STR720 datasheet(150/401 Pages) STMICROELECTRONICS

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STR720 - REAL TIME CLOCK (RTC)

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REAL TIME CLOCK (RTC)

14.1 Introduction

The RTC provides a set of continuously running counters which can be used, with suitable

software, to provide a clock-calendar function. The counters values can be written to set the

current time/date of the system.

The RTC includes an APB slave interface, to provide access by word to internal 48-bit

registers. this interface is properly disconnected by the APB bus when the main power supply

is removed.

14.2 Main Features

■

48-bit programmable counter for long term measurement

■

External clock input, CLK_AF, defining RTC ticks (must be at least 4 times slower than APB

clock) driven by two different sources:

•

32 kHz crystal oscillator connected to OSCIN and OSCOUT

•

External clock generator connected to OSCIN

■

Separate power supply

■

2 dedicated maskable interrupt lines:

•

alarm interrupt, to generate a software programmable alarm interrupt (up to 1 s)

•

periodic interrupt, to generate a software programmable periodic interrupt (up to 32 s)

14.3 Functional Description

14.3.1

Overview

The RTC consists of two main units (see Figure 18 on page 151), the first one (S-APB

Interface) is used to interface with the APB bus. This unit also contains a set of 16-bit

registers, synchronous to the S-APB clock and accessible from the APB bus in read or write

mode (for more details refer to Register description section). The APB interface is clocked by

S-APB clock in order to interface with APB bus.

The other unit (RTC Core) consists of three programmable registers. The first is made of a

48-bit programmable counter that may be initialized to the current system time. The system

time is incremented at CLK_AF rate and compared with a programmable date (stored in the

second 48-bit register, RTCA) in order to generate an alarm via an interrupt (Alarm), if

enabled in the RTCCR control register. The third register is the 20-bit RTCP register that is

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