摘要：在没有专用总线主机(如DS2480B、DS2482)的情况下，微处理器可以轻松地产生1-Wire时序信号。本应用笔记给出了一个采用‘C’语言编写、支持
标准
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速率的1-Wire主机通信基本子程序实例。1-Wire总线的四个基本操作是：复位、写“1”、写“0”和读数据位。字节操作可以通过反复调用位操作实现，本文提供了通过各种传输线与1-Wire器件进行可靠通信的时间参数。
引言
在没有专用总线主机的情况下，微处理器可以轻松地产生1-Wire时序信号。本应用笔记给出了一个采用C语言编写、支持标准速率的1-Wire主机通信基本子程序实例。此外，本文也讨论了高速通信模式。要使该实例中的代码正常运行，系统必须满足以下几点要求：
1. 微处理器的通信端口必须是双向的，其输出为漏极开路，且线上具有弱上拉。这也是所有1-Wire总线的基本要求。关于简单的1-Wire主机微处理器电路实例，请参见应用笔记4206："为嵌入式应用选择合适的1-Wire主机"中的1类部分。
2. 微处理器必须能产生标准速度1-Wire通信所需的精确1µs延时和高速通信所需要的0.25µs延时。
3. 通信过程不能被中断。
1-Wire总线有四种基本操作：复位、写1位、写0位和读位操作。在数据
资料
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中，将完成一位传输的时间称为一个时隙。于是字节传输可以通过多次调用位操作来实现，下面的
表
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1是各个操作的简要说明以及实现这些操作所必须的步骤列表。图1为其时序波形图。表2给出了通常线路条件下1-Wire主机与1-Wire器件通信的推荐时间。如果与1-Wire主机相连的器件比较特殊或者线路条件比较特殊，则可以采用最值。请参考可下载的工作表中的系统和器件参数，确定最小值和最大值。
表1. 1-Wire操作
Operation
Description
Implementation
Write 1 bit
Send a '1' bit to the 1-Wire slaves (Write 1 time slot)
Drive bus low, delay A
Release bus, delay B
Write 0 bit
send a '0' bit to the 1-Wire slaves (Write 0 time slot)
Drive bus low, delay C
Release bus, delay D
Read bit
Read a bit from the 1-Wire slaves (Read time slot)
Drive bus low, delay A
Release bus, delay E
Sample bus to read bit from slave
Delay F
Reset
Reset the 1-Wire bus slave devices and ready them for a command
Delay G
Drive bus low, delay H
Release bus, delay I
Sample bus, 0 = device(s) present, 1 = no device present
Delay J
图1. 1-Wire时序图
表2. 1-Wire主机时序
Parameter
Speed
Recommended (µs)
A
Standard
6
Overdrive
1.0
B
Standard
64
Overdrive
7.5
C
Standard
60
Overdrive
7.5
D
Standard
10
Overdrive
2.5
E
Standard
9
Overdrive
1.0
F
Standard
55
Overdrive
7
G
Standard
0
Overdrive
2.5
H
Standard
480
Overdrive
70
I
Standard
70
Overdrive
8.5
J
Standard
410
Overdrive
40
计算这些值的工作表可供下载。
代码实例
下面代码实例都依赖于两个通用的'C'函数outp和inp，从IO端口读写字节数据。他们通常位于
标准库中。当应用于其它平台时，可以采用合适的函数来替代它们。
// send 'databyte' to 'port'
int outp(unsigned port, int databyte);
// read byte from 'port'
int inp(unsigned port);
代码中的常量PORTADDRESS (图3)用来定义通信端口的地址。这里我们假定使用通信端口的第0位控制1-Wire总线。设定该位为1，将使1-Wire总线变为低电平；设定该位为0，1-Wire总线将被释放，此时1-Wire总线被电阻上拉，或被1-Wire从器件下拉。
代码中的tickDelay函数是一个用户编制的子程序，此函数用于产生一个1/4µs整数倍的延时。在不同的平台下，该函数的实现也是不同的，故在此不做具体描述。以下是tickDelay函数声明代码，以及一个SetSpeed函数，用于设定标准速度和高速模式的延时时间。
实例1. 1-Wire时序的生成
// Pause for exactly 'tick' number of ticks = 0.25us
void tickDelay(int tick); // Implementation is platform specific
// 'tick' values
int A,B,C,D,E,F,G,H,I,J;
//-----------------------------------------------------------------------------
// Set the 1-Wire timing to 'standard' (standard=1) or 'overdrive' (standard=0).
//
void SetSpeed(int standard)
{
// Adjust tick values depending on speed
if (standard)
{
// Standard Speed
A = 6 * 4;
B = 64 * 4;
C = 60 * 4;
D = 10 * 4;
E = 9 * 4;
F = 55 * 4;
G = 0;
H = 480 * 4;
I = 70 * 4;
J = 410 * 4;
}
else
{
// Overdrive Speed
A = 1.5 * 4;
B = 7.5 * 4;
C = 7.5 * 4;
D = 2.5 * 4;
E = 0.75 * 4;
F = 7 * 4;
G = 2.5 * 4;
H = 70 * 4;
I = 8.5 * 4;
J = 40 * 4;
}
}
1-Wire基本操作的代码程序如实例2所示。
实例2. 基本的1-Wire函数
//-----------------------------------------------------------------------------
// Generate a 1-Wire reset, return 1 if no presence detect was found,
// return 0 otherwise.
