M41T82-83-93 series RTCs

AN2678

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1

M41T82-83-93 series RTCs

ST has long offered RTCs with digital calibration, and the new M41T82-83-93 series follows

suit. In addition to digital calibration utilizing periodic counter correction, this new series

features analog calibration wherein the oscillator frequency can be adjusted by adding or

removing load capacitance via a programmable capacitance array. Analog calibration has

the added benefit of instantaneous feedback for the user; adjustments to the oscillator

speed are immediately visible in the 512 Hz test output signal built into the devices.

1.1

Oscillator temperature characteristics

As shown in Figure 1, the frequency error of typical 32 KHz oscillators is around –25 parts

per million (ppm) at both 0°C and 50°C. That is, at 25° above and below room temperature,

the device exhibits a frequency deviation of –25 ppm, and it only gets worse as the

temperature deviates farther from +/–25°C.

Conversely, for an AT cut crystal, as is commonly used with microprocessors, the frequency

deviation is much less pronounced over a wide range of temperatures. The trade-off is that

AT cut crystals usually operate at frequencies much, much higher than watch crystals and

thus require much more power - too much for battery backup as is typically employed with

real-time clocks (RTCs).

But it is possible to get the low power performance of watch crystals along with the wide

temperature accuracy of AT cut crystals if the RTC can be periodically calibrated by the

microprocessor, almost all of which nowadays include timers that can be employed to

measure the frequency test (FT) pin of the RTC and correct any error in the timing utilizing

the calibration features of the RTC.

The approach discussed herein utilizes both of the calibration circuits in ST's M41T82,

M41T83 and M41T93 serial RTCs. This application note describes how to perform an initial

calibration, using the analog circuit, followed by occasional calibration updates using the

digital calibration circuit.

Initially, the analog calibration circuit is adjusted at room temperature, in a closed loop

fashion wherein successively smaller and smaller adjustments are made until the error is

minimized. After that, during normal operation, the microprocessor will periodically measure

the error as before, then use a look-up table, and write adjustments to the digital calibration

circuit.

1.2

Frequency test output

In the case of ST's M41T82, M41T83 and M41T93 serial RTCs, the frequency test signal is

output on the IRQ/FT/OUT pin. It is enabled by setting the FT bit (bit 6, register 0x08) to 1 in

the M41T82 and M41T83. (Refer to the datasheets for more information and for the

M41T93 details.) The FT signal is measured by test equipment and/or the microprocessor.

The calibration circuits are then adjusted accordingly.

The frequency test signal is nominally 512 Hz and is derived from the 32,768 Hz oscillator.

During the analog calibration sequence, changes in the oscillator will appear in real-time in

the FT signal. Thus, analog calibration changes can be measured and errors can be

minimized on-the-fly by the system microprocessor.