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DS12885 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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DS12885 Datasheet(HTML) 11 Page - Maxim Integrated Products |
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11 / 22 page  Real-Time Clock ____________________________________________________________________ 11 Detailed Description The DS12885 family of RTCs provide 14 bytes of real- time clock/calendar, alarm, and control/status registers and 114 bytes (113 bytes for DS12C887 and DS12C887A) of nonvolatile, battery-backed static RAM. A time-of-day alarm, three maskable interrupts with a common interrupt output, and a programmable square- wave output are available. The devices also operate in either 24-hour or 12-hour format with an AM/PM indica- tor. A precision temperature-compensated circuit moni- tors the status of VCC. If a primary power-supply failure is detected, the devices automatically switch to a back- up supply. The backup supply input supports a primary battery, such as lithium coin cell. The devices are accessed through a multiplexed address/data bus that supports Intel and Motorola modes. Oscillator Circuit The DS12885 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or capacitors to operate. Table 1 specifies several crys- tal parameters for the external crystal. Figure 1 shows a functional schematic of the oscillator circuit. An enable bit in the control register controls the oscillator. Oscillator startup times are highly dependent upon crystal characteristics, PC board leakage, and layout. High ESR and excessive capacitive loads are the major contributors to long startup times. A circuit using a crystal with the recommended characteristics and proper layout usually starts within one second. An external 32.768kHz oscillator can also drive the DS12885. In this configuration, the X1 pin is connected to the external oscillator signal and the X2 pin is floated. COUNTDOWN CHAIN X1 X2 CRYSTAL CL1CL2 RTC REGISTERS DS12885 Figure 1. Oscillator Circuit Showing Internal Bias Network PARAMETER SYMBOL MIN TYP MAX UNITS Nominal Frequency fO 32.768 kHz Series Resistance ESR 50 k Ω Load Capacitance CL 6pF Table 1. Crystal Specifications* * The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to Application Note 58: Crystal Considerations for Dallas Real-Time Clocks for additional specifications. |
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