摘要：DS2482是I2C到1-Wire的橋接器件。DS2482可以使任何具備I2C通信功能的主機產生正確時序和具有擺率控制的1-Wire波形。本應用筆記是DS2482 I2C至1-Wire線驅動器的用戶指南，詳細介紹了通用1-Wire主控制器操作的通信過程。

引言

1-Wire網絡包括一個主機和一個或多個從機器件，1-Wire主機可以由微處理器的一個I/O引腳構成，手動產生定時脈沖。DS2482 I2C至1-Wire網絡的橋接器件可以產生詳細的1-Wire通信時序，無需工程師參與設計。圖1所示為DS2482配置的簡化框圖。本文介紹了采用DS2482實現應用程序接口(API)有效方法，支持基本的和擴展的1-Wire操作。詳細介紹了對應于1-Wire操作的I2C通信。這些操作為執行當前和未來1-Wire器件的所有功能打下了全面的基礎，但基于DS250x系列EPROM的器件編程除外。以這種方式概要介紹1-Wire操作，適合不依賴1-Wire主機的1-Wire應用。

本文僅作為DS2482數據資料的補充，并不能替代數據資料。DS2482可提供兩種配置，單通道1-Wire主控制器(DS2482-100)和帶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)。


圖1. 實現I2C與1-Wire網絡通信的DS2482橋接器功能簡圖

1-Wire接口

下面給出幾個基本的1-Wire函數，稱之為原函數，也就是為了執行所有1-Wire操作，應用中必須具備的函數。第一個函數(OWReset)是使網絡上所有1-Wire從器件復位，為接收來自1-Wire主控制器的指令做好準備。第二個函數(OWWriteBit)完成1-Wire主控制器向從器件寫入一位的操作，而第三個函數(OWReadBit)完成從1-Wire從器件中讀取一位的操作。由于必須由1-Wire主控制器啟動所有的1-Wire位通信，所以“讀取”實際上是在“寫入”一位后采樣得到的結果。幾乎所有其他1-Wire操作都可以由這三個操作構成。例如，向1-Wire網絡寫1個字節相當于8次寫一位操作。

1-Wire搜索算法也可以利用相同的三個原始函數構成。通1-Wire的三個命令，DS2482可以執行搜索功能，大大降低了搜索操作所需的通信量。同樣，單字節的1-Wire通信命令要比八次逐位操作效率更高。

表1所示是三個基本原函數(OWReset、OWWriteBit/OWReadBit和OWWriteByte/OWReadByte)以及其它三個非常有用的函數(OWBlock、OWSearch和msDelay)，它們構成了一系列主要的基本1-Wire操作。這些操作名稱將在下文中使用。

表1. 基本的1-Wire操作

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.

許多1-Wire從器件可以工作在兩種不同的通信速率下：標準速率和高速模式。所有器件都支持標準速率模式。高速速率大約是標準速率的10倍。DS2482同時支持這兩種1-Wire速率。

1-Wire器件通常從1-Wire總線上獲取部分或全部工作電源。不過有些器件在協議的特定操作中需要額外供電。例如，某個器件可能需要進行溫度轉換或執行SHA-1散列算法。這種操作的電源是通過使能1-Wire總線的強上拉提供的。這種供電方式下，不能進行正常通信。DS2482通過設置強上拉標志(SPU)位供電，這將在1-Wire的下一個字節/位通信完成后啟動強上拉。DS2482-100和DS2482-101還具有一個外部引腳(PCTLZ)，可控制大電流強上拉。

表2列出了用于1-Wire速率設定、供電和編程脈沖的擴展1-Wire操作。

表2. 擴展的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.

主機配置

DS2482的主機應具有一個I2C通信口，本文并沒有講述主機的配置。然而，主機必須提供標準I2C接口操作。需要注意的是，主機接口具有包含一些I2C接口操作的高級函數。所需操作請參考表3。

表3. 所需要的I2C主機操作

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

在嘗試1-Wire操作之前，主機必須設置I2C至1-Wire線驅動器DS2482，并與其同步。要與DS2482通信，主機必須知道其從地址。圖2所示為DS2482-100、DS2482-101和DS2482-800的從地址。


圖2. DS2482的I2C從地址

DS2482的I2C命令

下列符號說明來自于DS2482數據資料，以簡寫符號表示，用來說明器件的I2C通信過程。接下來，我們會重復這些通信過程，并為實現基本的和擴展的1-Wire操作提供附加注釋和C語言例程。

