PC87303 datasheet(91/114 Pages) NSC

91 / 114 page

90 Keyboard Controller and Real-Time Clock (Continued)

92 REAL-TIME CLOCK FUNCTION

The RTC in the PC87303 is a low-power clock that provides

a time-of-day clock and 100-year calendar with alarm fea-

tures and battery operation Other features include three

maskable interrupt sources and 242 bytes of general pur-

pose RAM Valid RAM and time can be maintained through

the use of an external battery source It is software compati-

ble with the DS1287 and MC146818

921 Memory Map

The RTC contains 14 time and control registers It also con-

tains 242 bytes of general purpose battery backed static

RAM in two banks RAMSEL is used to select the active

bank Refer to Table 9-2

922 Bus Interface

The RTC function is mapped to IO locations 70h (index)

and 71h (data)

This decode is done internal to the

PC87303

923 Time Generation

The Time Generation function divides the 32768 kHz from

the external clock pins (X1C X2C) down to a one Hertz

signal The divider chain is controlled by bits 6 – 4 of Control

fifteen taps from the divider chain to be used as a Periodic

Interrupt See Control Register A in the Control Register and

Interrupt section for divider configurations and rate selec-

tions

During divider reset (bits 6 – 4 of Control Register A e 11x)

the divider chain is reset to 0 An update will occur 500 ms

after the divider chain is activated into normal operational

mode (bits 6 – 4 of Control Register A e 010) The periodic

flag also becomes active 12 of the programmed value

when the divider chain is activated

Figure 9-13 represents the internal and external circuitry

that comprise the oscillator The oscillator input may be driv-

en from an external source If this is desired the input

should be driven rail to rail and be approximately a 50%

duty cycle The oscillator output should be open in this case

The external capacitor values should be chosen to provide

the manufacturer’s specified load capacitance for the crys-

tal when combined with the parasitic capacitance of the

trace socket and package which can vary from 0 pF to

8 pF The rule of thumb in choosing these capacitors is

CL e (C1 C2)(C1 a C2) a Cparasitic

C2 l C1

C1 can be trimmed to obtain the 327680 Hz

TABLE 9-2 RTC Memory Map

Index

Function

BCD Format

Binary Format

Comments

(RAMSEL e 0)

Seconds

00 – 59

00 – 3b

Seconds Alarm

00 – 59

00 – 3b

Minutes

00 – 59

00 – 3b

Minutes Alarm

00 – 59

00 – 3b

Hours

12 hr e 01 – 12 (AM)

01 – 0c (AM)

12 hr e 81 – 92 (PM)

81 – 8c (PM)

24 hr e 00–23

00–17

R W

Hours Alarm

12 hr e 01 – 12 (AM)

01 – 0c (AM)

12 hr e 81 – 92 (PM)

81 – 8c (PM)

24 hr e 00–23

00–17

R W

Day of Week

01 – 07

01 – 07 (Sunday e 1)

Date of Month

01 – 31

01 – 1f

Month

01–12

01 – 0c

Year

00 – 99

00 – 63

Control Register A

RW (Bit 7 is Read Only)

Control Register B

RW (Bit 3 is Read Only)

Control Register C

Control Register D

0E – 7F

Battery-Backed RAM

(114 Bytes)

(RAMSEL e 1)

00 – 7F

Battery-Backed RAM

(128 Bytes)

PC87303 Datasheet(PDF) 91 Page - National Semiconductor (TI)

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