December 2004, Issue 173

H8/38024F-Based Programmable Timer

HARDWARE

Photo 2 shows the modified demo board. Note the battery holder located where the power supply components used to be and the piezo speaker just below it. An array of push buttons is located in the prototyping area at the bottom. The schematic is shown in Figure 2 (page 16).

The H8/38024F is a good fit for this application. It has a built-in LCD controller/driver with just enough segment pins to drive a four-digit display in Static mode, which allows it to work over a wider voltage range than with multiplexed drive. The H8 is set to output the same signal on all four LCD common outputs. Two of these drive the display, which uses a pair of pins connected to a single backplane. The H8’s low-frequency oscillator allows it to keep time and drive the display while using a miniscule amount of power. There are plenty of I/O pins allowing the push buttons to be connected without multiplexing them. This saves time and power during button scanning.

The 10-bit A/D converter is used to sense battery voltage in an unconventional way. Because its reference voltage is internally connected to the supply, and the supply is connected directly to the battery without any regulation, the ADC’s reference will rise and fall with the battery voltage. I used a 1.235-V reference diode to feed one of the analog inputs, so the converted value will be the ratio between the diode voltage and the battery voltage. Thus, as the battery drops, the converted value will increase. An output port controls power to the reference diode. Even though it consumes only 10 µA, I didn’t want this extra drain to run continuously.

Each of the switches connects to a port with an internal pull-up resistor—there are just enough of them. The switches’ common terminals are controlled by an output port, which I enable only when they’re being read. The exception is the On button, which has to be active when the MCU is in Standby mode. When the H8 enters Standby mode, all of its port drivers are disabled, even though the pull-up resistors are still active. I took advantage of the IRQ0 input to wake up the MCU. The datasheet states that either it or IRQ1 should work, but I was unable to get any response from IRQ1 in Standby mode. It worked fine in Sub-Sleep mode, so I used it for the Start/Stop button instead.

One of the PWM outputs drives a piezo speaker element. By setting the PWM to a 50% duty cycle, I got a square wave output at the conversion rate. This allows the piezo to generate a single frequency beep, which is all that’s needed. I originally wanted to use both PWM outputs to drive the piezo differentially and effectively double the drive voltage, but I couldn’t find a way to synchronize the two channels. It turned out to be unnecessary, because the beep ended up loud enough even with a single-ended drive.

I wanted the timer to operate on a single lithium coin cell, which is typically rated for discharge down to 2 V at room temperature. The MCU won’t run that low, but it will work down to 2.7 V, which takes advantage of about 90% of the cell’s capacity. The maximum cell voltage is around 3.3 V, so no voltage regulator is needed. I used a 2.7-V reset supervisor (U2) to keep the MCU honest, and set the low battery threshold to 2.8 V for plenty of warning.

As I selected U2, I discovered the interesting fact that some parts have power consumption specs that vary with the package type. For example, the Fairchild FM809R uses only 2 µA in the SOT-23 package, but needs 6 µA in an SC-70 for an otherwise identical part. The Microchip TCM809R uses 17 µA in an SOT-23, but the SC-70 needs only 9 µA. I avoided the issue by using a Seiko S-80927CL, which is rated at 1.2 µA in any package. It doesn’t have the built-in time delay that the others do, but the MCU has its own 131,072-cycle (approximately 26 ms) reset timer that does the job.

When the timer is awake, it’s almost always in Sub-Sleep or Sub-Active modes, measured at 10 and 22 µA, respectively. When asleep, it’s in Standby mode drawing only 5 µA. Occasional short bursts of Active mode (each time a button is pressed or released) use little enough power to be ignored. I chose a CR2032 cell because it’s commonly available and has a rated capacity of 220 mAh. This allows for more than a full year of continuous operation. It can operate for several years if it’s used infrequently.

The demo board includes a flash memory MCU, which allows for instant reprogramming and easy debugging using a serial port. I used what was once the demo board’s power switch to control power to a 3-V RS-232 converter so it would only draw power when needed. The switch also puts the MCU in Boot mode, thereby allowing the flash memory programming routine to be downloaded into RAM and executed. I ended up using a power supply when downloading, because the battery couldn’t run the serial port for very long.