What's the fuss about RTCs?
RTCs are everywhere. It's estimated (by our RTC manufacturers who need to know this stuff) that at least 50% of all applications have some form of RTC on board. And with good reason. Almost any data-driven, time-sensitive embedded system features some form of time-based function. Specific actions and recorded events depend on the accuracy of the date and time an event occurs, or when an activity is triggered. For example, a door security system may enforce different access privileges based on the current time of day.
Real time clocks are often critical in embedded systems that record events or alarms when they are tasked with being a reliable audit trail. These could include a fire alarm system that needs to keep a record of alarm events. Often the accuracy of a system's time keeping can have significant financial implications so, despite their diminutive size, an RTC's role in your design should not be underestimated. In fact, a failure to implement best practice in RTC electronic circuit design can be expensive, especially if you have to later rectify a problem within a live system already out in the field.
RTC Module Vs RTC in MCU
Most MCUs these days have a built in RTC, ADC, DAC and various other functions which do a phenomenal job within the confines of a single compact module. However, there are several areas where the dedicated RTC offers a superior solution to having on-board convenience.
Built-in MCU RTC
A larger backup power supply is needed to maintain RTC function in case of a power outage
Overall power consumption is higher
Greater vulnerability to external factors
A built-in RTC can result in a slightly lower cost, but as we've explored - the implications of an inaccurate or even failed time function could have far greater financial implications
Benefits of a dedicated RTC
At Board Level
Single device: No external components required like quartz, capacitors or resistor
Reduced BOM: just one single component needed: RTC-Module
Small footprint, low overall height
Ideal for wearable and portable applications
At Signal and Software Level
Plug and play, factory calibrated, internal components are tuned where necessary
Insensitive to external disturbances, no special board layout required
Quality & Reliability Standards
Production facilities already comply with medical standards
Modules qualified according automotive standards
Varies by device but can range from +/- 35ppm +/-20s/wk or +/-3ppm or +/-2s/wk
Built-in periodical time correction can keep the system accurate
What are real-world examples of ppm accuracy?
100m world record 9.58s measures with 1/100s resolution requires <100ppm accuracy
Chronometer - mechanical, better than 2s/day = 23ppm
Best RTC module: 3ppm
Low Power Consumption
From 350nA down to as little as 60nA
60nA equates to just 1mAh in 14 months
A back-up coin cell battery has 16mAh capacity, whilst a small 0.022F Supercap can supply an RTC for 90 days
Image of the RV8803-C7 RTC module from Micro Crystal
Useful Functions an RTC can perform
Light in the car stays on for 20 more seconds after car key is pulled, several similar not very visible applications, like fan control, blinker timing and there more
EV Charging management:
Charging of batteries depends on a number of factors: voltage, temperature, balancing of cells, how fast and how long they are charged
Smart charge management - charge off-peak, sell back to the grid during peak demand
Max. charge time is also monitored, even it is not the actual switch-off criteria
Set once the actual time and forget
A system has to reset the countdown timer periodically. If not, an interrupt signal is generated. The length of the max period can be programmed from 10s of milliseconds to a few hours
Length for a specific task
Time between tasks
Wake up of the microcontroller from hibernation or sleep mode to safe power and charge of the battery
32kHz, 1kHz, 1Hz … square wave
Reference clock for Microcontroller, for Blue-tooth and for other low power applications
Tone generations, beep
Timing of long time intervals:
RTC alarm facility can be used to accurately interrupt the controller after seconds, minutes or even after days. A microcontroller would require cascaded loops and depends on actual clock frequency; modern Micro-controller architectures have dynamic operating clocks: cumbersome programming
Timing of short intervals
Accurate timing of time intervals in the order of ms to seconds or minutes using the timer
• Remote time access:
Have one Real-Time Clock to be accessed by different microcontrollers. I2C-bus is multi-master capable
Best practice in RTC design
So, you've decided on a dedicated RTC and, whilst it can offer a superior solution to a built-in Microcontroller, you can help improve the capability and performance of your RTC by observing these simple rules..
Place your crystal as close as possible to the RTC and keep the trace as short as possible to reduce the possibility of noise coupling.
Avoid routing high-speed signals within 200mm of the RTC circuitry.
Minimise interference being coupled into the clock signal by not routing any traces under or between the RTC and the crystal.
Place a ground plane underneath the RTC circuitry and isolate it from other ground planes by no less than 40mm.
Bear in mind that your product will probably not have optimal conditions in the field. External factors like temperature, dirty power supplies and interference call affect RTC performance and anticipating these in the lab could save considerable expense if the unit fails in the field.
Applications for your RTC
Medical & Healthcare
Home medical treating Glucose dispenser or Gluco-meter
Internet cameras Industrial computer Embedded systems
Battery charging management e-vehicle
Home & Building automation
Door lock systems
Smart watering systems
Burglar alarm systems
Mid and high end digital cameras and camcorders
Our RTC Manufacturers
Ineltek works with two global leaders in RTC manufacture - Epson and Micro Crystal. They have a comprehensive portfolio of RTC solutions and, in partnership with Ineltek's FAEs, provide valuable support in helping to get your RTC projects running like clockwork.
If you would like to know more, please contact us for more details.