www.maxim-ic.com DS12885/DS12885Q/DS12885T Real-Time Clock www.maxim-ic.com FEATURES § § § § § § § § § § § § § § § § § § DS12885, 24 DIP DS12885S, 24 SO 300mil § PIN ASSIGNMENT (Top View) Drop- in replacement for IBM AT computer clock/calendar Pin configuration closely matches MC146818B and DS1285 Counts seconds, minutes, hours, days, day of the week, date, month, and year with leapyear compensation valid up to 2100 Binary or BCD representation of time, calendar, and alarm 12-hour or 24-hour clock with AM and PM in 12-hour mode Daylight Savings Time option Selectable between Motorola and Intel bus timing Multiplex bus for pin efficiency Interfaced with software as 128 RAM locations 14 bytes of clock and control registers 114 bytes of general purpose RAM Programmable square-wave output signal bus-compatible interrupt signals ( IRQ ) Three interrupts are separately softwaremaskable and testable Time-of-day alarm once/second to once/day Periodic rates from 122µs to 500ms End-of-clock update cycle Optional 28-pin PLCC surface mount package or 32-pin TQFP Optional industrial temperature range available Underwriters Laboratory (UL) recognized DS12885Q 28 PLCC DS12885T 32 TQFP Package Dimension Information http://www.maxim-ic.com/TechSupport/DallasPackInfo.htm Note: Some revisions of this device may incorporate deviations from published specifications known as errata. Multiple revisions of any device may be simultaneously available through various sales channels. For information about device errata, click here: http://www.maxim-ic.com/errata. 1 of 6 072902 DS12885/DS12885Q/DS12885T DESCRIPTION The DS12885 real-time clock plus RAM is designed to be a direct replacement for the DS1285. The DS12885 is identical in form, fit, and function to the DS1285, and has an additional 64 bytes of general purpose RAM. Access to this additional RAM space is determined by the logic level presented on AD6 during the address portion of an access cycle. An external crystal and battery are the only components required to maintain time-of-day and memory status in the absence of power. For a complete description of operating conditions, electrical characteristics, bus timing, and pin descriptions other than X1, X2, VBAT , and RCLR , see the DS12887 data sheet. TYPICAL OPERATING CIRCUIT ORDERING INFORMATION PART DS12885 DS12885N DS12885S DS12885N DS12885Q DS12885QN DS12885Q/T&R DS12885T DS12885TN DS12885T/T&R PIN-PACKAGE 24 DIP 24 DIP 24 SO 24 SO 28 PLCC 28 PLCC 28 PLCC/Tape and Reel 32 TQFP 32 TQFP 32 TQFP/Tape and Reel TEMP RANGE 0°C to +70°C -40°C to +85°C 0°C to +70°C -40°C to +85°C 0°C to +70°C -40°C to +85°C 0°C to +70°C 0°C to +70°C -40°C to +85°C 0°C to +70°C 2 of 4 DS12885/DS12885Q/DS12885T PIN DESCRIPTION AD0–AD7 N.C. MOT CS AS R/ W DS RESET – Multiplexed Address/Data Bus – No Connection – Bus Type Selection – Chip Select – Address Strobe – Read/Write Input – Data Strobe – Reset Input IRQ SQW VCC GND X1, X2 VBAT RCLR – Interrupt Request Output (Open Drain) – Square-Wave Output – +5V Supply – Ground – 32.768kHz Crystal Connections – +3V Battery Input – RAM Clear PIN DESCRIPTION X1, X2 – Connections for a standard 32.768kHz quartz crystal. The internal oscillator circuitry is designed for operation with a crystal having a specified load capacitance (C L) of 6pF. The crystal is connected directly to the X1 and X2 pins. There is no need for external capacitors or resistors. Note: X1 and X2 are very high- impedance nodes. It is recommended that they and the crystal be guard-ringed with ground and that high- frequency signals be kept away from the crystal area. For more information about crystal selection and crystal layout considerations, refer to Application Note 58 “Crystal Considerations with Dallas Real Time Clocks.” Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and escessive capactitive loads are the major contributors to long startup times. A circuit using a crystal with the recommended characteristics and following the recommended layout usually start within one second. VBAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be held between 2.5V and 4V for proper operation. A maximum load of 0.5µA at +25°C in the absence of power should be used to size the external energy source. Maximum load is measured using a recommended crystal type connected to X1 and X2. The battery should be connected directly to the VBAT pin. A diode must not be placed in series with the battery to the VBAT pin. Furthermore, a diode is not necessary because reverse charging-current protection circuitry is provided internally to the device and has passed the requirements of Underwriters Laboratories for UL listing. See “Conditions of Acceptability” at http://www.maxim- ic.com/TechSupport/QA/ntrl.htm. RCLR – The RCLR pin is used to clear (set to logic 1) all 114 bytes of general purpose RAM but does not affect the RAM associated with the real-time clock. In order to clear the RAM, RCLR must be forced to an input logic “0” (-0.3V to +0.8V) during battery-backup mode when VCC is not applied. The RCLR function is designed to be used by human interface (shorting to ground manually or by switch) and not to be driven with external buffers. This pin is internally pulled up. Do not use an external pullup resistor on this pin. 3 of 4 DS12885/DS12885Q/DS12885T CLOCK ACCURACY The accuracy of the clock is dependent upon the accuracy 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 is added by crystal freque ncy drift caused by temperature shifts. External circuit noise coupled into the oscillator circuit can result in the clock running fast. Refer to Application Note 58 “Crystal Considerations with Dallas Real-Time Clocks” for detailed information. RECOMMENDED LAYOUT FOR CRYSTAL 4 of 4