ICS87972I Datasheet, PDF (10/17 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-12 LVCMOS CLOCK MULTIPLIER/ZERO DELAY BUFFER

ICS87972I
LOW SKEW, 1-TO-12
LVCMOS/LVTTL CLOCK MULTIPLIER/ZERO DELAY BUFFER
APPLICATION INFORMATION
USING THE OUTPUT FREEZE CIRCUITRY
OVERVIEW
To enable low power states within a system, each output of
ICS87972I (Except QC0 and QFB) can be individually frozen
(stopped in the logic “0” state) using a simple serial interface
to a 12 bit shift register. A serial interface was chosen to elimi-
nate the need for each output to have its own Output Enable
pin, which would dramatically increase pin count and package
cost. Common sources in a system that can be used to drive
the ICS87972I serial interface are FPGA’s and ASICs.
PROTOCOL
The Serial interface consists of two pins, FRZ_Data (Freeze
Data) and FRZ_CLK (Freeze Clock). Each of the outputs which
can be frozen has its own freeze enable bit in the 12 bit shift
register. The sequence is started by supplying a logic “0” start
bit followed by 12NRZ freeze enable bits. The period of each
FRZ_DATA bit equals the period of the FRZ_CLK signal. The
FRZ_DATA serial transmission should be timed so the ICS87972I
can sample each FRZ_DATA bit with the rising edge of the
FRZ_CLK signal. To place an output in the freeze state, a logic
“0” must be written to the respective freeze enable bit in the shift
register. To unfreeze an output, a logic “1” must be written to the
respective freeze enable bit. Outputs will not become enabled/
disabled until all 12 data bits are shifted into the shift register.
When all 12 data bits are shifted in the register, the next rising
edge of FRZ_CLK will enable or disable the outputs. If the bit
that is following the 12th bit in the register is a logic “0”, it is used
for the start bit of the next cycle; otherwise, the device will wait
and won’t start the next cycle until it sees a logic “0” bit. Freez-
ing and unfreezing of the output clock is synchronous (see the
timing diagram below). When going into a frozen state, the out-
put clock will go LOW at the time it would normally go LOW, and
the freeze logic will keep the output low until unfrozen. Likewise,
when coming out of the frozen state, the output will go HIGH
only when it would normally go HIGH. This logic, therefore, pre-
vents runt pulses when going into and out of the frozen state.
FRZ_DATA
StaBritt _{QA0 QA1 QA2 QA3 QB0 QB1 QB2 QB3 QC1 QC2 QC3 QSYNC}
FRZ_CLK
FIGURE 2A. FREEZE DATA INPUT PROTOCOL
Qx FREEZE Internal
Qx Internal
87972DYI
Qx Out
FIGURE 2B. OUTPUT DISABLE TIMING
www.idt.com
10
REV. E JUNE 25, 2010