Advanced Clock Drivers Device Data

Freescale Semiconductor

9

MPC9773

MPC9773 Individual Output Disable (Clock Stop)

Circuitry

The individual clock stop (output enable) control of the

MPC9773 allows designers, under software control, to

implement power management into the clock distribution

design. A simple serial interface and a clock stop control logic

provides a mechanism through which the MPC9773 clock

outputs can be individually stopped in the logic ‘0’ state: The

clock stop mechanism allows serial loading of a 12-bit serial

input register. This register contains one programmable clock

stop bit for 12 of the 14 output clocks. The QC0 and QFB

outputs cannot be stopped (disabled) with the serial port.

The user can program an output clock to stop (disable) by

writing logic ‘0’ to the respective stop enable bit. Likewise, the

user may programmably enable an output clock by writing

logic ‘1’ to the respective enable bit. The clock stop logic

enables or disables clock outputs during the time when the

output would normally be in logic low state, eliminating the

possibility of short or ‘runt’ clock pulses.

The user can write to the serial input register through the

STOP_DATA input by supplying a logic ‘0’ start bit followed

serially by 12 NRZ disable/enable bits. The period of each

STOP_DATA bit equals the period of the free-running

STOP_CLK signal. The STOP_DATA serial transmission

should be timed so the MPC9773 can sample each

STOP_DATA bit with the rising edge of the free-running

STOP_CLK signal. (See Figure 5.)

Figure 5. Clock Stop Circuit Programing

STOP_CLK

STOP_DATA

START

QA0

QA1

QA2

QA3

QB0

QB1

QB2

QB3

QC1

QC2

QC3

QSYNC