
94/324
ST92F120 - RESET AND CLOCK CONTROL UNIT (RCCU)
7 RESET AND CLOCK CONTROL UNIT (RCCU)
7.1 INTRODUCTION
The Reset and Clock Control Unit (RCCU) com-
prises two distinct sections:
– The Clock Control Unit, which generates and
manages the internal clock signals.
– The Reset/Stop Manager, which detects and
flags Hardware, Software and Watchdog gener-
ated resets.
On ST9 devices where the external Stop pin is
available, this circuit also detects and manages
the externally triggered Stop mode, during which
all oscillators are frozen in order to achieve the
lowest possible power consumption.
7.2 CLOCK CONTROL UNIT
The Clock Control Unit generates the internal
clocks for the CPU core (CPUCLK) and for the on-
chip peripherals (INTCLK). The Clock Control Unit
may be driven by an external crystal circuit, con-
nected to the OSCIN and OSCOUT pins, or by an
external pulse generator, connected to OSCIN
(see
Figure 49
and
Figure 51
). A low frequency ex-
ternal clock may be connected to the CK_AF pin,
and this clock source may be selected when low
power operation is required.
7.2.1 Clock Control Unit Overview
As shown in
Figure 43
a programmable divider
can divide the CLOCK1 input clock signal by two.
In practice, the divide-by-two is virtually always
used in order to ensure a 50% duty cycle signal to
the PLL multiplier circuit. The resulting signal,
CLOCK2, is the reference input clock to the pro-
grammable Phase Locked Loop frequency multi-
plier, which is capable of multiplying the clock fre-
quency by a factor of 6, 8, 10 or 14; the multiplied
clock is then divided by a programmable divider,
by a factor of 1 to 7. By this means, the ST9 can
operate with cheaper, medium frequency (3-5
MHz) crystals, while still providing a high frequen-
cy internal clock for maximum system perform-
ance; the range of available multiplication and divi-
sion factors allow a great number of operating
clock frequencies to be derived from a single crys-
tal frequency.
For low power operation, especially in Wait for In-
terrupt mode, the Clock Multiplier unit may be
turned off, whereupon the output clock signal may
be programmed as CLOCK2 divided by 16. For
further power reduction, a low frequency external
clock connected to the CK_AF pin may be select-
ed, whereupon the crystal controlled main oscilla-
tor may be turned off.
The internal system clock, INTCLK, is routed to all
on-chip peripherals, as well as to the programma-
ble Clock Prescaler Unit which generates the clock
for the CPU core (CPUCLK). (See
Figure 43
)
The Clock Prescaler is programmable and can
slow the CPU clock by a factor of up to 8, allowing
the programmer to reduce CPU processing speed,
and thus power consumption, while maintaining a
high speed clock to the peripherals. This is partic-
ularly useful when little actual processing is being
done by the CPU and the peripherals are doing
most of the work.
9