DIGITAL CLOCK MODEL CLK100
GEKCO INC.
Rev 1.1
25
11.2.
Main Board Circuit Description
The main circuit board has the microcontroller, real time clock (RTC), switch inputs and connectors for
the display board, programming interface and the input power.
The microcontroller U3 is the heart of the clock. This design uses the popular Atmel ATMEGA328P.
This is a powerful microcontroller with 32Kbytes of flash memory, and 2Kbytes of SRAM as well as
numerous other peripherals. See the ATMEGA328P data sheet for details.
The crystal Y1 provides the clock for the microcontroller and C1 and C2 are required pad capacitors for
the crystal to operate. The display board interface connectors are J4 and J5. J4 is the only one used in
the model CLK100. This connector provides the standard serial peripheral interface (SPI) interface to
the display board. The signal SS is asserted low when serial data is present on the MISO signal. The
MISO signal is the serial data stream from the microcontroller to the MAX7219 on the display board.
The signal SCK provides the clock timing for the interface. More details of the SPI interface can be
found on the web.
Connector J3 provides the interface to the real time clock (RTC) module. The clock time accuracy and
the time keeping function are provided by the RTC module, which has a Maxim DS3231 real time
clock. This device uses a 3 wire, I2C interface to communicate with the microcontroller.
Connector J2 is the programming interface connector. This allows the firmware to be modified and the
ability to reprogram the microcontroller. The CLK100 board looks just like a Arduino Uno and can use
the popular Arduino IDE.
The path for the input power for the clock is provided by connector J1. The input voltage should be 5V
+/- 5%. Diode D1 is a schottky diode which provides protection of the internal circuits if the polarity of
the applied voltage is incorrect.
Resistor network RN1 provides the 10kohm pull up resistors for the switch inputs SW1 through SW4.
SW1, SW2 and SW3 provide the clock setup capabilities and SW4 allows the user to reset the device.
The piezo electric buzzer LS1 provides an audible alert for the alarm function.
11.3.
Display Board Circuit Description
The display board used in the model CLK100 utilizes the MAX7219 as the display driver and controller.
This part provides a 4 wire interface to the main board to interface to the 6 digit display. The LED
displays are multiplexed by the logic in the MAX7219. The microcontroller on the main board only
needs to communicate with the display board when a change in the display is desired.
The seven segment common cathode LED display modules are from Sun LED part number
XDUR14C2. There are two digits per display module and each digit has separate interface pins. In our
design, we have the segment sections wired in parallel and each digits common cathode is wired
separately to the display controller. Resistor R1 sets the current drive for the LED displays.
