAPU); return MODE_STANDARD; }
OWReadBitPower
例13是OWReadBitPower所使用的程序,用于读取一个1-Wire位并执行供电功能。当配置寄存器中的强上拉(SPU)位使能时,在下一位或字节通信完成后,DS2482将有源拉高1-Wire总线。该操作可通过是否获得相应的响应来验证读位的正确性,如果响应不正确,1-Wire电平将返回到标准的上拉状态。
//-------------------------------------------------------------------------- // Send 1 bit of communication to the 1-Wire Net and verify that the // response matches the 'applyPowerResponse' bit and apply power delivery // to the 1-Wire net. Note that some implementations may apply the power // first and then turn it off if the response is incorrect. // // 'applyPowerResponse' - 1 bit response to check, if correct then start // power delivery // // Returns: TRUE: bit written and response correct, strong pullup now on // FALSE: response incorrect // int OWReadBitPower(int applyPowerResponse) { unsigned char rdbit; // set strong pullup enable cSPU = CONFIG_SPU; // write the new config if (!DS2482_write_config(c1WS | cSPU | cPPM | cAPU)) return FALSE; // perform read bit rdbit = OWReadBit(); // check if response was correct, if not then turn off strong pullup if (rdbit != applyPowerResponse) { OWLevel(MODE_STANDARD); return FALSE; } return TRUE; }
OWWriteBytePower
例14为OWWriteBytePower所采用的程序,用于写入1-Wire字节并执行强上拉供电功能。当配置寄存器中的强上拉(SPU)位使能时,在下一位或字节通信完成后,DS2482将由源拉高1-Wire总线。
//-------------------------------------------------------------------------- // Send 8 bits of communication to the 1-Wire Net and verify that the // 8 bits read from the 1-Wire Net are the same (write operation). // The parameter 'sendbyte' least significant 8 bits are used. After the // 8 bits are sent change the level of the 1-Wire net. // // 'sendbyte' - 8 bits to send (least significant bit) // // Returns: TRUE: bytes written and echo was the same, strong pullup now on // FALSE: echo was not the same // int OWWriteBytePower(int sendbyte) { // set strong pullup enable cSPU = CONFIG_SPU; // write the new config if (!DS2482_write_config(c1WS | cSPU | cPPM | cAPU)) return FALSE; // perform write byte OWWriteByte(sendbyte); return TRUE; }
结论
DS2482已经过测试,可成功完成I2C接口至1-Wire网络的转换。本文给出了基于DS2482 I2C 1-Wire线驱动器的完整的1-Wire接口方案,该接口适用于所有1-Wire器件,所提供的例程可以在任何一个具有I2C通信口的主机上轻松实现。另外还提供可供下载的完整的C语言执行程序。测试平台是CMAXQUSB评估套件。
修订历史
11/2005版本1.0–首次发布。
9/2008版本2.0–修改程序以使用低电平I2C通信子程序,修改相应的文字内容。