![Infineon Technologies TC1796 User Manual Download Page 159](http://html1.mh-extra.com/html/infineon-technologies/tc1796/tc1796_user-manual_2055437159.webp)
TC1796
System Units (Vol. 1 of 2)
Clock System and Control
User’s Manual
3-10
V2.0, 2007-07
Clock, V2.0
3.2.2
Phase Looked Loop (PLL) Circuitry
The PLL is a main component of the CGU that is dedicated to generate the CPU and
system clock inside the TC1796. The PLL basically converts a low-frequency external
clock signal into high-speed internal CPU and system clocks for maximum performance.
The PLL consists of a Voltage Controlled Oscillator (VCO) with a feedback path. A
divider in the feedback path (N-Divider) divides the VCO frequency down. The resulting
frequency is then compared with the externally provided and divided frequency (P-
Divider). The phase detection logic determines the difference between the two clock
signals and accordingly controls the frequency of the VCO (
f
VCO
). During start-up, the
VCO increases its frequency until the divided feedback clock matches the external clock
frequency. A PLL lock detection unit monitors and signals this condition. The phase
detection logic continues to monitor the two clock signals and adjusts the VCO clock if
required. The CGU output clocks
f
CPU
and
f
SYS
are derived from the VCO clock by the K-
Divider.
3.2.2.1
Clock Source Control
The CPU clock
f
CPU
and the system clock
f
SYS
are generated from
f
OSC
in one of four
hardware/software selectable modes:
•
Direct Drive Mode (PLL Bypass Mode)
•
VCO Bypass Mode (Prescaler Mode)
•
PLL Mode
•
PLL Base Mode
Direct Drive Mode (PLL Bypass Operation)
In Direct Drive Mode, the PLL is bypassed and the CGU clock outputs are directly fed
from the clock signal
f
OSC
, i.e.
f
CPU
=
f
OSC
and
f
SYS
=
f
OSC
/2 or
f
OSC
. This mode can be only
selected by hardware when pin BYPASS = 1 during the rising edge of PORST.
VCO Bypass Mode (Prescaler Mode)
In VCO Bypass Mode,
f
CPU
and
f
SYS
are derived from
f
OSC
by the two divider stages, P-
Divider and K-Divider. This mode is selected by setting PLL_CLC.VCOBYP = 1. The
system clock
f
SYS
can be equal to
f
CPU
(PLL_CLC.SYSFS = 1) or equal to
f
CPU
/2
(PLL_CLC.SYSFS = 0).
(3.1)
f
CPU
1
P K
×
--------------
f
OSC
×
=
f
SYS
=
f
CPU
or
f
CPU
/2