April 1999, Issue 105

Dallas 1-Wire Devices, (Part 1):
One on One

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ONE ON ONE

Each device has a unique serial number associated with it, but when only one device is connected on the 1-wire bus, communication is simplified. The SKIP_ROM command (CCH) is the only command necessary to activate the device. SKIP_ROM commands the attached device to immediately wake up and take notice.

It’s not necessary to know the ID number of the device. If you don’t know the family of the device you're talking to (it may be any touch device), you can issue READ_ROM and the attached device will respond with its family code, ID number, and cyclic redundancy check (CRC) byte. From the family code byte you can determine what kind of device is connected.

The 8-byte ROM code contains the family code byte, 6 bytes of ID code, and a CRC byte. The CRC byte is based on the previous 7 bytes and can be used to authenticate the validity of the 8-byte transmission. Although a wrong CRC byte might be because of a bad data transfer, it could also mean that multiple devices are talking at the same time.

This time around, I’m limiting the discussion to single devices tied to an I/O pin. Figure 2 shows how the CRC byte is calculated. Each bit shifted into the CRC byte is XOR’d with bits 4, 5, and 8 of the CRC before shifting.

Although six assembly-language routines are shown in Listing 1—reset, write-1, write-0, read, write-byte, and read-byte—only the first four of these routines are necessary. With these four routines, you can make contact with almost any 1-wire device.

My first program was written as a generic program enabling you to experiment with various 1-wire devices. It was written for the PicStic using PicBasic but it can be used with almost any PIC microprocessor.

I like to use the PicStic because it contains all the essentials you need every time you build a PIC circuit—a 5-V regulator, crystal, and supporting capacitors. And because the PicStic is electrically reprogrammable (it uses a ’16F84), my debugging time is reduced. PicBasic lets me embed assembly language where necessary and still use the higher level commands to quickly build the framework.

You’ll also need to know the specifics for each 1-wire device (i.e., which commands to use for which devices and how many bytes a command will return, if any). Refer to the individual 1-wire datasheets for this information.

After wiring up the circuit in Figure 3, programming the PicStic, and applying power, you are presented with three choices—reset the 1-wire bus, send a command, and request data.

Commands are broken down into function and memory commands. A reset is required before any function command, and a memory command may follow a function command. The function and memory commands are write only, but either command may offer a response.

Let’s try using the program on a DS1820 connected to the 1-wire bus. Following these steps, we receive the responses displayed in Figure 4.

RESET <presence> Command (hex)? CC (SKIP_ROM) Response (bits-dec)? 0 Command (hex)? BE (READ_SCRATCHPAD) Response (bits-dec)? 72 <temp> <pos/neg> <user1> <user2> <res1> <res2> <cr> <cpc> <crc>   48 0 0 0 255 255 41 81 51 Figure 4—My first program was flexible enough to enable me to enter any of the 1-wire command bytes and receive responses from any of the devices connected. Here you can a SKIP_ROM command with no response and a READ_SCRATCHPAD command with a 9-byte (72-bit) response.

If the device’s VCC lead is connected to ground (true 1-wire connection) you must not use the 1-wire bus for 500 ms after sending a convert temp command (44h). Because the device uses parasitic power in this configuration, it must see a high for this duration.

After that, you can proceed with reading the conversion. However, if you connect its VCC lead to VCC, you can poll the device for end of conversion status with Response (bits-dec)? 8 <busy>.

The response to READ_SCRATCHPAD has the temperature in the first byte. If you know it’s above 0°, you can stop reading after the first byte.

The conversation the master has with a 1-wire device can be terminated at any time. The temperature reply is in 0.5°C increments. To get a true temperature in Celsius degrees, divide this by 2 (ditch the 0.5 bit).

To convert this temperature to Fahrenheit, perform the conversion calculation:

or, in our case (because Celsius is in 0.5° increments):

This circuitry and software can easily be massaged into displaying the present temperature in both Celsius and Fahrenheit with a bit more software.

Using a serial LCD can eliminate the serial connection to your PC that would be used in the experimental and debugging process. Listing 2 shows you what my BASIC program looks like (minus the same assembly-language routines that are used in Listing 1).

Next month, I’m going to look at the 1-wire bus with multiple sensors. I’ll show you how to get these devices to act civilly with one another.

You might want to check out Dallas’s web site for documentation on 1-wire devices. They have serial and parallel port dongles for reading external touch memory devices.

Software for reading these on the PC is also available, if you’re into that. I prefer to provide my micros with the ability to use these devices directly. I hope that’s your direction, too.

Jeff Bachiochi (pronounced"BAH-key-AH-key") is an electrical engineer on Circuit Cellar INK’s engineering staff. His background includes product design and manufacturing. He may be reached at jeff.bachiochi@circuitcellar.com.