Real time clocks with self-cal facilities

Real-time clock ICs speed up assembly and test by not requiring calibration during the assembly process.

STMicroelectronics has brought out real-time clock (RTC) chips with factory calibration, enabling manufacturers using the parts to simplify assembly and test operations by omitting the time-consuming calibration step.

The new M41T83 and M41T93 RTCs, in embedded-crystal packages, are precalibrated to plus or minus 5part/million, delivering a new level of accuracy to users of time-of-day clock and calendar devices.

Using a one-time programmable (OTP) register, ST trims the internal load capacitance of the 32.768kHz on-chip oscillator during factory test, ensuring that the parts are accurate to within 12s per month.

Apart from the serial interfaces, these two new devices share the same specifications: the M41T83 includes a 400 kHz I2C interface, and the M41T93 uses a 10MHz SPI bus, thought to be the world's first RTC to operate at this speed.

The parts introduce a number of advanced timekeeping features including analogue calibration, a programmable auto-reload counter/timer, and dual alarms.

In addition to the factory-programmed register, an additional analogue calibration register is included which allows the user to override the factory calibration settings.

This can be beneficial to adjust for the ambient temperature or to compensate for the effects of stray capacitance on the board.

This second register is also needed in applications using the crystal-less QFN16 package option.

Oscillator adjustments appear immediately in the test output to give instantaneous feedback to the user.

The auto-reload counter/timer can be used with one of four user-selectable source clocks to generate timing waveforms and to provide periodic processor interrupts.

The dual programmable alarms output on separate pins for applications requiring multiple alarm times.

Both devices include an onboard precision voltage reference which exactly defines the threshold for the automatic back-up switchover circuit.

The RTCs' low 450nA backup current (typical) makes them suitable for capacitor-backed applications as well as for battery backup, and with ST's low-power process technology, accurate timekeeping is ensured down to 1.8V.

A built-in battery monitor checks the back-up voltage and indicates when the battery should be replaced.

Other features include 12byte of NVRAM, and an oscillator-fail detection circuit which sets a bit if the oscillator drops more than three pulses, indicating that the back-up voltage might have fallen low enough to affect timekeeping -- a situation that might occur during brownout in capacitor-backed applications.

A programmable square-wave generator and a watchdog are also included.

When Vcc fails and the device automatically switches to backup, it generates a Reset signal which is held for 200ms after power is restored.

This Power-on Reset/Low-Voltage Detect function can save the designer from needing a separate Reset IC on the board.

Three voltage levels are available for the switchover and reset thresholds (nominally 2.93V, 2.63V and 2.32V) which are specified in the part number when ordering.

In addition to the precalibrated embedded crystal package, the M41T83 and M41T93 are also available in ST's space-saving, low-cost 4 x 4mm QFN16 package for use with an external crystal.

For cost-sensitive applications that nevertheless require mid-range real-time clock features, the low-cost M41T82 variant offers a reduced feature set of the M41T83.

It eliminates the watchdog, square-wave generator and alarms, but retains the 400kHz I2C interface, analogue user calibration, precision switchover reference, battery monitor, 12byte of NVRAM and reset output, and is supplied in a small footprint, low-cost 8-lead SOIC.

The M41T82, M41T83 and M41T93 are all sampling now with volume production slated for March 2007.

The M41T82 is priced at US $0.70 in volume.

The M41T83/93 in the QFN package will start at $0.90, and the factory calibrated embedded crystal versions are $1.20 in volume.

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