DS12C887.pdf

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www.dalsemi.com FEATURES Drop-in replacement for IBM AT computer Pin compatible with the MC146818B and clock/calendar DS1287 Totally nonvolatile with over 10 years of operation in the absence of power Self-contained subsystem includes lithium, quartz, and support circuitry. Counts seconds, minutes, hours, days, day of the week, date, month, and year with leap year compensation valid up to 2100 Binary or BCD representation of time, 12– or 24–hour clock with AM and PM in calendar, and alarm 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 – 15 bytes of clock and control registers – 113 bytes of general purpose RAM Programmable square wave output signal Bus–compatible interrupt signals (IRQ) Three interrupts are separately software maskable and testable – Time–of–day alarm once/second to once/day – Periodic rates from 122 ms to 500 ms – End of clock update cycle DS12C887 Real Time Clock PIN ASSIGNMENT MOT NC NC AD0 AD1 AD2 AD3 AD4 AD5 AD6 AD7 GND 1 2 3 4 5 6 7 8 9 10 11 12 24 23 22 21 20 19 18 17 16 15 14 13 VCC SQW NC NC NC IRQ RESET DS NC R/W AS CS DS12C887 24-Pin ENCAPSULATED PACKAGE PIN DESCRIPTION AD0-AD7 - Multiplexed Address/Data Bus NC MOT CS AS R/W DS RESET IRQ SQW VCC GND - No Connect - Bus Type Selection - RTC Chip Select Input - Address Strobe - Read/Write Input - Data Strobe - Reset Input - - Square Wave Output - +5 Volt Main Supply - Ground Interrupt Request Output Century register DESCRIPTION The DS12C887 Real Time Clock plus RAM is designed as a direct upgrade replacement for the DS12887 in existing IBM compatible personal computers to add hardware year 2000 compliance. A century byte was added to memory location 50, 32h, as called out by the PC AT specification. A lithium energy source, quartz crystal, and write-protection circuitry are contained within a 24–pin dual in-line package. As such, the DS12C887 is a complete subsystem replacing 16 components in a typical application. The functions include a nonvolatile time-of-day clock, an alarm, a one-hundred-year calendar, programmable interrupt, square wave generator, and 113 bytes of nonvolatile static RAM. The real time clock is distinctive in that time-of-day and memory are maintained even in the absence of power. 1 of 19 020900

OPERATION The block diagram in Figure 1 shows the pin connections with the major internal functions of the DS12C887. The following paragraphs describe the function of each pin. DS12C887 SIGNAL DESCRIPTIONS GND, VCC - DC power is provided to the device on these pins. VCC is the +5 volt input. When 5 volts are applied within normal limits, the device is fully accessible and data can be written and read. When VCC is below 4.25 volts typical, reads and writes are inhibited. However, the timekeeping function continues unaffected by the lower input voltage. As VCC falls below 3 volts typical, the RAM and timekeeper are switched over to an internal lithium energy source. The timekeeping function maintains an accuracy of ±1 minute per month at 25°C regardless of the voltage input on the VCC pin. MOT (Mode Select) – The MOT pin offers the flexibility to choose between two bus types. When connected to VCC, Motorola bus timing is selected. When connected to GND or left disconnected, Intel bus timing is selected. The pin has an internal pull-down resistance of approximately 20KΩ. SQW (Square Wave Output) – The SQW pin can output a signal from one of 13 taps provided by the 15 internal divider stages of the Real Time Clock. The frequency of the SQW pin can be changed by programming Register A as shown in Table 1. The SQW signal can be turned on and off using the SQWE bit in Register B. The SQW signal is not available when VCC is less than 4.25 volts typical. AD0-AD7 (Multiplexed Bidirectional Address/Data Bus) – Multiplexed buses save pins because address information and data information time share the same signal paths. The addresses are present during the first portion of the bus cycle and the same pins and signal paths are used for data in the second portion of the cycle. Address/data multiplexing does not slow the access time of the DS12C887 since the bus change from address to data occurs during the internal RAM access time. Addresses must be valid prior to the falling edge of AS/ALE, at which time the DS12C887 latches the address from AD0 to AD6. Valid write data must be present and held stable during the latter portion of the DS or WR pulses. In a read cycle the DS12C887 outputs 8 bits of data during the latter portion of the DS or RD pulses. The read cycle is terminated and the bus returns to a high impedance state as DS transitions low in the case of Motorola timing or as RD transitions high in the case of Intel timing. AS (Address Strobe Input) – A positive going address strobe pulse serves to demultiplex the bus. The falling edge of AS/ALE causes the address to be latched within the DS12C887. The next rising edge that occurs on the AS bus will clear the address regardless of whether CS is asserted. Access commands should be sent in pairs. DS (Data Strobe or Read Input) – The DS/RD pin has two modes of operation depending on the level of the MOT pin. When the MOT pin is connected to VCC, Motorola bus timing is selected. In this mode DS is a positive pulse during the latter portion of the bus cycle and is called Data Strobe. During read cycles, DS signifies the time that the DS12C887 is to drive the bidirectional bus. In write cycles the trailing edge of DS causes the DS12C887 to latch the written data. When the MOT pin is connected to GND, Intel bus timing is selected. In this mode the DS pin is called Read(RD). RD identifies the time period when the DS12C887 drives the bus with read data. The RD signal is the same definition as the Output Enable (OE) signal on a typical memory. 2 of 19

