C167CS datasheet(310/517 Pages) INFINEON

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C167CS

Derivatives

The Real Time Clock

User’s Manual

14-4

V2.0, 2000-07

Defining the RTC Time Base

The reload timer T14 determines the input frequency of the RTC timer, i.e. the RTC time

base, as well as the T14 interrupt cycle time. Table 14-2 lists the interrupt period range

and the T14 reload values (for a time base of 1 s and 1 ms) for several oscillator

frequencies:

Increased RTC Accuracy through Software Correction

The accuracy of the C167CS’s RTC is determined by the oscillator frequency and by the

respective prescaling factor (excluding or including T14). The accuracy limit generated

by the prescaler is due to the quantization of a binary counter (where the average is

zero), while the accuracy limit generated by the oscillator frequency is due to the

difference between ideal and real frequency (and therefore accumulates over time). The

total accuracy of the RTC can be further increased via software for specific applications

that demand a high time accuracy.

The key to the improved accuracy is the knowledge of the exact oscillator frequency. The

relation of this frequency to the expected ideal frequency is a measure for the RTC’s

deviation. The number N of cycles after which this deviation causes an error of ± 1 cycle

can be easily computed. So the only action is to correct the count by ± 1 after each series

of N cycles.

This correction may be applied to the RTC register as well as to T14.

Also the correction may be done cyclic, e.g. within T14’s interrupt service routine, or by

evaluating a formula when the RTC registers are read (for this the respective “last” RTC

value must be available somewhere).

Note: For the majority of applications, however, the standard accuracy provided by the

RTC’s structure will be more than sufficient.

Table 14-2

RTC Interrupt Periods and Reload Values

Oscillator

Frequency

RTC Interrupt Period Reload Value A

Reload Value B

Minimum

Maximum T14REL

Base

T14REL

Base

32.768 kHz

Aux. 244.14

µs 16.0 s

F000H

1.000 s

FFFCH

0.977 ms

32 kHz

Aux. 250

µs

16.38 s

F060H

1.000 s

FFFCH

1.000 ms

32 kHz

Main 8000

µs

524.29 s

FF83H

1.000 s

----

4 MHz

Main 64.0

µs

4.19 s

C2F7H

1.000 s

FFF0H

1.024 ms

5 MHz

Main 51.2

µs

3.35 s

B3B5H

0.999 s

FFECH

1.024 ms

8 MHz

Main 32.0

µs

2.10 s

85EEH

1.000 s

FFE1H

0.992 ms

10 MHz

Main 25.6

µs

1.68 s

676AH

0.999 s

FFD9H

0.998 ms

12 MHz

Main 21.3

µs

1.40 s

48E5H

1.000 s

FFD2H

1.003 ms

16 MHz

Main 16.0

µs1.05 s

0BDCH

1.000 s

FFC2H

0.992 ms

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