ICS580-01

Glitch-Free Clock Multiplexer

MDS 580-01 A

4

Revision 030300

Printed 11/28/00

Integrated Circuit Systems, Inc. • 525 Race Street • San Jose •CA•95126• (408) 295-9800tel • www.icst.com

In the third example, the ICS580-01 is configured to automatically switch clocks when an an input stops.

The clock that could stop is connected to INA while the backup, always running, clock is connected to INB.

The output NO_INA is connected to SELB. This means that when the clock on INA stops, NO_INA

goes high selecting the clock on INB which is muxed to the output after 3 cycles. When the clock on

INA restarts, NO_INA immediately goes low, selecting the clock on INA. The output then switches in

the manner described in the first example.

SELB

DIV

VDDI

INA

INB

GND

OE4

OE3

OE1

VDDC

CLK1

CLK2

GND

OE2

NO_INA

VDD

0.01µF

33

Ω

0.01µF

Normal

Clock

Backup

Clock

NO_INB

Output

Clock

Figure 3

The circuit diagram in Figure 3 shows a typical connection for this example. Note that CLK2 and

NO_INB are unused and so are disabled by grounding OE2 and OE4. A 33

Ω series termination resistor

is used on the clock output and 2 decoupling capacitors of 0.01µF are used. All other inputs are left

floating and are therefore pulled high by the on-chip pull-ups.

Output Enable

Each output has a dedicated output enable pin. If an output is unused, it should be tri-stated by tying the

appropriate output enable pin to ground.

External Components

The ICS580-01 requires two 0.01µF decoupling capacitors, one between VDDI and GND and one between

VDDC and GND. Series termination resistors of 33

Ω can be used on CLK1 and CLK2.

Split Power Supplies

The VDDI pin provides the power for the INA and INB input buffers only. All the other inputs and the

rest of the chip are connected to VDDC. This allows for supply voltage translation. For example, INA and

INB could be 5 V clocks (VDDI=5 V) and the rest of the chip could use a 3.3 V supply on VDDC giving

3.3 V output clocks. For correct operation VDDI must always be greater than or equal to VDDC.