DS12C887 R/ W (Read/Write Input) – The R/ W pin also has two modes of operation. When the MOT pin is connected to VCC for Motorola timing, R/W is at a level which indicates whether the current cycle is a read or write. A read cycle is indicated with a high level on R/W while DS is high. A write cycle is indicated when R/ W is low during DS. When the MOT pin is connected to GND for Intel timing, the R/ W signal is an active low signal called WR. In this mode the R/ W pin has the same meaning as the Write Enable signal (WE) on generic RAMs. CS (Chip Select Input) – The Chip Select signal must be asserted low for a bus cycle in the DS12C887 to be accessed. CS must be kept in the active state during DS and AS for Motorola timing and during RD and WR for Intel timing. Bus cycles which take place without asserting CS will latch addresses but no access will occur. When VCC is below 4.25 volts, the DS12C887 internally inhibits access cycles by internally disabling the CS input. This action protects both the real time clock data and RAM data during power outages. IRQ (Interrupt Request Output) - The IRQ pin is an active low output of the DS12C887 that can be used as an interrupt input to a processor. The IRQ output remains low as long as the status bit causing the interrupt is present and the corresponding interrupt-enable bit is set. To clear the IRQ pin the processor program normally reads the C register. The RESET pin also clears pending interrupts. When no interrupt conditions are present, the IRQ level is in the high impedance state. Multiple interrupting devices can be connected to an IRQ bus. The IRQ bus is an open drain output and requires an external pull-up resistor. RESET (Reset Input) – The RESET pin has no effect on the clock, calendar, or RAM. On power-up the RESET pin can be held low for a time in order to allow the power supply to stabilize. The amount of time that RESET is held low is dependent on the application. However, if RESET is used on power–up, the time RESET is low should exceed 200 ms to make sure that the internal timer that controls the DS12C887 on power-up has timed out. When RESET is low and VCC is above 4.25 volts, the following occurs: A. Periodic Interrupt Enable (PEI) bit is cleared to zero. B. Alarm Interrupt Enable (AIE) bit is cleared to zero. C. Update Ended Interrupt Flag (UF) bit is cleared to zero. D. Interrupt Request Status Flag (IRQF) bit is cleared to zero. E. Periodic Interrupt Flag (PF) bit is cleared to zero. F. The device is not accessible until RESET is returned high. G. Alarm Interrupt Flag (AF) bit is cleared to zero. H. IRQ pin is in the high impedance state. I. Square Wave Output Enable (SQWE) bit is cleared to zero. J. Update Ended Interrupt Enable (UIE) is cleared to zero. In a typical application RESET can be connected to VCC . This connection will allow the DS12C887 to go in and out of power fail without affecting any of the control registers. 3 of 19

