CONTENTS i Contents 1 AVR Libc 1 1.1 Introduction. 1 1.2 General information about this library. This package contains the AVR C library docs in html and pdf format, the main package already contains the docs in man-page format (use 'avr-man xxxx' to access these).
The implementation aspires to conform with ISO/IEC 9899 (C90). However, due to limitations of thetarget processor and the nature of its development environment, a practical implementation mustof necessity deviate from the standard.
Section 7.23.2.1 clock()The type clock_t, the macro CLOCKS_PER_SEC, and the function clock() are not implemented. Weconsider these items belong to operating system code, or to application code when no operatingsystem is present.
Section 7.23.2.3 mktime()The standard specifies that mktime() should return (time_t) -1, if the time cannot be represented.This implementation always returns a 'best effort' representation.
Section 7.23.2.4 time()The standard specifies that time() should return (time_t) -1, if the time is not available.Since the application must initialize the time system, this functionality is not implemented.
Section 7.23.2.2, difftime()Due to the lack of a 64 bit double, the function difftime() returns a long integer. Database. In most casesthis change will be invisible to the user, handled automatically by the compiler.
Section 7.23.1.4 struct tmPer the standard, struct tm->tm_isdst is greater than zero when Daylight Saving time is in effect.This implementation further specifies that, when positive, the value of tm_isdst representsthe amount time is advanced during Daylight Saving time.
Section 7.23.3.5 strftime()Only the 'C' locale is supported, therefore the modifiers 'E' and 'O' are ignored.The 'Z' conversion is also ignored, due to the lack of time zone name.
In addition to the above departures from the standard, there are some behaviors which are differentfrom what is often expected, though allowed under the standard.
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There is no 'platform standard' method to obtain the current time, time zone, ordaylight savings 'rules' in the AVR environment. Therefore the application must initializethe time system with this information. The functions set_zone(), set_dst(), andset_system_time() are provided for initialization. Once initialized, system time is maintained bycalling the function system_tick() at one second intervals.
Though not specified in the standard, it is often expected that time_t is a signed integerrepresenting an offset in seconds from Midnight Jan 1 1970.. i.e. 'Unix time'. This implementationuses an unsigned 32 bit integer offset from Midnight Jan 1 2000. The use of this 'epoch' helps tosimplify the conversion functions, while the 32 bit value allows time to be properly representeduntil Tue Feb 7 06:28:15 2136 UTC. The macros UNIX_OFFSET and NTP_OFFSET are defined to assist inconverting to and from Unix and NTP time stamps.
Unlike desktop counterparts, it is impractical to implement or maintain the 'zoneinfo' database.Therefore no attempt is made to account for time zone, daylight saving, or leap seconds in past dates.All calculations are made according to the currently configured time zone and daylight saving 'rule'.
In addition to C standard functions, re-entrant versions of ctime(), asctime(), gmtime() andlocaltime() are provided which, in addition to being re-entrant, have the property of claimingless permanent storage in RAM. An additional time conversion, isotime() and its re-entrant version,uses far less storage than either ctime() or asctime().
Along with the usual smattering of utility functions, such as is_leap_year(), this library includesa set of functions related the sun and moon, as well as sidereal time functions.
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The tm structure contains a representation of time 'broken down' into components of theGregorian calendar.
The normal ranges of the elements of tm are.
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Compatibility
Avr Libc User Manual 1.8.0 Update
ATmega1284p @ 24.576MHz : Seeed Studio Goldilocks, Seeed Studio Goldilocks Analogue
ATmega2560 @ 16MHz : Arduino Mega, Arduino ADK
ATmega2560 @ 16MHz : Seeed Studio ADK
Files & Configuration
time.h : contains the definitions for all functions.
setup_RTC_interrupt() : this initialises Timer2, and the 'once per second' RTC interrupt.