DS1495 Datasheet, PDF (3/19 Pages) Dallas Semiconductor – RAMified Real Time Clock

DS1495/DS1497
ADDITIONAL PIN DESCRIPTION
(FOR DS1495, DS1495S)
X1, X2 – Connections for a standard 32.768 KHz quartz
crystal, Daiwa part number DT-26S or equivalent. 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 resis-
tors. 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
on crystal selection and crystal layout considerations,
please consult Application Note 58, “Crystal Consider-
ations with Dallas Real Time Clocks”.
V_BAT – Battery input for any standard +3 volt lithium cell
or other energy source. Battery voltage must be held
between 2.5 and 3.7 volts for proper operation. The
nominal write protect trip point voltage at which access
to the real time clock and user RAM is denied is set by
the internal circuitry at 4.25 volts typical. A maximum
load of 1 µA at 25^oC and 3.0V on V_BAT in the absence of
power should be used to size the external energy
source.
The battery should be connected directly to the V_BAT
pin. A diode must not be placed in series with the battery
to the V_BAT pin. Furthermore, a diode is not necessary
because reverse charging current protection circuitry is
provided internal to the device and has passed the
requirements of Underwriters Laboratories for UL list-
ing.
B_GND – Battery ground: This pin or pin 14 can be used
for the battery ground return.
OPERATION
Power-Down/Power-Up: The real time clock will con-
tinue to operate and all of the RAM, time, and calendar
and alarm memory locations will remain non-volatile re-
gardless of the voltage level of V_DD. When the voltage
level applied to the V_DD input is greater than 4.25 volts
(typical), the module becomes accessible after 200 ms
provided that the oscillator and countdown chain have
been programmed to be running. This time period al-
lows the module to stabilize after power is applied.
When V_DD falls below the CE_THR (4.25 volts typical), the
chip select inputs RTC and XRAM are forced to an inac-
tive state regardless of the state of the pin signals. This
puts the module into a write protected mode in which all
inputs are ignored and all outputs are in a high imped-
ance state. When V_DD falls below 3.2 volts (typical), the
module is switched over to an internal power source in
the case of the DS1497, or to an external battery con-
nected to the V_BAT and BGND pins in the case of the
DS1495 and DS1495S, so that power is not interrupted
to timekeeping and nonvolatile RAM functions.
Address Map: The registers of the device appear in two
distinct address ranges. One set of registers is active
when RTC is asserted low and represents the real time
clock. The second set of registers is active when XRAM
is asserted low and represents the extended RAM.
RTC Address Map: The address map of the RTC mod-
ule is shown in Figure 2. The address map consists of
50 bytes of general purpose RAM, 10 bytes of RTC/cal-
endar information, and 4 bytes of status and control in-
formation. All 64 bytes can be accessed as read/write
registers except for the following:
1. Registers C and D are Read Only (status informa-
tion)
2. Bit 7 of register A is Read Only
3. Bit 7 of the “Seconds” byte (00) is Read Only
The first byte of the real time clock address map is the
RTC indirect address register, accessible when A0 is
low. The second byte is the RTC data register, accessi-
ble when A0 is high. The function of the RTC indirect ad-
dress register is to point to one of the 64 RTC registers
that are indirectly accessible through the RTC data reg-
ister.
Extended RAM Address Map: The first 32 bytes of the
extended RAM represent one of 256 pages of general
purpose nonvolatile memory. These 32 bytes on a page
are addressed by A0 through A4 when A5 is low. When
A5 is high, the XRAM page register is accessible. The
value in the XRAM page register points to one of 256
pages of nonvolatile memory available. The address of
the XRAM page register is dependent only on A5 being
high; thus, there are 31 aliases of this register in I/O
spaces. (See Figure 3.)
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