Does system time tick on Arduino? - time

Question

After setting system time via a call to a real-time-clock (RTC), repeated calls to time() always return the same value. Does the system time actually progress on Arduinos or do I need to continue querying the RTC every time I need the time?
To illustrate:
void InitRTC(void) {
DateTime rtcDT; // type defined by the RTC library
time_t bin_time;
rtcDT = rtc.now();
bin_time = rtcDT.secondstime(); // returns unixlike time
set_system_time(bin_time); //AVR call to set the sys time
}
void Dump(time_t t) {
char debug_log_message[MAX_DEBUG_LENGTH];
sprintf(debug_log_message, " time_t:\t %lu", t);
DebugLog(debug_log_message); //....routine to print to the serial port
}
void setup() {
InitRTC();
time_t now;
while (1) {
now = time(0);
Dump(now);
}
}
(safety checks omitted, serial code omitted).
This simply prints the same time for ever - it never progresses!

Answer 1

The standard library has few platform dependencies, so is very portable. However one of those dependencies is the source for real-time, which is entirely platform dependent, and as such it is common for libraries to leave the function as a stub to be re-defined by the user to suit the specific platform or to implement hook functions or call-backs to run the standard library clock.
With the avr-libc used by AVR based Arduino platformsfFor time() to advance it is necessary to call the hook function system_tick() at 1 second intervals (typically from a timer or RTC interrupt) (refer to the avr-libc time.h documentation. It is also necessary to set the time at initialisation using the set_system_time() function.
Invoking system_tick() from a 1Hz timer will then maintain time(), but it is also possible to use an RTC alarm interrupt, by advancing the alarm match target on each interrupt. So you might for example have:
void rtc_interrupt()
{
// Set next interrupt for 1 seconds time.
RTC rtc ;
int next = rtc.getSeconds() + 1 ;
if( next == 60 )
{
next = 0 ;
}
rtc.setAlarmSeconds( next ) ;
// update std time
system_tick() ;
}
void init_system_time()
{
tm component_time ;
RTC rtc ;
// Get *consistent* time components - i.e ensure the
// components are not read either side of a minute boundary
do
{
component_time.tm_sec = rtc.getSeconds() ;
component_time.tm_min = rtc.getMInutes(),
component_time.tm_hour = rtc.getHours(),
component_time.tm_mday = rtc.getDay(),
component_time.tm_mon = rtc.getMonth() - 1, // January = 0 in struct tm
component_time.tm_year = rtc.getYear() + 100 // Years since 1900
} while( component_time.tm_min != rtc.getMinutes() ) ;
set_system_time( mktime( &component_time ) - UNIX_OFFSET ) ;
// Set alarm for now + one second
rtc.attachInterrupt( rtc_interrupt ) ;
rtc.setAlarmSeconds( rtc.getSeconds() + 1 ) ;
rtc.enableAlarm( rtc.MATCH_SS ) ;
}
An alternative is to override time() completely and read the RTC directly on each call - this has the advantage of not requiring any interrupt handlers for example:
#include <time.h>
extern "C" time_t time( time_t* time )
{
tm component_time ;
RTC rtc ;
// Get *consistent* time components - i.e ensure the
// components are not read either side of a minute boundary
do
{
component_time.tm_sec = rtc.getSeconds() ;
component_time.tm_min = rtc.getMInutes(),
component_time.tm_hour = rtc.getHours(),
component_time.tm_mday = rtc.getDay(),
component_time.tm_mon = rtc.getMonth() - 1, // January = 0 in struct tm
component_time.tm_year = rtc.getYear() + 100 // Years since 1900
} while( component_time.tm_min != rtc.getMinutes() ) ;
return mktime( &component_time ) ;
}
I have assumed that the Arduino library getYear() returns years since 2000, and that getMonth() returns 1-12, but neither is documented, so modify as necessary.
Linking the above function before linking libc will cause the library version to be overridden.