PICclock June 2015

My fishkeeping hobby has influenced me in making my own "real-time" clock timer, because I wanted to remove the hassle of annual replacement of T8 fluorescent lamps that quickly degrade and have harmful effects of UV spectrum. Furthermore, AC power line timers with relays have failed on me multiple times because of mechanical relay contacts. Therefore an idea of creating a custom timer and perfect spectrum LED lights for aquarium was born.

PCB design

Since I have some leftovers of PICs from my Helipad project, I decided to use them also in PICclock design. This brought all the hassle of using Assembly with limited MCU functionality (2-level stack, 12-bit counter, no interrupts...), but the project could be executed quickly and with available components.

The circuit's schematics and layout were drawn in KiCad, which was a huge improvement over the Eagle with open-source base and no feature limitation.

2 MCUs were used, one for dual channel timer control, 6 LEDs to display hours, minutes or seconds and 2 buttons to select mode and increase clock/change display. The second MCU was used to generate light transition on ON/OFF signal from the channel by PWM output MOSFETs. The schematic and layout are shown in Figures 1 and 2.

Figure 1: Schematics of PICclock

Figure 2: Layout of PICclock

PCB fabrication

Last time I have made my own PCBs I have used the phototransfer method but due to its time consumption and equipment used. Therefore I have decided to try the heat transfer method from laser copy on paper. The finished PCB is not as precise as with phototransfer method, as there are some parts on the PCB where copper of traces was etched but the circuit was nevertheless a success as shown on Figures 3 and 4. Lastly all the components were soldered and the circuit was ready to be populated with programmed PICs. The finished product is on Figure 5.

Figure 3: Etching a heat-transferred mask

Figure 4: Etched PCB with some imperfections

Figure 5: Final PICclock PCB with soldered components

Programming 8-bit PIC MCUs in Assembly

The 12-bit timer of PIC16F505 which is too small for counting 4,000,000 cycles using 4MHz clock leaded to construction of 1 second while iterations which can be precisely constructed because Assembly instructions are precisely defined how many cycles each takes.

The 1 second iteration was built from the following parts:

increment seconds and check for 60s overflow
increment minutes if second's overflow happens and check for 60min overflow
increment hours if minute's overflow happens and check for 24h overflow
cycle LEDs if SET button pressed: NONE > seconds > minutes > hours
check if timer1 needs to turn ON/OFF
check if timer2 needs to turn ON/OFF
check if MODE button pressed
- increment hours by 1 if SET button pressed
- check if MODE button pressed
  - increment minutes by 1 if SET button pressed
delay to compensate the empty cycles for 1 second iteration

Code snippets from PICs running "real-time" clock and timer and PWM timer are shown in Figures 6 and 7.

Figure 6: Assembly code snippet for "real-time" clock

Figure 7: Assembly code snippet for PWM transition

Conclusion

The design and production of a PCB was quick and hassle free where as coding the PICs consumed the most time because of the 1s iteration criterion. Later I have found out that 8-bit counter could be used to count seconds since it has additional 8-bit multiplier to count MCU's ticks. If a binary RTC oscillator such as 32.768kHz was used, the need for precise 1s iteration could be dropped.

Update - February 2023

Since my Advanced Clock project was taking a lot of time, I decided to add some upgrades to my PICclock in the meanwhile. First is to use a RTC clock oscillator with 32.768 kHz resonant frequency together with timer to count time in a more precise manner. This will allow changes to code with minimal effort, since I would not need to count instructions for one second iteration, and even improve power consumption of MCU, but this is not a major thing, because chip consumes under 2mA with 4MHz resonator.

The code needed to implement timer and one second instruction is shown in Figure 8. And in Figure 9 and 10 a slow start up and shut down filter and current regulator for one channel of LEDs are presented. A slow transition allows more natural feeling for the fish and was made using third order passive RC filter and some offset to current sensing was added, that way when MCU signal is off, the LED current will definitely go to zero.

Figure 8: Assembly code for TMR0 implementation for counting one second

Figure 9: Third order RC filter for current transition

Figure 10: Filter and regulator circuit for slow current transition