82C836 Datasheet, PDF (60/205 Pages) List of Unclassifed Manufacturers – Single-Chip 386sx AT

Section 6
Real Time Clock and
Interval Timer Registers
Real Time Clock Interface----MC146818 Compatible
The 82C836 contains an internal, MC146818-compatible Real Time Clock (RTC) with
a total of 128 bytes of CMOS RAM (including the dedicated bytes used for clock
functions). The internal RTC may be disabled so that an external RTC can be used.
Internal/external RTC mode is selected by the state of the -EXTRTC signal (-DACK5)
during power-up. If this pin is pulled low with a 4.7K ohm resistor, external RTC mode
is selected. A 4.7K ohm pull-up resistor puts the 82C836 into internal RTC mode.
When the internal RTC is used:
• 82C836 Vcc pins should be connected to battery-backed power
• MFP4 is used as the 32KHz clock input
• MFP5 is used as the power sense input.
When an external RTC is used:
• 82C836 Vcc pins should be connected to the normal power source
• MFP4 is used as the IRQ8 input from the RTC
• MFP5 is used as the RTCCS- chip select output for the RTC
• -RTCCS should be connected to PWRGOOD through a diode (see Figure 3-2 shown
earlier).
The internal RTC combines a complete time-of-day clock with alarm, one hundred year
calendar, a programmable periodic interrupt, and 114 bytes of low-power static RAM.
Provisions are made to enable the device to operate in a low-power (battery powered)
mode and protect the contents of both the RAM and clock during system power-down.
Chips and Technologies, Inc.
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Revision 3.0 6-1