Datasheet

ACPI Devices
Intel
®
Atom™ Processor E6xx Series Datasheet
230
11.6 Real Time Clock
11.6.1 Overview
The Real Time Clock (RTC) module provides a battery backed-up date and time keeping
device. Three interrupt features are available: time of day alarm with once a second to
once a month range, periodic rates of 122 μs to 500 ms, and end of update cycle
notification. Seconds, minutes, hours, days, day of week, month, and year are counted.
The hour is represented in twelve or twenty-four hour format, and data can be
represented in BCD or binary format. The design is meant to be functionally compatible
with the Motorola MS146818B. The time keeping comes from a 32.768 kHz oscillating
source, which is divided to achieve an update every second. The lower 14 bytes on the
lower RAM block have very specific functions. The first ten are for time and date
information. The next four (0Ah to 0Dh) are registers, which configure and report RTC
functions. A host-initiated write takes precedence over a hardware update in the event
of a collision.
11.6.2 I/O Registers
The RTC internal registers and RAM are organized as two banks of 128 bytes each,
called the standard and extended banks. The first 14 bytes of the standard bank
contain the RTC time and date information along with four registers, A - D, that are
used for configuration of the RTC. The extended bank contains a full 128 bytes of
battery backed SRAM. All data movement between the host CPU and the RTC is done
through registers mapped to the standard I/O space. The register map appears below.
Notes:
1. I/O locations 70h and 71h are the standard ISA location for the real-time clock. Locations 72h and 73h
are for accessing the extended RAM. The extended RAM bank is also accessed using an indexed
scheme. I/O address 72h is used as the address pointer and I/O address 73h is used as the data
register. Index addresses above 127h are not valid.
2. Writes to 72h, 74h, and 76h do not affect the NMI enable (bit 7 of 70h).
Note: Port 70h is not directly readable. The only way to read this register is through Alt
Access mode. Although RTC Index bits 6:0 are readable from port 74h, bit 7 will always
return 0. If the NMI# enable is not changed during normal operation, software can
alternatively read this bit once and then retain the value for all subsequent writes to
port 70h.
11.6.3 Indexed Registers
The RTC contains two sets of indexed registers that are accessed using the two
separate Index and Target registers (70/71h or 72/73h), as shown below.
Table 334. RTC Registers
I/O Locations If U128E bit = 0 Function
70h and 74h Also alias to 72h and 76h Real-Time Clock (Standard RAM) Index Register
71h and 75h Also alias to 73h and 77h Real-Time Clock (Standard RAM) Target Register
72h and 76h Extended RAM Index Register (if enabled)
73h and 77h Extended RAM Target Register (if enabled)