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SH3000UM Datasheet(PDF) 11 Page - Semtech Corporation |
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SH3000UM Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 24 page  SH3000 MicroBuddy™ SYSTEM MANAGEMENT Copyright ©2003-2005 Semtech Corporation 11 V1.15 www.semtech.com The crystal oscillator has the useful feature of adjustable load capacitors. It permits tuning of the circuit for initial tolerance of the crystal (often ±20 ppm) as well as an adjustment for the required load capacitance (with possible variations from the PCB layout). While the oscillator was designed for a crystal with a nominal load capacitance of 12.5 pF, the circuit accommodates any value from ~7 pF to 22 pF (depending on parasitics of the layout). All of these corrections can be performed when the part is already installed on the PCB, in the actual circuit. The default value for load capacitance (12.5 pF) loaded on power-up from the factory-programmed OTP nonvolatile memory can be re-programmed at any time (following a secure process of unlocking the load capacitance value register and immediately writing a new setting), or it can be completely protected from any changes by a permanent OTP write-protect flag. This adjustment can set the frequency of the crystal oscillator to within ±4ppm of the ideal value. As a reference, a typical 32.768 kHz crystal changes its frequency 4ppm for a 10°C change in temperature. Since the temperature characteristics of crystals are well known and stable, the host processor is free to implement an algorithm for temperature compensation of the crystal oscillator using the adjustable load capacitors, with resulting accuracy of ±4ppm over the entire temperature range. Real Time Clock Using the ±4ppm, 32.768 kHz clock from the LF oscillator, the Real Time Clock module keeps time with a maximum error as low as 2 minutes per year. This compares favorably with a conventional error of 2 minutes per month for the typical RTC chip. The hardware of the Real Time Clock is capable of 179-years of calendar operations (see Figure 6). All counting-chain values are loaded at the same time into corresponding registers when the Fractions register is read. All values from registers are loaded into the counting-chain when the Fractions register is written. The RTC continues normal operations when VDD is absent, if backup power is available. 14 Figure 6. Real Time Clock and Periodic Interrupt / Wakeup timer IO/INT 16-bit Counter Days (BIN) 0 - 65535 256 Hz 32-bit Time Interval 32-bit Counter 32-bit Comparator 32-bit Latch Interrupt Logic Serial I/O RESET ÷ 256 ÷ 60 ÷ 60 ÷ 24 1 Hz LSB MSB Minutes (BCD) 0 - 59 Seconds (BCD) 0 - 59 Fractions (BIN) 0 - 255 Hours (BCD) 0 - 23 32-bit Latch LOAD LOAD LSB Current Timer Value LSB MSB 32768 Hz ÷ 128 |
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