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M48T35AV-10PC1 Datasheet(PDF) 11 Page - STMicroelectronics |
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M48T35AV-10PC1 Datasheet(HTML) 11 Page - STMicroelectronics |
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11 / 19 page  11/19 M48T35AY, M48T35AV Calibrating the Clock The M48T35AY/35AV is driven by a quartz con- trolled oscillator with a nominal frequency of 32,768Hz. The devices are tested not to exceed 35 ppm (parts per million) oscillator frequency er- ror at 25°C, which equates to about ±1.53 minutes per month. With the calibration bits properly set, the accuracy of each M48T35AY/35AV improves to better than ±4 ppm at 25°C. The oscillation rate of any crystal changes with temperature (see Figure 11). Most clock chips compensate for crystal frequency and tempera- ture shift error with cumbersome trim capacitors. The M48T35AY/35AV design, however, employs periodic counter correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as shown in Fig- ure 9. The number of times pulses are blanked (subtracted, negative calibration) or split (added, positive calibration) depends upon the value load- ed into the five Calibration bits found in the Control Register. Adding counts speeds the clock up, sub- tracting counts slows the clock down. The Calibration byte occupies the five lower order bits (D4-D0) in the Control Register 7FF8h. These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a Sign bit; '1' indi- cates positive calibration, '0' indicates negative calibration. Calibration occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per minute, have one second either shortened by 128 or lengthened by 256 oscillator cycles. If a bi- nary '1' is loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a binary 6 is loaded, the first 12 will be affected, and so on. Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator cycles for every 125,829,120 actual oscillator cycles, that is +4.068 or –2.034 ppm of adjustment per calibra- tion step in the calibration register. Assuming that the oscillator is in fact running at exactly 32,768Hz, each of the 31 increments in the Calibration byte would represent +10.7 or –5.35 seconds per month which corresponds to a total range of +5.5 or –2.75 minutes per month. Two methods are available for ascertaining how much calibration a given M48T35AY/35AV may require. The first involves simply setting the clock, letting it run for a month and comparing it to a known accurate reference (like WWV broadcasts). While that may seem crude, it allows the designer to give the end user the ability to calibrate his clock as his environment may require, even after the fi- nal product is packaged in a non-user serviceable enclosure. All the designer has to do is provide a simple utility that accesses the Calibration byte. The second approach is better suited to a manu- facturing environment, and involves the use of some test equipment. When the Frequency Test (FT) bit, the seventh-most significant bit in the Day Register is set to a '1', and D7 of the Seconds Reg- ister is a '0' (Oscillator Running), DQ0 will toggle at 512Hz during a read of the Seconds Register. Any deviation from 512Hz indicates the degree and di- rection of oscillator frequency shift at the test tem- perature. For example, a reading of 512.01024Hz would indicate a +20 ppm oscillator frequency er- ror, requiring a –10 (WR001010) to be loaded into the Calibration Byte for correction. Note that set- ting or changing the Calibration Byte does not af- fect the Frequency test output frequency. The FT bit MUST be reset to '0' for normal clock operations to resume. The FT bit is automatically Reset on power-up. For more information on calibration, see the Appli- cation Note AN934 "TIMEKEEPER Calibration". Figure 9. Checking the BOK Flag Status READ DATA AT ANY ADDRESS AI00607 IS DATA COMPLEMENT OF FIRST READ? (BATTERY OK) POWER-UP YES NO WRITE DATA COMPLEMENT BACK TO SAME ADDRESS READ DATA AT SAME ADDRESS AGAIN NOTIFY SYSTEM OF LOW BATTERY (DATA MAY BE CORRUPTED) WRITE ORIGINAL DATA BACK TO SAME ADDRESS (BATTERY LOW) CONTINUE |
Similar Part No. - M48T35AV-10PC1 |
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Similar Description - M48T35AV-10PC1 |
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