ST10F271 datasheet(76/173 Pages) STMICROELECTRONICS

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Real time clock

ST10F271

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Real time clock

The Real Time Clock is an independent timer, in which the clock is derived directly from the

clock oscillator on XTAL1 (main oscillator) input or XTAL3 input (32 kHz low-power oscillator)

so that it can be kept on running even in Idle or Power down mode (if enabled to). Registers

access is implemented onto the XBUS. This module is designed with the following

characteristics:

●

Generation of the current time and date for the system

●

Cyclic time based interrupt, on Port2 external interrupts every ’RTC basic clock tick’

and after n ’RTC basic clock ticks’ (n is programmable) if enabled

●

58-bit timer for long term measurement

●

Capability to exit the ST10 chip from Power down mode (if PWDCFG of SYSCON set)

after a programmed delay

The real time clock is based on two main blocks of counters. The first block is a prescaler

which generates a basic reference clock (for example a 1 second period). This basic

reference clock is coming out of a 20-bit DIVIDER. This 20-bit counter is driven by an input

clock derived from the on-chip CPU clock, pre-divided by a 1/64 fixed counter. This 20-bit

counter is loaded at each basic reference clock period with the value of the 20-bit

PRESCALER register. The value of the 20-bit RTCP register determines the period of the

basic reference clock.

A timed interrupt request (RTCSI) may be sent on each basic reference clock period. The

second block of the RTC is a 32-bit counter that may be initialized with the current system

time. This counter is driven with the basic reference clock signal. In order to provide an

alarm function the contents of the counter is compared with a 32-bit alarm register. The

alarm register may be loaded with a reference date. An alarm interrupt request (RTCAI),

may be generated when the value of the counter matches the alarm register.

The timed RTCSI and the alarm RTCAI interrupt requests can trigger a fast external

interrupt via EXISEL register of port 2 and wake-up the ST10 chip when running power

down mode. Using the RTCOFF bit of RTCCON register, the user may switch off the clock

oscillator when entering the power down mode.

The last function implemented in the RTC is to switch off the main on-chip oscillator and the

32 kHz on chip oscillator if the ST10 enters the Power Down mode, so that the chip can be

fully switched off (if RTC is disabled).

At power on, and after Reset phase, if the presence of a 32 kHz oscillation on XTAL3 /

XTAL4 pins is detected, then the RTC counter is driven by this low frequency reference

clock: when Power Down mode is entered, the RTC can either be stopped or left running,

and in both the cases the main oscillator is turned off, reducing the power consumption of

the device to the minimum required to keep on running the RTC counter and relative

reference oscillator. This is valid also if Stand-by mode is entered (switching off the main

supply VDD), since both the RTC and the low power oscillator (32 kHz) are biased by the

VSTBY. Vice versa, when at power on and after Reset, the 32 kHz is not present, the main

oscillator drives the RTC counter, and since it is powered by the main power supply, it

cannot be maintained running in Stand-by mode, while in Power Down mode the main

oscillator is maintained running to provide the reference to the RTC module (if not disabled).

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