ICS2694

Motherboard Clock Generator

Integrated

Circuit

Systems, Inc.

Description

The ICS2694 Motherboard Clock Generator is an integrated

circuit using PLL and VCO technology to generate virtually all

the clock signals required in a PC. The use of the device can be

generalized to satisfy the timing needs of most digital systems

by reprogramming the VCO or reconfiguring the counter stages

which derive the output frequencies from the VCO’s.

The primary VCO is customarily used to generate the CPU

clock and is so labeled on the ICS2694. Pre-programmed

frequency sets are listed on page 6. These choices were made

to match the major microprocessor families. CPUSEL (0-3)

allow the user to select the appropriate frequency for the

application.

Due to the filter in the phase-locked loop, the CPUCLK will

move in a linear fashion from one frequency to a newly-

selected frequency without glitches. If a fixed CPUCLK value

is desired, CPUSEL (0-3) may be hard wired to the desired

address with STROBE tied high. (It has a pull-up.) For board

test and debug, pulling OUTPUTE to Ground will tristate all

the outputs.

Features

•• Low cost - eliminates multiple oscillators and Count

Down Logic

•• Primary VCO has 16 Mask Programmable frequencies

(normally CPU clock)

•• Secondary VCO has 1 Mask Programmable frequency

(usually 96 MHz)

•• Pre-programmed versions for typical PC applications

•• 10 Outputs in addition to the primary CPU clock

•• Capability to reconfigure counter stages to change the

frequencies of the outputs via mask options

•• Advanced PLL design

•• On-chip PLL filters

•• Very Flexible Architecture

OUT2

1

24

OUT3

OUT1

2

23

OUT4

OUT0

3

22

OUT5

OUT9

4

21

OUT6

CPUCLK

5

20

VSS

6

19

OUT8

DVDD

7

18

AVDD

STROBE

8

17

XTAL2

CPUSEL0

9

16

XTAL1

CPUSEL1

10

15

AVSS

CPUSEL2

11

14

OUTPUTE

CPUSEL3

12

13

CLKIN

24-Pin DIP or SOIC

Pin Configuration

ICS2694RevA1094

Applications

•• CPU clock and Co-processor clock

•• Hard Disk and Floppy Disk clock

•• Keyboard clock

•• Serial Port clock

•• Bus clock

•• System counting or timing functions