AN923 Application note Managing century information using serial real-time clocks and TIMEKEEPER® NVRAMs By Doug Sams Introduction ® ST's serial real-time clocks (RTCs) and TIMEKEEPER NVRAMs all include at least one byte of year data in binary-coded decimal (BCD) format. Many devices include additional bits for tracking the century, effectively extending the year register. Applications can use software to further extend the century/year data to any desired resolution. The amount of software required depends on the resources built into the chip and the resolution needed by the application. January 2015 DocID005227 Rev 4 1/9 www.st.com Devices with no century data 1 AN923 Devices with no century data M48T02/12, M48T08/08Y/18 A few devices have only a single byte of year information. The BCD year register represents a 2-digit number in the range 00 to 99. The result is that the software must interpret what the 2-digit number means. For example, 85 might be interpreted as 1985, or 2085, or 2185. So the software must maintain the upper two digits of the year, outside of the RTC. The reader should note that the upper two digits of the 4-digit year are considered the century value. So, for the devices listed above, the software must maintain the century information in non-volatile memory such as flash or EEPROM, and increment the century value whenever the year value, in the RTC, rolls over from 99 to 00. 2/9 DocID005227 Rev 4 AN923 2 Devices with one bit of century data Devices with one bit of century data M41ST85W, M41T0, M41T00, M41T00AUD, M41T00CAP, M41T00S, M41T11, M41T56, M41T80, M41T81, M41T81S, M41T94, M48T35AV, M48T35/Y, M48T58 Most ST clock devices include at least one bit of century information. When enabled (CEB is set), the century bit will toggle at the end of every century, at midnight of December 31 of the year ending in 99. User software must interpret the bit's meaning. For example, users can let 0 represent 2000-2099 and 1 represent 2100-2199, but other mappings may also be used. By adding more bits in software, the century information can be extended to whatever resolution is desired. DocID005227 Rev 4 3/9 Devices with two bits of century data 3 AN923 Devices with two bits of century data M41ST87W, M41T60, M41T62, M41T63, M41T64, M41T65, M41T66, M41T82, M41T83, M41T93 Many of ST's newer RTCs include 2 century bits (CB1, CB0) which function as a 2-bit binary counter that increments at the end of each century. The user may arbitrarily assign the meaning of CB1:CB0 to represent any century value, but the simplest way of using these bits is to extend the year register by mapping them directly to bits 9 and 8 (with the year register comprising bits 7:0). Higher order century bits can be maintained in the application software. Figure 1: Two-bit binary counter (century bits CB1:CB0) Example: 16-bit year value MAINTAIN� ADDITIONAL YEAR BITS IN SOFTWARE LOWER 8 BITS CONTAINED IN YEAR REGISTER (07h) 00 01 11 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CB1 CB0 Century 2000 -2099 2100 - 2199 2200 - 2299 2300 - 2399 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 CB1:CB0 00 01 10 11 CB1:CB0 LET CB1:CB0 REPRESENT BITS 9 AND 8 TO EXTEND THE YEAR REGISTER In this example, CB1:CB0 represent the two lower bits of the century byte. 4/9 DocID005227 Rev 4 AN923 4 Devices with a full byte of century data Devices with a full byte of century data M48T201V/Y, M48T37V/Y Lastly, a few ST TIMEKEEPER devices include a full byte of century information. In addition to the byte of year data, there is another, upper byte for the century, thus comprising a full 4-digit (16-bit BCD) year parameter. These devices will automatically track the century through the year 9999. However, to go beyond that, user software would still need to add a bit to support the years 10000-19999. The following table summarizes the type of century data provided by the various ST RTC and TIMEKEEPER devices. DocID005227 Rev 4 5/9 Devices with a full byte of century data AN923 Table 1: Century data provided according to device Device Century support Byte (1) CB1/CB0 (2) CEB/CB (3) ● M41ST87W M41T0 ● M41T00 ● M41T00AUD ● M41T00CAP ● M41T00S ● M41T11 ● M41T56 ● M41T60 ● M41T62 ● M41T63 ● M41T64 ● M41T65 ● M41T66 ● Serial RTCs M41T80 ● M41T81 ● M41T81S ● M41T82 ● M41T83 ● M41T93 ● ● M41T94 ● TIMEKEEPER supervisor M48T02/12 ● M48T08/08Y/18 ● M48T35/Y ● M48T35AV ● M48T37V/Y ● ● M48T58 Notes: (1) (2) (3) (4) 6/9 Product family ● M41ST85W M48T201V/Y None (4) 1 byte (BCD) which increments once per century 2 century bits which increment once per century 1 century bit which toggles once per century No century information DocID005227 Rev 4 TIMEKEEPER NVRAM AN923 5 Support for leap year Support for leap year Leap year occurs every four years, in years which are multiples of 4. For example, 2012 was a leap year. An exception to that is any year which is a multiple of 100. For example, the year 2100 is not a leap year. A contradiction to that is that years which are multiples of 400 are indeed leap years. Hence, while 2100 is not a leap year, 2400 is. During any year which is a multiple of 4, ST RTC and TIMEKEEPER devices will automatically insert leap day, February 29. Therefore, the application software must correct for this during the exception years (2100, 2200, etc.) as noted above. For more information about ST's TIMEKEEPER NVRAM's and serial real-time clocks, please contact a local ST sales office or visit www.st.com. DocID005227 Rev 4 7/9 Revision history 6 AN923 Revision history Table 2: Document revision history Date Revision Changes May-2000 1 Initial release 08-Apr-2013 2 Updated title, updated devices in Table 1: "Century data provided according to device", removed obsolete contact information 8/9 14-Aug-2013 3 Complete rewrite of document 07-Jan-2015 4 Removed M41TC8025 device from document DocID005227 Rev 4 AN923 IMPORTANT NOTICE – PLEASE READ CAREFULLY STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgement. Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or the design of Purchasers’ products. No license, express or implied, to any intellectual property right is granted by ST herein. Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product. ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners. Information in this document supersedes and replaces information previously supplied in any prior versions of this document. © 2015 STMicroelectronics – All rights reserved DocID005227 Rev 4 9/9