DS12C887 BLOCK DIAGRAM Figure 1 DS12C887 4 of 19

DS12C887 POWER-DOWN/POWER-UP CONSIDERATIONS The Real Time Clock function will continue to operate and all of the RAM, time, calendar, and alarm memory locations remain nonvolatile regardless of the level of the VCC input. When VCC is applied to the DS12C887 and reaches a level of greater than 4.25 volts, the device becomes accessible after 200 ms, provided that the oscillator is running and the oscillator countdown chain is not in reset (see Register A). This time period allows the system to stabilize after power is applied. When VCC falls below 4.25 volts, the chip select input is internally forced to an inactive level regardless of the value of CS at the input pin. The DS12C887 is, therefore, write-protected. When the DS12C887 is in a write-protected state, all inputs are ignored and all outputs are in a high impedance state. When VCC falls below a level of approximately 3 volts, the external VCC supply is switched off and an internal lithium energy source supplies power to the Real Time Clock and the RAM memory. RTC ADDRESS MAP The address map for the DS12C885 is shown in Figure 2. The address map consists of 113 bytes of user RAM, 11 bytes of RAM that contain the RTC time, calendar, and alarm data, and 4 bytes which are used for control and status. All 128 bytes can be directly written or read except for the following: 1. 2. 3. Registers C and D are read-only. Bit-7 of Register A is read-only. The high order bit of the seconds byte is read-only. DS12C887 REAL TIME CLOCK ADDRESS MAP Figure 2 5 of 19

DS12C887 TIME, CALENDAR AND ALARM LOCATIONS The time and calendar information is obtained by reading the appropriate memory bytes. The time, calendar, and alarm are set or initialized by writing the appropriate RAM bytes. The contents of the ten time, calendar, and alarm bytes can be either Binary or Binary-Coded Decimal (BCD) format. Before writing the internal time, calendar, and alarm registers, the SET bit in Register B should be written to a logic one to prevent updates from occurring while access is being attempted. In addition to writing the ten time, calendar, and alarm registers in a selected format (binary or BCD), the data mode bit (DM) of Register B must be set to the appropriate logic level. All ten time, calendar, and alarm bytes must use the same data mode. The set bit in Register B should be cleared after the data mode bit has been written to allow the real time clock to update the time and calendar bytes. Once initialized, the real time clock makes all updates in the selected mode. The data mode cannot be changed without reinitializing the ten data bytes. Table 2 shows the binary and BCD formats of the ten time, calendar, and alarm locations. The 24–12 bit cannot be changed without reinitializing the hour locations. When the 12–hour format is selected, the high order bit of the hours byte represents PM when it is a logic one. The time, calendar, and alarm bytes are always accessible because they are double buffered. Once per second the eleven bytes are advanced by one second and checked for an alarm condition. If a read of the time and calendar data occurs during an update, a problem exists where seconds, minutes, hours, etc. may not correlate. The probability of reading incorrect time and calendar data is low. Several methods of avoiding any possible incorrect time and calendar reads are covered later in this text. The three alarm bytes can be used in two ways. First, when the alarm time is written in the appropriate hours, minutes, and seconds alarm locations, the alarm interrupt is initiated at the specified time each day if the alarm enable bit is high . The second use condition is to insert a “don’t care” state in one or more of the three alarm bytes. The “don’t care” code is any hexadecimal value from C0 to FF. The two most significant bits of each byte set the “don’t care” condition when at logic 1. An alarm will be generated each hour when the “don’t care” bits are set in the hours byte. Similarly, an alarm is generated every minute with “don’t care” codes in the hours and minute alarm bytes. The “don’t care” codes in all three alarm bytes create an interrupt every second. TIME, CALENDAR AND ALARM DATA MODES Table 1 BINARY DATA MODE ADDRESS LOCATION FUNCTION DECIMAL RANGE O 1 2 3 4 5 6 7 8 9 50 Seconds Seconds Alarm Minutes Minutes Alarm Hours 12-hr, Mode Hours 24-hr, Mode Hours Alarm 12-hr, Mode Hours Alarm 24-hr, Mode Day of the week Sunday=1 Date of Month Month Year Century 0-59 0-59 0-59 0-59 1-12 0-23 1-12 0-23 1-7 1-31 1-12 0-99 0-99 RANGE BCD DATA MODE 00-59 00-59 00-59 00-59 00-3B 00-3B 00-3B 00-3B 01-0C AM, 81-8C PM 01-12 AM, 81-92 PM 00-17 00-23 01-0C AM, 81-8C PM 01-12 AM, 81-92 PM 00-17 01-07 01-1F 01-0C 00-63 NA 00-23 01-07 01-31 01-12 00-99 19,20 6 of 19