I2C通信過程—符號說明

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
	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

數據方向表示法

Master-to-Slave

Slave-to-Master

本文中許多圖中的數據方向表示方法指出了通信方向是主機到從機(灰色)，還是從機到主機(白色)。

DS2482配置操作

以下操作用于設置、配置DS2482，有些操作需要調用1-Wire操作的子程序。

DS2482檢測

例1所示C程序提供檢測和配置流程，寫入DS2482的默認值包括1-Wire速率(標準)、強上拉(關斷)、在線脈沖屏蔽(關斷)和有源上拉(通)。該狀態保持為通用變量，如果器件需要復位到該默認狀態可以進行恢復。針對不同應用可以選擇不同的默認值。

// 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;
}

例1. DS2482檢測

DS2482器件復位

圖3是DS2482器件復位I2C通信的流程。例2給出了DS2482復位命令的C程序，可實現器件狀態機邏輯的完全復位，并終止所有正在進行的1-Wire通信。器件的復位命令代碼是F0h。


圖3. 上電后進行器件復位。該實例包括可選的讀訪問，以檢驗命令是否成功執行。

//--------------------------------------------------------------------------
// 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);
}

例2. 復位器件代碼

DS2482寫配置

圖4所示為DS2482寫配置的I2C通信例程；例3所示為實現DS2482寫配置命令時序的C程序。寫配置命令代碼為D2h。


圖4. 寫配置寄存器，該實例中包括可選擇的讀操作，用于驗證是否成功執行命令。

//--------------------------------------------------------------------------
// 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;
}

例3. DS2482寫配置

DS2482通道選擇

圖5所示為實現DS2482-800通道選擇的I2C通信實例，有效通道為0至7。注意，該操作不適合DS2482-100或DS2482-101。例4給出了執行DS2482-800通道選擇命令的C程序，通道選擇命令碼為C3h。選擇通道后，選中通道可執行所有1-Wire操作。


圖5. 寫通道選擇寄存器，該實例中包括可選擇的讀操作，用于驗證是否成功執行命令。

//--------------------------------------------------------------------------
// 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);
}

例4. DS2482-800通道選擇

DS2482的1-Wire操作

OWReset

1-Wire Reset命令(B4h)在1-Wire總線上執行1-Wire復位和1-Wire器件在線應答脈沖檢測。通過狀態寄存器中的在線應答脈沖檢測(PPD)和短路檢測(SD)字段，可以采樣和報告1-Wire總線的狀態。圖6所示為1-Wire復位命令的I2C通信流程。例5為發送命令的C程序，將檢查狀態寄存器以確定在線應答脈沖狀態。


圖6. 1-Wire復位。開始或終止1-Wire通信。連續檢測1-Wire的空閑(1WB = 0)、忙狀態，直到1-Wire命令完成為止，等到1-Wire命令執行完成，然后讀取結果。

//--------------------------------------------------------------------------
// 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;
}

例5. OWReset代碼

OWWriteBit/OWReadBit

1-Wire位命令(0x87)生成一個1-Wire位時隙。圖7所示是1-Wire位命令的I2C通信C程序。圖8時字節的位分配，V是發送位。例6給出了OWWriteBit、OWReadBit和OWTouchBit程序。


圖7. 1-Wire位，在1-Wire總線上生成一個時隙，當1WB從1變為0時，狀態寄存器保持1-Wire位命令的有效結果。


圖8. 1-Wire的一個數據字節

//--------------------------------------------------------------------------
// 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;
}

例6. 1-Wire位命令

OWWriteByte

1-Wire寫字節命令(A5h)向1-Wire總線寫入單個數據字節。在DS2482執行該命令之前，必須結束1-Wire的工作狀態。圖9所示為通過I2C寫1-Wire字節的情況。例7中的程序在從該操作返回前檢查1-Wire是否完成有效的操作。


圖9. 1-Wire寫字節。向1-Wire總線發送命令代碼。當1WB從1變為0時，完成1-Wire寫字節命令。

//--------------------------------------------------------------------------
// 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();
   }
}

例7. OWWriteByte程序

OWReadByte

1-Wire讀字節命令(96h)從1-Wire網絡讀取單個數據字節。在DS2482執行該命令之前，必須等1-Wire的工作狀態結束。圖10所示為I2C通信流程，1-Wire讀字節命令程序如例8。發送讀字節命令之前，必須檢查1-Wire的工作狀態是否結束。