// (NOTE: Does not handle alarm presence from DS2404/DS1994)
//
int OWTouchReset(void)
{
int result;
tickDelay(G);
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(H);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(I);
result = inp(PORTADDRESS) ^ 0x01; // Sample for presence pulse from slave
tickDelay(J); // Complete the reset sequence recovery
return result; // Return sample presence pulse result
}
//-----------------------------------------------------------------------------
// Send a 1-Wire write bit. Provide 10us recovery time.
//
void OWWriteBit(int bit)
{
if (bit)
{
// Write '1' bit
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(A);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(B); // Complete the time slot and 10us recovery
}
else
{
// Write '0' bit
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(C);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(D);
}
}
//-----------------------------------------------------------------------------
// Read a bit from the 1-Wire bus and return it. Provide 10us recovery time.
//
int OWReadBit(void)
{
int result;
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(A);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(E);
result = inp(PORTADDRESS) & 0x01; // Sample the bit value from the slave
tickDelay(F); // Complete the time slot and 10us recovery
return result;
}
该程序包括了1-Wire总线的所有位操作，通过调用该程序可以构成以字节为处理对象的函数，见实例3。
实例3. 派生的1-Wire函数
//-----------------------------------------------------------------------------
// Write 1-Wire data byte
//
void OWWriteByte(int data)
{
int loop;
// Loop to write each bit in the byte, LS-bit first
for (loop = 0; loop < 8; loop++)
{
OWWriteBit(data & 0x01);
// shift the data byte for the next bit
data >>= 1;
}
}
//-----------------------------------------------------------------------------
// Read 1-Wire data byte and return it
//
int OWReadByte(void)
{
int loop, result=0;
for (loop = 0; loop < 8; loop++)
{
// shift the result to get it ready for the next bit
result >>= 1;
// if result is one, then set MS bit
if (OWReadBit())
result |= 0x80;
}
return result;
}
//-----------------------------------------------------------------------------
// Write a 1-Wire data byte and return the sampled result.
//
int OWTouchByte(int data)
{
int loop, result=0;
for (loop = 0; loop < 8; loop++)
{
// shift the result to get it ready for the next bit
result >>= 1;
// If sending a '1' then read a bit else write a '0'
if (data & 0x01)
{
if (OWReadBit())
result |= 0x80;
}
else
OWWriteBit(0);
// shift the data byte for the next bit
data >>= 1;
}
return result;
}
//-----------------------------------------------------------------------------
// Write a block 1-Wire data bytes and return the sampled result in the same
// buffer.
//
void OWBlock(unsigned char *data, int data_len)
{
int loop;
for (loop = 0; loop < data_len; loop++)
{
data[loop] = OWTouchByte(data[loop]);
}
}
//-----------------------------------------------------------------------------
// Set all devices on 1-Wire to overdrive speed. Return '1' if at least one
// overdrive capable device is detected.
//
int OWOverdriveSkip(unsigned char *data, int data_len)
{
// set the speed to 'standard'
SetSpeed(1);
// reset all devices
if (OWTouchReset()) // Reset the 1-Wire bus
return 0; // Return if no devices found
// overdrive skip command
OWWriteByte(0x3C);
// set the speed to 'overdrive'
SetSpeed(0);
// do a 1-Wire reset in 'overdrive' and return presence result
return OWTouchReset();
}
OWTouchByte函数可以同时完成读写1-Wire总线数据，通过该函数可以实现数据块的读写。在一些平台上执行效率更高，Maxim提供的API就采用了这种函数。通过OWTouchByte函数，OWBlock函数简化了1-Wire总线的数据块发送和接收。注意：OWTouchByte(0xFF)与OWReadByte()等效，OWTouchByte(data)与OWWriteByte(data)等效。
这些函数和tickDelay函数一起构成了1-Wire总线进行位、字节和块操作时所必需的全部函数。实例4给出了利用这些函数读取DS2432的SHA-1认证页的实例。
实例4. 读DS2432实例
//-----------------------------------------------------------------------------
// Read and return the page data and SHA-1 message authentication code (MAC)
// from a DS2432.
//
int ReadPageMAC(int page, unsigned char *page_data, unsigned char *mac)
{
int i;
unsigned short data_crc16, mac_crc16;
// set the speed to 'standard'
SetSpeed(1);
// select the device
if (OWTouchReset()) // Reset the 1-Wire bus
return 0; // Return if no devices found
OWWriteByte(0xCC); // Send Skip ROM command to select single device
// read the page
OWWriteByte(0xA5); // Read Authentication command
OWWriteByte((page << 5) & 0xFF); // TA1
OWWriteByte(0); // TA2 (always zero for DS2432)
// read the page data
for (i = 0; i < 32; i++)
page_data[i] = OWReadByte();
OWWriteByte(0xFF);
// read the CRC16 of command, address, and data
data_crc16 = OWReadByte();
data_crc16 |= (OWReadByte() << 8);
// delay 2ms for the device MAC computation
// read the MAC
for (i = 0; i < 20; i++)
mac[i] = OWReadByte();
// read CRC16 of the MAC
mac_crc16 = OWReadByte();
mac_crc16 |= (OWReadByte() << 8);
// check CRC16...
return 1;
}
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