Rev. 1.2
127
C8051T620/1/6/7 & C8051T320/1/2/3
21. Oscillators and Clock Selection
C8051T620/1/6/7 & C8051T320/1/2/3 devices include a programmable internal high-frequency oscillator, a
programmable internal low-frequency oscillator, and an external oscillator drive circuit. The internal high-
frequency oscillator can be enabled/disabled and calibrated using the OSCICN and OSCICL registers, as
shown in Figure 21.1. The internal low-frequency oscillator can be enabled/disabled and calibrated using
the OSCLCN register. The system clock can be sourced by the external oscillator circuit or either internal
oscillator. Both internal oscillators offer a selectable post-scaling feature. The USB clock (USBCLK) can be
derived from the internal oscillators or external oscillator.
Figure 21.1. Oscillator Options
OSC
Programmable
Internal Clock
Generator
Input
Circuit
EN
SYSCLK
OSCICL
OSCICN
IO
SCE
N
IF
RD
Y
SU
S
P
E
N
D
IF
CN
1
IF
CN
0
OSCXCN
XO
SCMD
2
XO
SCMD
1
XO
SCMD
0
XFC
N
2
XFC
N
1
XFC
N
0
CLKSEL
USBC
LK2
USBC
LK1
USBC
LK0
CLK
S
L2
CLK
S
L1
CLK
S
L0
OSCLCN
OS
CLEN
O
S
C
L
RDY
OS
C
L
F3
OS
C
L
F2
OS
C
L
F1
OS
C
L
F0
OS
CLD
1
OS
CLD
0
Low Frequency
Oscillator
EN
n
OSCLF
OSCLF OSCLD
USBCLK
CLKMUL
MU
LE
N
MU
LIN
T
MU
L
R
D
Y
MU
LSEL1
MU
LSEL0
US
B
C
LK2-
0
Internal HFO / 8
EXOSC
EXOSC / 2
EXOSC / 3
EXOSC / 4
Internal LFO
Internal HFO
n
4
2
OSCLD
XTAL1
XTAL2
RC Mode
VDD
XTAL2
Option 1
10M
C Mode
XTAL2
CMOS Mode
XTAL2