Your Own STOPWATCH

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One of the basic usage of the TIMER peripheral on every microcontroller is to provide the accurate timing mechanism. Using the TIMER peripheral as the basic timing, we could easily develop a stopwatch and display it to the 8-Digit seven segment numeric LED display. Thanks to the Maxim MAX7219 chip which enable us to interface this 8-Digit seven segment LED display much easier using just three wires of the SPI (serial peripheral interface) to display the hour, minute, second, and hundredth of seconds to the 8-Digit seven segments LED display.
The following is the list of hardware, software, and references used to build this project:
1. One Maxim MAX7219: Serially Interfaced, 8-Digit LED Display Drivers
2. Two common cathode 4-Digits seven segment LED display
3. One Resistor: 10K Ohm
4. One Capacitors 0.1uF
5. AVRJazz 28PIN development board from ermicro which is based on the AVR ATmega328P microcontroller.
6. Atmel AVR Studio 6.0 for coding and debugging environment
7. STK500 programmer from AVR Studio 6.0, using the AVRJazz 28PIN board STK500 v2.0 bootloader
8. Atmel AVR ATmega328 and Maxim MAX7219 Datasheet
The stopwatch project that we are going to build has these following features:
Stopwatch counting up to hundredth of second when the SW1 is pressed
Pressing the SW1 once will freeze the counting display while continuing counting in the background, pressing the SW1  again will continue to display the stopwatch counting
Adjust the intensity of the 8-Digits seven segment LED display using the trimmer potentiometer (TP).
Reset the stopwatch counting by pressing the SW0.
In order to accomplish this project, we will use the AVR ATmega328P 16-bit TIMER2 peripheral in compare match mode as the heart beat of the counting mechanism, the pin change interrupt is used to detect the SW1 switch, and the ADC peripheral is used to adjust the 8-Digit seven segment LED display brightness. This project also serves as a good example of how we use many of the powerful AVR ATmega328P microcontroller features at the same time.
The Maxim MAX7219
The MAX7219 chip from maxim is a powerful serial input/output common-cathode display driver that interfaces microcontroller to 7-segment numeric LED displays of up to 8 digits. It has a build-in BCD (binary code decimal) decoder and a brightness control. Although the main function is to drive the 8-Digits seven segment LED display but because it also capable to drive an individual LED segment i.e. segment A to segment G and DP (decimal point), therefore you could also use this chip to drive the individual LED, the LED bar-graph, or the 8×8 LED matrix display. The MAX7219 could easily be daisy-chained (cascaded) to another MAX7219 chip using the DOUT pin which is useful when you want to drive 16-Digits LED display or several LED matrix display.
The AVR ATmega328P TIMER1 Peripheral

The 16-bits TIMER1 peripheral is the heart beat of this stopwatch project. We used this 16-bit TIMER1 as it give more flexibility to implement the stopwatch, because of the 16-bit counter length, therefore it could counting up to 65.536 before overflow. We will use TIMER1 16-bit output compare register OCR1A (OCR1AH – OCR1AL) as the base of our hundredth of seconds counter.

Bhawna Sachdeva
Assistant Professor
ECE Department

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