CONTROL REGISTERS The DS12C887 has four control registers which are accessible at all times, even during the update cycle. DS12C887 REGISTER A MSB BIT 7 UIP BIT 6 DV2 BIT 5 DV1 BIT 4 DV0 BIT 3 RS3 BIT 2 RS2 BIT 1 RS1 LSB BIT 0 RS0 UIP - The Update In Progress (UIP) bit is a status flag that can be monitored. When the UIP bit is a 1, the update transfer will soon occur. When UIP is a 0, the update transfer will not occur for at least 244µs. The time, calendar, and alarm information in RAM is fully available for access when the UIP bit is 0. The UIP bit is read-only and is not affected by RESET . Writing the SET bit in Register B to a 1 inhibits any update transfer and clears the UIP status bit. DV2, DV1, DV0 - These three bits are used to turn the oscillator on or off and to reset the countdown chain. A pattern of 010 is the only combination of bits that will turn the oscillator on and allow the RTC to keep time. A pattern of 11X will enable the oscillator but holds the countdown chain in reset. The next update will occur at 500 ms after a pattern of 010 is written to DV0, DV1, and DV2. RS3, RS2, RS1, RS0 - These four rate-selection bits select one of the 13 taps on the 15-stage divider or disable the divider output. The tap selected can be used to generate an output square wave (SQW pin) and/or a periodic interrupt. The user can do one of the following: 1. Enable the interrupt with the PIE bit; 2. Enable the SQW output pin with the SQWE bit; 3. Enable both at the same time and the same rate; or 4. Enable neither Table 1 lists the periodic interrupt rates and the square wave frequencies that can be chosen with the RS bits. These four read/write bits are not affected by RESET . 7 of 19

REGISTER B MSB BIT 7 SET BIT 6 PIE BIT 5 AIE BIT 4 UIE BIT 3 SQWE BIT 2 DM BIT 1 24/12 DS12C887 LSB BIT 0 DSE SET - When the SET bit is a 0, the update transfer functions normally by advancing the counts once per second. When the SET bit is written to a 1, any update transfer is inhibited and the program can initialize the time and calendar bytes without an update occurring in the midst of initializing. Read cycles can be executed in a similar manner. SET is a read/write bit and is not affected by RESET or internal functions of the DS12C887. PIE - The Periodic Interrupt Enable bit is a read/write bit which allows the Periodic Interrupt Flag (PF) bit in Register C to drive the IRQ pin low. When the PIE bit is set to 1, periodic interrupts are generated by driving the IRQ pin low at a rate specified by the RS3-RS0 bits of Register A. A 0 in the PIE bit blocks the IRQ output from being driven by a periodic interrupt, but the Periodic Flag (PF) bit is still set at the periodic rate. PIE is not modified by any internal DS12C887 functions but is cleared to 0 on RESET . AIE - The Alarm Interrupt Enable (AIE) bit is a read/write bit which, when set to a 1, permits the Alarm Flag (AF) bit in register C to assert IRQ . An alarm interrupt occurs for each second that the 3 time bytes equal the 3 alarm bytes including a “don’t care” alarm code of binary 11XXXXXX. When the AIE bit is set to 0, the AF bit does not initiate the IRQ signal. The internal functions of the DS12C887 not affect the AIE bit. UIE - The Update Ended Interrupt Enable (UIE) bit is a read/write bit that enables the Update End Flag (UF) bit in Register C to assert IRQ . The RESET pin going low or the SET bit going high clears the UIE bit. SQWE - When the Square Wave Enable (SQWE) bit is set to a 1, a square wave signal at the frequency set by the rate-selection bits RS3 through RS0 is driven out on the SQW pin. When the SQWE bit is set to 0, the SQW pin is held low. SQWE is a read/write bit and is cleared by RESET . SQWE is set to a 1 when VCC is powered up. DM - The Data Mode (DM) bit indicates whether time and calendar information is in binary or BCD format. The DM bit is set by the program to the appropriate format and can be read as required. This bit is not modified by internal functions or RESET . A 1 in DM signifies binary data while a 0 in DM specifies Binary Coded Decimal (BCD) data. 24/12 - The 24/12 control bit establishes the format of the hours byte. A 1 indicates the 24-hour mode and a 0 indicates the 12-hour mode. This bit is read/write and is not affected by internal functions or RESET . DSE - The Daylight Savings Enable (DSE) bit is a read/write bit which enables two special updates when DSE is set to 1. On the first Sunday in April the time increments from 1:59:59 AM to 3:00:00 AM. On the last Sunday in October when the time first reaches 1:59:59 AM it changes to 1:00:00 AM. These special updates do not occur when the DSE bit is a zero. This bit is not affected by internal functions or RESET . 8 of 19

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