Serial Real-Time Clocks
M41T6x Ultra-Low Current Family of Serial Real-Time Clocks

Many applications require the date and time, and Real-Time Clocks provide those functions. For example, in data storage applications, all files have date and time attributes that are updated every time a file is created or modified, and the RTC data must be accessed every time that occurs.

The M41T6x family boasts typical standby currents of only 350nA at 3.0V, which makes ST a leader in the marketplace.

These RTCs do not include battery switchover circuits, but are targeted at applications where they are not needed. In many of today’s applications, the entire system is often backed up, so individual components such as the RTCs do not need backup circuitry on chip. Capacitor backup is another example where switchover is not required. The capacitor’s charging diode inherently provides the switchover mechanism.

Handheld applications are a case where the RTC may run off the system battery and also be backed up by a capacitor. And since there is no backup circuitry, the RTC can be made smaller and less costly, and use even lower power. Since current draw is very important in handheld and battery operated systems, the RTC's standby current is a critical parameter. It determines the rate of battery consumption - how long the battery will last - as well as the backup life when using capacitor backup. Drawing only 350nA, users can get from minutes to months of backup time with a capacitor. And they will not encounter the regulatory issues often associated with batteries.

While 5V applications are still around, there is shrinking demand for them, but there is growing demand for 3V and 3.3V solutions. Furthermore, the next sweetspot is expected to be 1.8V. Operating from 1.3V to 3.6V, the M41T6x family meets today’s demands, and will meet tomorrow’s, too. Plus, they will keep time all the way down to 1.0V. These are outstanding numbers that should please any user.

In capacitor-backed applications (because they keep time all the way down to 1.0V), the M41T6x series RTCs are able to use more of the charge stored in the backup capacitor, thereby extending the backup life even further. This means less capacitance is required for a given backup time. For the double layer capacitors (“SuperCaps”) often used in these applications, reducing the capacitance lowers the cost.

Lower timekeeping voltage also means that in applications where the system voltage often sags - such as in battery operated and handheld devices - the timekeeping function is made more robust because the voltage must drop to a lower level before the clock begins to lose time. At the end of the day when a handheld unit’s rechargeable batteries have begun to sag, the clock will still be running, and the user will not need to reprogram it.

Brown-out occurs when Vcc drops too low for reliable operation of the circuit, but not low enough to ensure a proper power down/power up sequence. Some registers may get corrupted thereby losing track of the real time.

To detect this brown-out condition, the M41T6x parts include an Oscillator Fail Detect circuit. If, due to low Vcc, the oscillator begins to run intermittently, or stops altogether, a special filter detects this and sets a bit which indicates this event has occurred. The system software now has a bit it can check to confirm whether the time is still good or has possibly been corrupted. This is a key feature for many users, and a major selling point for these devices.