Digital clock with an ATmega16

• stabilized with a standard crystal,
• operating voltage from the grid, in case of failure buffering of clock and alarm by an accu pack,
• simple operation with three buttons,
• double display separate for clock and alarm time,
• fast adjust of clock and alarm time with a potentiometer,
• course adjust for brightness via a foto transistor,
• snooze repeat every five minutes with single-button,
• alarm via programmable melody in EEPROM,
• interface for in-system-programming.

0 Content

• 1. Hardware
  • 1.1 Processor-Hardware
  • 1.2 Display-Hardware
• 2. Use of the buttons
• 3. Software
  • 3.1 Commented source code
  • 3.2 Melody programming
  • 3.3 Fuse settings>

1 Hardware

1.1 Processor-Hardware

• the processor ATmega16 with a crystal 2,457600 MHz and the two startup capacitors,
• the ISP10 interface connector for programming within the system,
• the four anode driver PNP transistors BD238,
• the ports PC and PD as catode drivers for the displays,
• the AD converter connections for potentiometer and brightness signals,
• the speaker for alarm and the capacitor for DC separation,
• the two-row-connector for plugging the display unit on top of the processor unit.

• the power supply via a transformer with fuse protection on the primary side,
• the rectifier with two diodes and a capacitor,
• the voltage regulator with a 7805 and the two tantal capacitors for suppressing swinging,
• a Schottky diode and a stabilizing capacitor for processor supply,
• the emergency power supply with the NiMH accumulators, feeding into the processor supply, and the compensation for the self discharge via a resistor and a diode from the unregulated supply voltage.

1.2 Display-Hardware

• four large seven-segment-display devices, displaying the current time,
• four smaller seven-segment-display devices, displaying the alarm time,
• three keys for adjusting times and switching the state of the clock,
• the potentiometer for adjusting time and alarm,
• the foto transistor for measuring ambient light and adjusting the brightness of the displays,
• the resistors for limiting currents through the displays,
• the 2-row angle plug for plugging the display unit onto the processor unit.

2 Using the keys

3 Software

3.1 Software source code

1. Display and display modes
   • The upper, large display shows the time,
   • the lower, smaller display shows the alarm time,
   • the upper colon blinks in a second pulse when the clock is running,
   • the lower colon blinks if the alarm is armed and doesn't blink if the alarm is disarmed.
2. Display multiplexing
   • The display control uses timer/counter 1 in CTC mode.
   • Every 5 ms an interrupt occurs, when TC1 reaches the top value and TC1 is restarted.
   • The four display segments are refreshed every 20 ms, the multiplex frequency hence is 50 Hz.
   • After interrupt, the current segment is switched off first, the pointer to the SRAM position of the multiplex info is incremented and the SRAM content is written to port C.
   • The SRAM content on the pointer position +4 is read and written to port D.
   • The respective portbit for the anode driver is activated (cleared).
3. Display dim
   • The dim function (reduced brightness of the displays) works with the Compare Match Interrupt B of TC1.
   • The Compare-Match value is calculated from the AD conversion value of the foto transistor input.
   • If TC1 reaches Compare-Match B, the respective interrupt service routine switches off the anode driver.
   • Measuring the light and changing the Compare-Match value is only done in large intervals to avoid display flickering.
4. Time and alarm
   • The time counting also is derived from TC1's 5 ms pulse. For that, the Compare-Match A is used to relieve the CTC multiplex interrupt mode from that burden.
   • Within that interrupt service routine a counter, that has been set to 200, is downcounted. When reaching zero, the counter is restarted and a "second complete" flag is set.
   • Outside the service routine, the time is advanced by a second and, after 60 seconds, the minute counter is advanced. After 60 minutes the hour counter is also advanced, and set to 00:00 if 24 hours are reached.
   • If the alarm is armed, but not activated yet, the alarm is activatet.
5. Keys
   • The key inputs are read after each interrupt, at least every 5 ms. If a certain key is activated for at least 15 ms long (input pin clear for at least three times), the key is stored.
   • If for at least 15 ms none of the keys is activated, the respective key action is executed.
   • The following table shows how the keys act in different modes:
     
     

     Mode	Key	Action
     Normal	Black	Arming/disarming the alarm
     	Red	Start time adjust
     	White	Start alarm time adjust
     Time adjust	Black	Skip time adjust
     	Red	Store adjusted time (Hour, Minute)
     Alarm time adjust	Black	Skip alarm time adjust
     	White	Store adjusted alarm time (Hour, Minute), if minute adjusted arm the alarm
     Alarm	Black	Deactivate and disarm the alarm, reset the original alarm time
     	Red	Deactivate the alarm, add five minutes to the alarm time

6. AD conversion
   • The AD converter runs with a prescaler value of 128, measures two channels alternately and works with conversion complete interrupts.
   • The analog voltages of the potentiometer (channel ADC0) and of the collector of the foto transistor (channel ADC1) are measured, read left-adjusted and the upper 8 bits are added to a 16-bit sum.
   • If the results of 256 measurements are summed up, the upper byte of the sum are transferred and a flag is set.
   • Outside the service routine, the results of
     • ADC0 are converted to hours (*24) or minutes (*60) and the respective time or alarm time is set, if the time or alarm time adjustment is activated,
     • ADC1 is converted to set the Compare Match B value.
7. Alarm
   • The timer/counter TC0 is used as programmable audio frequency generator.
   • The prescaler of TC0 is set to 8, the timer works with OCR0 in CTC mode.
   • The port output works in toggle mode, so that OCR0 is the duration of a half rectangle swing.
   • With OCR0 values between 255 and 16 audio frequencies from 600 to 9600 Hz can be generated.
   • An interrupt by reaching the top in OCR0 triggers downcounting of a counter that determines the duration of a tone.
   • If the end of a tone has been reached, the next tone and its duration is is read from the EEPROM, the duration is converted and the next tone is played.
   • If the tone and its duration is read as zero, the counter TC0 is stopped and the toggle output pin deactivated.

3.2 Programming melodies

1. The melody is stored as a table in the EEPROM.
2. Each note requires two bytes storage space: the first byte codes the frequency or divider rate of TC0 in CTC mode, the second byte codes the tone duration and gives the number of timer runs, divided by 32.
3. The table starts at address 0000 in EEPROM and ends with the combination 0,0.
4. Because the ATmega16 provides 512 bytes EEPROM, the melody can have 255 notes (512 / 2 - 1).
5. The coding in the source code starts with the directive "e;.eseg"e;. The playable gamut reaches from E2 (659 Hz) to C5 (4186 Hz), available as constants named "cfxx".
6. The tone duration of the tones of that gamut are given as constants "cdxx", so encoding the tone E2 should add the two constants cfE2 and cdE2 to the table in the EEPROM.
7. From these constants ccxx and cdxx the complete melody can be composed and with the directive "e;.db"e; a table can be generated.
8. The hex file "e;digiclock.eep"e; that is generated during assembling the source code has to be written to the EEPROM after flashing the processor's code.

3.3 Fuse adjusting