Issue 113 December 1999
Being Cool is Easy
A Temperature-Sensing Control Device

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THE LCD

The LCD is an Optrex DMC16207 2-line by 16-character display. The device operates with either a 4- or 8-bit bidirectional parallel data interface and three control lines. The X-10 temperature sensor initializes the LCD to its 4-bit mode. Only seven microcontroller I/O lines are required in 4-bit mode instead of 11, which would be required in 8-bit mode. An external 10 kW potentiometer controls LCD contrast.

The LCD accepts 8-bit ASCII data and control characters. In 4-bit mode, two consecutive output operations are necessary to transfer each ASCII or control character to the display. The four data lines are bidirectional and are also used to read busy status from the LCD.

The LCD is not necessary to the operation of the X-10 temperature sensor. It can be eliminated with no other modification to the hardware or software. When present, the LCD displays the selected house code at power-on, current local temperature (alternating between Fahrenheit and Celsius), received requests, and transmitted responses.

The local temperature is displayed on the first line of the LCD and the second line is displayed on the selected House Code or the X-10 controller query and transmitted responses.

The selected House Code is displayed for about 60 s following power-on. A query received from the X-10 controller and the response transmitted by the X-10 temperature sensor are displayed for about 120 s. The second line is blanked after the specified time period has elapsed. The LCD is controlled by the foreground software component.

The LCD was also invaluable during debug of the X-10 temperature sensor software. I used the second line of the display to output information to help troubleshoot problems.

TEMPERATURE SENSOR

The DS1820 1-Wire digital thermometer from Dallas Semiconductor is available in both a PR35 (3-pin) and a 16-pin SSOP package. The DS1820 provides a 9-bit digital value that represents the device’s temperature in 0.5°C increments over a –55°C to +125°C range.

I used the PR35 package for the X-10 temperature sensor. There are three connections with either package: power, ground, and data in/out. An unusual feature of the device is that the power connection is optional. The power and ground pins can be tied together to operate the device in its parasite power mode.

In this mode, which I use in the X-10 temperature sensor, the DS1820 steals power from the data in/out pin when it’s high. The data in/out line is wired to the 16C73A Port A bit 0 (RA0) pin.

The DS1820 temperature conversion cycle requires 500 ms. During this time, up to 1 mA is required and the data in/out pin must be held high by the PIC. The 4.7-kW pull-up resistor, used when reading from the DS1820, will not supply sufficient current during the conversion. The X-10 temperature sensor software performs the DS1820 read temperature sequence shown in Table 2.

The sequence begins with a device reset performed by pulling the data in/out line low for 720 µs (minimum 480 µs, maximum 960 µs). The DS1820 responds with a "device present" indication 15 to 60 µs following the release of the data in/out line. The DS1820 pulls the data in/out line low for 60 to 240 µs.

The X-10 temperature sensor software checks for the "device present" response. If no device is detected, a software flag is set to indicate that a temperature reading is not available, and the remainder of the read temperature sequence is skipped.

The sensor uses three of the six DS1820 commands. Each command consists of 8-bits, which are written serially a bit at a time. The bit write timing is illustrated in Figure 4. To write a one, the PIC drives the data in/out line low for at least 1 µs and then drives the line high.

(Click here to enlarge)

Figure 4— Command bytes are written to and data bytes are read from the DS1820 a bit at a time. Each bit read or write slot takes 60 µs with at least 1 µs between slots.

The DS1820 samples the data in/out line between 15 and 45 µs after the PIC first drives it low. To write a zero, the PIC drives the data in/out line low and keeps it low for at least 60 µs. The PIC drives the data in/out line high for at least 1 µs between bits.

Each DS1820 has a unique serial number in built-in ROM that enables several devices to be wired together on the same 1-wire bus. A match ROM command identifies the serial number of the selected device.

The X-10 temperature sensor uses the DS1820 in a single-drop configuration. The skip ROM command is used to select the device in a single-drop configuration. It works like the match ROM command without needing to provide the serial number.

The convert T command instructs the DS1820 to begin a temperature conversion cycle. The conversion requires a maximum of 500 ms during which the data in/out pin must be held high when using the parasite power mode.

The temperature conversion is read from the DS1820’s scratchpad memory by the last four steps in the sequence. The DS1820 sends one CRC byte following the eight bytes of scratchpad memory. This byte can be used to validate the data transfer.

The sensor software reads these nine bytes, one bit at a time. The software saves the first two bytes, which contain the 9-bit temperature value. The remaining six bytes and the CRC value are discarded. The DS1820 datasheet further details the reset, read, and write cycles, and has information pertaining to additional features, including the content of the remaining scratchpad memory locations.

The bit-read timing is also illustrated in Figure 4. To read each bit, the PIC first drives the data in/out line low for at least 1 µs and then puts the RA0 pin in the high-impedance mode.

The DS1820 will drive the data in/out line low to output a zero or put it in a high-impedance mode to output a one. The pull-up resistor causes the data in/out line to go high when it’s in the high-impedance mode. The PIC samples RA0 15 µs from the time it drives the line low.