Bailey O.H.Embedded systems.Desktop integration.2005

The same power used for the LCD is also used to power the I/O pins on the LCD, so no additional connections are needed. The following PBASIC program will alternate the relay on and off every 5 seconds.

GOTO TestRelay

Enter this program into the BASIC Stamp editor, connect the Stamp to the host via the RS-232 cable, apply power to the Stamp by attaching a 9-volt battery, and download and run this program. If you cannot feel the relay switching, set your multimeter to continuity or buzzer and attach the leads to the fan control terminals. You should get a tone or see the LED continuity light for 5 seconds and then go off for 5 seconds, then repeat the process. Once you’ve tested the circuit, disconnect the power as this will drain a 9-volt battery pretty quickly.

Note:

Earlier we defined fan control using a 1-Wire switch. Since we have chosen a smart LCD display that provides us with outputs that can drive a relay, this approach makes sense and saves the engineer the additional cost of a 1-Wire switch.

Wiring the Alarm LED

The alarm LED is connected to I/O pin 2 on the LCD display. Since we have a resistor already connected to the LED we need to bypass the current limiting resistor as we did on pin 6. Once this modification is completed, attach a wire between the + side of the I/O pin to the resistor for the alarm LED. To complete the circuit, add a second wire from the –I/O pin to the ground lead on the LED. Use the same software we developed for testing the fan

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Auto-Repeat is initiated by holding down the key just as with a PC keyboard.

I2COUT 8, $50, [254, 69] ‘ Flush Keyboard buffer.

This empties the keyboard buffer of any remaining keys that have not been received. This is useful when the time between fetching keyboard code is lengthy and multiple keyboard characters may be in the buffer.

We have not implemented all of the functions on the keyboard/alarm board we built in Chapter 7. The remaining fuctions are discussed next.

Menu Functionality

We have considerable flexibility in how we implement the menu details and menu navigation. Earlier we determined how our LCD display would be designed. Now, we will provide the details of how our menu should look. We have 12 characters to display our main menu. Following is a description of the needed menu items, the LCD display message, and the message length.

Table 8-1

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Using the DS2404 Time and Memory Chip

Now it’s time to integrate the Dallas 1- and 3-Wire components. Our design calls for a Dallas 1-Wire interface to be implemented. Earlier we built a board that has both the 1- and 3-Wire interfaces and here’s why. Accessing our temperature chip is fast since we are not sending or receiving large amounts of data. We have 512 bytes of RAM that is battery backed up. This is a great place to store nonvolatile information we want to retain. If we read and write large amounts of data using the 1-Wire interface, it will certainly slow things down. In addition, we can use the memory in the DS2404 as dual-port RAM, meaning it can be read from one interface and written to by the other. This has promise for future projects, so we will implement and test the 3-Wire interface even if we don’t use it in the final prototype right now.

The DS2404 Timers

Just as the DS1822 can tell us when a temperature threshold has been reached, the DS2404 can tell us when a timer has expired. The DS2404 time functions are simple free-running counters. We can set timed events; when they are reached, a bit in the corresponding status register is set. The DS2404 timers can trigger interrupts, but since the BASIC Stamp can’t process those interrupts, this feature is of little use to us right now. We can still set the interval timers and look for a change in the status register that indicates the event time has been reached. Let’s examine how the DS2404 timers work and how we can put them to effective use. First, the timing functions of the DS2404 don’t really store the time and date but are free-running elapsed counters. The DS2404 data sheet tells us there are three timers. They are:

Real-time clock — A 5-byte binary counter updated 256 times each second. The least significant byte contains a count of fractional seconds and the remaining four bytes holds the total seconds since the clock was started.

Chapter 8

DS2404 Interrupt Registers

In addition to the alarm registers, there are interrupt enable flags for each of the three counters. If the associated interrupt bit is set, an interrupt is generated when the event occurs. It’s important to note two items: First the interrupts are only available on the 1-Wire interface and second, they only fire once. The good news is that even though we may miss the actual interrupt, the interrupt bit for the associated timer will remain set until it is manually cleared. This means that if we enable the interrupt for the Stamp, we will have to read the status register and test to see if the interrupt was triggered.

Timers and the BASIC Stamp 2p

There are two ways to test if a timer has expired. The first is to create an endless program loop that continuously polls for keyboard input, updates the display, checks for host communications, and reads the 1-Wire and 3-Wire networks. This is a classical way for a developer to handle what we call “the do forever loop” and it works just fine. The only drawback to this technique is that we will never reach power conservation mode. That’s not a big issue right now since are keeping the clock going with a battery backup circuit. In the lab, the CR2032 3.3-volt battery has kept the timer going for well over a week with no external power. Figure 8-11 illustrates our flowchart for how the BASIC Stamp code will manage our thermostat. The emphasis is on how we will process temperature and time.

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accordingly. You’ve probably noticed we are not checking seconds; the reason is simple: If the system gets busy, it is too easy to miss more than one second, thereby putting our second counter out of sync. We should be able to check the minute interrupt with ease and keep the display updated. Additionally, if we trigger on each second we have to add the logic to check if seconds equals 60. If we lose track, our minute counter may be off so we will stick with a minute display for now.

Static RAM vs. EEPROM

At this point we need to consider where our data is stored. You’ll notice the date, hours, and minutes are read and stored back to the DS2404 battery-backed RAM. This assures we always have a copy of the current date, hours, and minutes images as they were last updated. If we lose power, the counters in the DS2404 will continue to run but we have no reference counter to check it against. The solution to this is simple: We retain the cycle counter copy in the DS2404 RAM to compare against. If the cycle counter has incremented since it was last stored, we know that a power outage has occurred. In that case we will compare the counter values to the last counter values read and adjust our minutes, hours, and days accordingly.

We haven’t discussed using the static RAM area of the BASIC Stamp (EEPROM). Since we have battery backed-up RAM in the DS2404 and there is a limited number of times we can write to the EEPROM on the BASIC Stamp, we’ve avoided having to use the EEPROM. The EEPROM could be used but we would need to rotate locations to extend the life of the part. We would also need to check each write with a read to verify the data was written to a good location. Rather than add those routines, we’ve chosen instead to use the battery-backed RAM, which does not have those limitations. We can use the EEPROM for storing our menu strings, since it won’t need to be updated very frequently. Reading the EEPROM has no effect on its life; only writing to the EEPROM is limited.

Chapter 8