XC2200 Derivatives
System Units (Vol. 1 of 2)
System Control Unit (SCU)
User’s Manual
6-20
V2.1, 2008-08
SCU, V1.13
6.1.6
External Clock Output
An external clock output can be provided via pin EXTCLK to clock an external system or
to observe one of the selectable device clocks. This external clock is enabled by setting
bit EXTCON.EN and by selecting the clock signal as alternate output function at pin
EXTCLK. Following clocks can be selected by EXTCON.SEL for external clock
f
EXT
:
•
System clock
f
SYS
•
Programmable clock output
f
OUT
•
Direct Clock from oscillator OSC_HP
f
OSC
•
PLL clock
f
PLL
•
Wake-up clock
f
WU
•
RTC clock
f
RTC
Note: Changing bit field EXTCON.SEL can lead to spikes at pin EXTCLK.
Figure 6-11 EXTCLK Generation
6.1.6.1
Programmable Frequency Output
The programmable frequency output
f
OUT
can be selected as clock output (EXTCLK).
This clock can be controlled via software, and so can be adapted to the requirements of
the connected external circuitry. The programmability also extends the power
management to a system level, as also circuitry (peripherals, etc.) outside the XC2200
can be run at a scalable frequency or can temporarily be left without a clock.
Clock
f
OUT
is generated via a reload counter, so the output frequency can be selected in
small steps.
CCU_EXTCLK.vsd
M
U
X
0000
EXTCON .SEL
EXTCON.EN
f
OUT
0001
0010
0011
0100
0101
0110
0111
1000
1001
...
1111
'0'
EXTCLK
Reload
Counter
f
WU
f
OSC
f
PLL
f
RTC
f
SYS
M
U
X
'0'
0
1
Reserved
Reserved
f
EXT
f
CLKIN1
