Real-Time Clock

Introduction

Your CPU has a real-time clock that:

provides the current date and time
displays the date and time of the last application shut-down

Clock Accuracy

The resolution of the real-time clock is 1 ms. The clock accuracy is affected by the operating temperature of the application:

Operating Temperature	Maximum Daily Drift (Seconds/Day)	Maximum Yearly Drift (Minutes/Year)
25 °C (77 °F) stabilized	+/- 2.6	+/- 17.4
0...60 °C (32...140 °F)	+/- 5.2	+/-33.1

Clock Back-Up

The accuracy of the real-time clock is maintained for 4 weeks when the CPU power is turned off if the temperature is below 45 °C (113 °F). If the temperature is higher, the back-up time is shorter. The real-time clock back-up does not need any maintenance.

If the back-up power is too low, system bit %S51 is set to 1. This value indicates a loss of time when the power supply was OFF.

Current Date and Time

The CPU updates the current date and time in the system words %SW49–%SW53 and %SW70. This data is in BCD.

NOTE: For M580 PACs, the current time is in universal coordinated time (UTC). If local time is needed, use the RRTC_DT function.

Accessing the Date and Time

You can access the date and time:

on the CPU debug screen
in the program
from the DTM diagnostics screen

To read the current date and time, read system words %SW49 through %SW53. This operation sets system bit %S50 to 0.

To write the current date and time, write system words %SW50 through %SW53. This operation sets system bit %S50 to 1.

When system bit %S59 is set to 1, you can increment or decrement the current date and time values with system word %SW59.

The function performed by each bit in word %SW59 is:

Bit	Function
0	increments the day of the week
1	increments the seconds
2	increments the minutes
3	increments the hours
4	increments the days
5	increments the months
6	increments the years
7	increments the centuries
8	decrements the day of the week
9	decrements the seconds
10	decrements the minutes
11	decrements the hours
12	decrements the days
13	decrements the months
14	decrements the years
15	decrements the centuries

NOTE: The preceeding functions are performed when system bit %S59 is set to 1.

Determining the Date and Time of the Last Application Shutdown

The local date and time of the last application shutdown are displayed in system words %SW54 through %SW58. They are displayed in BCD.

System Word	Most Significant Byte	Least Significant Byte
%SW54	seconds (0 to 59)	00
%SW55	hours (0 to 23)	minutes (0 to 59)
%SW56	month (1 to 12)	day in the month (1 to 31)
%SW57	century (0 to 99)	year (0 to 99)
%SW58	day of the week (1 to 7)	reason for the last application shutdown

The reason for the last application shutdown can be displayed by reading the least significant byte of system word %SW58, which can have these values (in BCD):

Word%SW58 Value	Definition
1	application switched to STOP mode
2	application stopped by watchdog
4	power loss
5	stop on detected hardware error
6	stop when errors such as these are detected: software error (HALT instruction) SFC error application CRC checksum error undefined system function call Details on the software detected fault type are stored in %SW125.