AT91SAM9G20 datasheet(101/832 Pages) ATMEL

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101

6384D–ATARM–04-May-09

AT91SAM9G20 Preliminary

15. Real-time Timer (RTT)

15.1

Overview

The Real-time Timer is built around a 32-bit counter and used to count elapsed seconds. It gen-

erates a periodic interrupt and/or triggers an alarm on a programmed value.

15.2

Block Diagram

Figure 15-1. Real-time Timer

15.3

Functional Description

The Real-time Timer is used to count elapsed seconds. It is built around a 32-bit counter fed by

Slow Clock divided by a programmable 16-bit value. The value can be programmed in the field

RTPRES of the Real-time Mode Register (RTT_MR).

Programming RTPRES at 0x00008000 corresponds to feeding the real-time counter with a 1 Hz

signal (if the Slow Clock is 32.768 Hz). The 32-bit counter can count up to 2

32 seconds, corre-

sponding to more than 136 years, then roll over to 0.

The Real-time Timer can also be used as a free-running timer with a lower time-base. The best

accuracy is achieved by writing RTPRES to 3. Programming RTPRES to 1 or 2 is possible, but

may result in losing status events because the status register is cleared two Slow Clock cycles

after read. Thus if the RTT is configured to trigger an interrupt, the interrupt occurs during 2 Slow

Clock cycles after reading RTT_SR. To prevent several executions of the interrupt handler, the

interrupt must be disabled in the interrupt handler and re-enabled when the status register is

clear.

SLCK

RTPRES

RTTINC

ALMS

16-bit

Divider

32-bit

Counter

ALMV

CRTV

RTT_MR

RTT_VR

RTT_AR

RTT_SR

RTTINCIEN

RTT_MR

ALMIEN

rtt_int

RTT_MR

set

RTT_SR

read

RTT_SR

reset

RTT_MR

reload

rtt_alarm

RTTRST

RTT_MR

RTTRST

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