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

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PROCESSING REQUESTS

The X-10 sensor recognizes a request when it receives two consecutive On commands for the selected House Code. Remember from the example in Table 1 that one On command requires two X-10 transmissions. The first provides the Key Code and the second provides the Function Code (On in this case). Combining the two Key Codes forms the query value. In the example of Table 1, the Key Codes of 7 and 5 represent a query value of 75°F.

The query value is compared to the last local temperature read from the DS1820, if any. If the local temperature is greater than or equal to the query value, a response of Unit 16 On is created. Otherwise, a response of Unit 16 Off is created. An internal software flag is set indicating that a response is ready to be transmitted.

If the sensor detects an error during DS1820 communication, all queries are ignored and no response is generated or transmitted.

TRANSMITTING RESPONSES

As in receiving X-10 commands, transmission must be synchronized to the zero crossing event. The TW-523 Tx input controls when a high-frequency (120 KHz) carrier is superimposed onto the power line. A "1" is represented by a 120-KHz burst and a "0" is represented by no burst. The TW-523 Tx input is driven by the 16C73A RA2 output through Q1.

For single-phase residential power, this burst begins at zero crossing and lasts for 1 ms. A three-phase setup requires three 1-ms bursts during each half cycle. The temperature sensor is designed to work in a three-phase setup.

The first burst begins at zero crossing. Subsequent Timer0 interrupts control when that burst ends or when the remaining two bursts begin and end. See Figure 6 for the exact timing.

The TW-523 doesn’t handle the duplicate transmissions on outgoing commands as it does for incoming transmissions. The X-10 software must handle transmitting each command twice with a gap of at least three cycles between pairs.

I use 10 half cycles between groups so it takes a total of 108 half cycles (zero crossing interrupts) to transmit a complete response. Breaking down the 108 figure, you get 44 half cycles per group of two (11 cycles × 2 zero crossings/cycle × 2 transmissions), 10 half cycle gaps between codes, × 2 codes (Unit/Key Code and Unit/Function Code).

SYNCHRONIZATION OF TEMPERATURE MONITORING

Remember from the high-level software flow in Figure 2 that DS1820 communication takes place in the main foreground loop and the interrupt-driven background loop deals with the TW-523. The DS1820 communication requires precise timing. A zero crossing or Timer0 interrupt during DS1820 communication would disturb this timing.

One way to deal with this would be to disable interrupts during the periods of time-critical DS1820 communication. However, this technique disturbs the precise timing required for the TW-523 interface.

The solution is to synchronize the time-critical DS1820 code with the zero crossing and Timer0 interrupts. Note in Figure 6 that there are three intervals in each half cycle between transmission bursts where no interrupts occur. The intervals are 1.778 ms long—more than enough time to deal with time-critical DS1820 communication.

An interrupt synchronization subroutine is called just before all time-critical DS1820 code. This subroutine sets an internal flag then waits for that flag to be reset.

The ’16C73A interrupt handler resets this flag at the start of each 1.778-ms interval. The interrupt synchronization subroutine has a 3-ms timeout for protection against a situation where the zero crossing interrupt has been lost.