In the digital world, time is everywhere. Not only the high speed clocks that creates the oscillation and timing reference for a microcontroller, but also Real Time Clocks (RTC), giving the date and time like any watch. The vast majority of microcontrollers come with an embedded RTC that meet many application demands. But for very high demanding applications, a stand-alone RTC must be used.
A decade ago, microcontrollers integrating an RTC consumed several µA and could oscillate only with large 32kHz crystal. Indeed, small crystals have a high serial resistance in the range of 70 kΩ versus 30 kΩ, and microcontroller oscillators were not powerful enough to guarantee the oscillation with a high resistance crystal. It’s at that time that the PCF8563 from NXP and the M41T00S from STMicroelectronics became standards on the market and are still widely used.
Microcontrollers’ RTC robustness improved and nowadays RTCs consume only in the 300nA range with a high resistance crystal, but new conditions can necessitate the use of a stand-alone RTC.
In wearable devices, battery voltage can be higher than 3.6V, a common maximum voltage components can withstand. To optimize battery lifetime, an RTC accepts higher voltage input and wakes up the application by turning on the power supply at a fixed time thanks to an interrupt. It’s not even necessary to allot extra space on the board. The smallest 32kHz crystal in the world is the FC12-M from Epson with a tiny size of 2.05 x 1.2 mm, but the oscillator needs two extra capacitors. It even takes more space if compared to the M41T62LC from STMicroelectronics, which is an RTC with embedded crystal in a 3.2 x 1.5 mm footprint.
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A 32kHz crystal is usually accurate at +/- 35ppm at 25C and a drift in temperature of more than 100pm across temperature. As a point of reference, 100ppm error is more than 4 minutes a month and this large error is unacceptable in some applications like utility billing that cannot synchronize with a network. An embedded crystal allows designing an RTC with a Temperature Compensated Crystal (TCXO). The RX8900 from Epson or the PCF2129T from NXP are RTCs with TCXO -- and the last one guarantees an accuracy of +/-8ppm (or 21 seconds per month) across -30 to 80C.
Stand-alone RTCs also continue to optimize for power. The PCF2123 from NXP can consume down to 100nA with a low resistance crystal at 25C. No microcontroller can reach this performance today and new security applications need ultralow power. For instance, in new smart cards, the CCV code (the 3 digits at the back of a payment card) is dynamically changed every 20 minutes. Obviously, the ultrathin battery inserted in the card is not removable and ultralow power is very critical.
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Finally, the oil industry uses RTC modules that face extreme temperatures. Indeed, oil is found deeper and deeper in earth’s crust where temperatures increase dramatically. 175C is a very common situation in this industry, but incredibly, there are industrial RTC modules that can withstand temperatures as high as 200-225C.