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DS17285E-3 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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DS17285E-3 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 30 page Detailed Description The DS17x85 is a successor to the DS1285 real-time clock (RTC). The device provides 18 bytes of real-time clock/calendar, alarm, and control/status registers and 114 bytes of nonvolatile battery-backed RAM. The device also provides additional extended RAM in either 2k/4k/8kbytes (DS17285/DS17485/DS17885). A time- of-day alarm, six maskable interrupts with a common interrupt output, and a programmable square-wave output are available. It also operates in either 24-hour or 12-hour format with an AM/PM indicator. A precision temperature-compensated circuit monitors the status of VCC. If a primary power-supply failure is detected, the device automatically switches to a backup supply. The backup supply input supports a primary battery, such as a lithium coin cell. The device is accessed by a mul- tiplexed address/data bus. Oscillator Circuit The DS17x85 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or capacitors to operate. Table 1 specifies several crystal parameters for the external crystal, and Figure 2 shows a functional schematic of the oscillator circuit. The oscillator is controlled by an enable bit in the con- trol register. Oscillator startup times are highly depen- dent upon crystal characteristics, PC board leakage, and layout. High ESR and excessive capacitive loads are the major contributors to long startup times. A cir- cuit using a crystal with the recommended characteris- tics and proper layout usually starts within one second. An external 32.768kHz oscillator can also drive the DS17x85. In this configuration, the X1 pin is connected to the external oscillator signal and the X2 pin is floated. Clock Accuracy The accuracy of the clock is dependent upon the accu- racy of the crystal and the accuracy of the match between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was trimmed. Additional error will be added by crystal frequency drift caused by temperature shifts. External circuit noise coupled into the oscillator circuit may result in the clock running fast. Figure 3 shows a typical PC board layout for isolation of the crystal and oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks for detailed information. Clock Accuracy (DS17287, DS17487, and DS17887) The encapsulated DIP (EDIP) modules are trimmed at the factory to ±1 minute per month accuracy at 25°C. Real-Time Clocks ____________________________________________________________________ 11 PARAMETER SYMBOL MIN TYP MAX UNITS Nominal fO 32.768 kHz Series Resistance ESR 50 kΩ Load Capacitance CL 6 or 12.5 pF Table 1. Crystal Specifications* (DS17x85 Only) *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 addi- tional specifications. COUNTDOWN CHAIN X1 X2 CRYSTAL CL1CL2 RTC REGISTERS DS17285/87 DS17485/87 DS17885/87 Figure 2. Oscillator Circuit Showing Internal Bias Network LOCAL GROUND PLANE (TOP LAYER) CRYSTAL GND X2 X1 NOTE: AVOID ROUTING SIGNAL LINES IN THE CROSSHATCHED AREA (UPPER LEFT QUADRANT) OF THE PACKAGE UNLESS THERE IS A GROUND PLANE BETWEEN THE SIGNAL LINE AND THE DEVICE PACKAGE. Figure 3. Layout Example |
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Similar Description - DS17285E-3 |
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