by Chris Sontag

---

Start • Intelligent Architechture • Putting the Pieces Together • Programming Brains • Building and Interface • Education Possible • Sources & PDF

PUTTING THE PIECES TOGETHER

Everything sounds good in theory. But what about putting this smart device in place?

In addition to about $20 worth of garden-variety sprinkler components, I used an AT89C2051 microcontroller. This 20-pin 8051 derivative from Atmel includes 2 KB of program space.

I installed an RS-232 transceiver to provide external communications. The software handles a real-time clock/calendar and an alarm clock that sequences throughout the program and manages the watering start times.

The emMicro module provides a remote interface for manually programming the system and customizing watering schedules. One of the beauties of the remote interface is that it can be easier to use than the physical interface on the device itself. Some sprinkler controls seem to require a college course just to get your lawn watered at the right time.

A serial EEPROM device stores interface documents and program times in a nonvolatile array to ensure that the device retains essential information if it loses power. A 9-V battery serves as a power backup to keep the clock running during a power failure.

All you need onboard is the microcontroller and the hardware it needs to turn on 24-V AC devices. I used a 74HC138 decoder so eight valves could be controlled using three lines from the microcontroller. The schematic provides for an additional decoder that gives control for up to 16 valves (see Figure 2).

Figure 3 shows the programming logic. As I mentioned, the real-time clock/calendar was implemented completely in software, and it uses the microcontroller’s 18,432-MHz crystal as its time base.

The clock has two main software components—an interrupt-service routine (ISR) and the clock routine. The 8051’s hardware timer 0 executes the ISR once every 10 ms. The ISR must reset the hardware timer for the next interrupt and account for interrupt latency, which would cause the clock to drift off.

Unfortunately, the 8051 architecture doesn’t support a 16-bit auto-reload mode, which would make it unnecessary to correct for interrupt latency. This situation isn’t hard to correct.

Because T0LOW (an 8051 special function) is small (zero), there’s no real possibility of a rollover when the addition is done and, therefore, no reason to account for a carry into the upper portion of the counter. The most important thing the ISR does is set the bit variable called temMSbit, which tells the real-time clock routine that a time tick occurred.

The clock routine is responsible for counting the ticks created by the ISR. Basically, it just counts hours, minutes, seconds, and days, and it sets a few bits here and there to tell other routines when to operate.

For instance, once every minute, the bit variable CheckWater is set. This way, the alarm-clock routine knows to check all the scheduled watering times to see if any are supposed to begin.

There are two day counters. One is used for watering schedules based on the day of the week (Monday through Friday) and the other handles periodic watering (every other day, every third day, etc.).

The counting routine is called by the main calling loop in a round-robin multitasking methodology. It is called much more frequently than the 10-ms rate of the timer ISR, so it is sure not to miss a tick.

Whenever the ISR sets temMSbit, the clock routine increments its clock registers as necessary. It then clears temMSbit so it won’t respond more than once to each tick.