圖10. 1-Wire讀字節。從1-Wire總線上讀取一個字節。連續檢測狀態寄存器直到1WB位從1變為0。然后設置讀指針指向讀數據寄存器(代碼E1h)，并再次訪問該器件，讀取從1-Wire總線上得到的數據字節。

//--------------------------------------------------------------------------
// 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;
}

例8. OWReadByte程序

OWBlock

OWBlock用于執行一組1-Wire字節操作，對于1-Wire網絡的數據塊傳輸非常有用。例9所示為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;
   }
}

例9. OWBlock程序

OWSearch/1-Wire的三合一命令

1-Wire搜索命令用于搜索1-Wire網絡中每個器件唯一的64位注冊碼，這個唯一的注冊碼通常在數據資料中用ROM碼表示，因為它存儲在只讀存儲器。搜索從1-Wire復位開始，隨后是搜索命令。所有1-Wire器件應答的搜索命令是F0h。搜索命令之后，1-Wire主機將進行二叉樹搜索，找到一個器件。二叉樹搜索通過以下操作判斷64位的每一位：首先讀一位，讀取該位的補碼，然后寫入一位以確定留在搜索中的器件。這三個單獨的位操作時序稱為三合一操作。DS2482帶有一個簡短命令，利用該三合一命令可以更高效地執行1-Wire搜索。

三合一命令(78h)可在1-Wire總線上產生3個時隙，其中包括兩個讀時隙和一個寫時隙。狀態寄存器中的方向字節(DIR)決定了寫時隙的類型(圖11)。例10介紹了完整的1-Wire搜索過程，采用1-Wire三合一命令。調用OWFirst，然后重復調用OWNext，可以查找到1-Wire網絡的所有器件。有關1-Wire搜索算法的詳細介紹請參考應用筆記187，“1-Wire搜索算法”。


圖11. 1-Wire三合一命令，在1-Wire總線上實現搜索ROM功能。完成該1-Wire功能需要空閑時間。然后在讀模式下訪問器件，通過1-Wire三合一命令得到結果。

// 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;
}

例10. OWSearch程序

擴展的1-Wire工作模式

OWSpeed

例11給出了如何用DS2482改變1-Wire總線速率的例程，所有1-Wire器件默認工作在標準通信速率，有些器件可以通過Overdrive-Match-ROM或Overdrive-Skip-ROM命令轉移到高速工作模式。一旦器件工作在高速模式，所有1-Wire通信將采用高速時序。標準速率的1-Wire復位命令將使所有器件恢復到標準速率。

//--------------------------------------------------------------------------
// 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;
}

例11. OWSpeed程序

OWLevel

例12給出了如何用DS2482改變1-Wire總線電平的程序，DS2482可以在執行一次位或字節通信后使能強上拉。隨后，OWLevel程序將1-Wire網絡返回到標準上拉，利用OWWriteBytePower或OWReadBitPower操作使能強上拉。

//--------------------------------------------------------------------------
// 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;
}

例12. OWLevel程序

OWReadBitPower

例13是OWReadBitPower所使用的程序，用于讀取一個1-Wire位并執行供電功能。當配置寄存器中的強上拉(SPU)位使能時，在下一位或字節通信完成后，DS2482將有源拉高1-Wire總線。該操作可通過是否獲得相應的響應來驗證讀位的正確性，如果響應不正確，1-Wire電平將返回到標準的上拉狀態。

//--------------------------------------------------------------------------
// 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;
}

例13. OWReadBitPower程序

OWWriteBytePower

例14為OWWriteBytePower所采用的程序，用于寫入1-Wire字節并執行強上拉供電功能。當配置寄存器中的強上拉(SPU)位使能時，在下一位或字節通信完成后，DS2482將由源拉高1-Wire總線。

//--------------------------------------------------------------------------
// 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_SU;

   // write the new config
   if (!DS2482_write_config(c1WS | cSPU | cPPM | cAPU))
      return FALSE;

   // perform write byte
   OWWriteByte(sendbyte);

   return TRUE;
}

例14. OWWriteBytePower程序

結論

DS2482已經過測試，可成功完成I2C接口至1-Wire網絡的轉換。本文給出了基于DS2482 I2C 1-Wire線驅動器的完整的1-Wire接口方案，該接口適用于所有1-Wire器件，所提供的例程可以在任何一個具有I2C通信口的主機上輕松實現。另外還提供可供下載的完整的C語言執行程序。測試平臺是CMAXQUSB評估套件。