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MS51
Nov. 28, 2019
Page
201
of 491
Rev 1.00
MS51
32K
SE
RIES
TE
CHNICAL RE
F
EREN
CE MA
N
UAL
6.2
System Manager
6.2.1
Clock System
Overview
6.2.1.1
The MS51 has a wide variety of clock sources and selection features that allow it to be used in a wide
range of applications while maximizing performance and minimizing power consumption. The MS51
provides three options of the system clock sources including internal oscillator, or external clock from
X
IN
pin via software. The MS51 is embedded with two internal oscillators: one 10 kHz low-speed and
one 16 MHz or 24 MHz high-speed, which is factory trimmed to ±2% under all conditions. A clock
divider CKDIV is also available on MS51 for adjustment of the flexibility between power consumption
and operating performance.
16/24 MHz
Internal
Oscillator
X
OUT
(P01)
X
IN
(P00)
OSC[1:0]
(CKSWT[2:1])
F
OSC
CPU
Peripherals
10 kHz
Internal
Oscillator
Flash
Memory
F
HIRC
F
LIRC
Clock
Divider
F
SYS
Watchdog
Timer
CKDIV
Clock
Filter
CLO pin
(P33 or P11)
CLOEN
4~24 MHz
Oscillating
Circuit
F
HXT
F
ECLK
10
01
00
EXTEN[1:0]
(CKEN[7:6])
01
11
Self
Wake-up
Timer
[1]
Default system clock source after power-on
[1]
10
OSCIN(P30)
Figure 6.2-1 Clock System Block Diagram
System Clock Sources
6.2.1.2
There are a total of three system clock sources selectable in the MS51 including high-speed internal
oscillator, low-speed internal oscillator and external clock input. Each of them can be the system clock
source in the MS51. Different active system clock sources also affect multi-function of P0.0/X
IN
.
Internal Oscillators
6.2.1.3
There are two internal oscillators in the MS51
– one high-speed internal oscillator (HIRC) 16 MHz or
24 MHz selectable base on HIRC24 (RCTRIM1.4) define. And one 10 kHz low-speed (LIRC). Both of
them can be selected as the system clock. HIRC can be enabled by setting HIRCEN (CKEN.5). LIRC
is enabled after device is powered up. User can set OSC[1:0] (CKSWT [2:1]) as [1,1] to select the
HIRC as the system clock. By setting OSC[1:0] as [1,0], LIRC will be selected as the system clock.
Note that after the MS51 is powered, HIRC and LIRC will be both enabled and HIRC is default
selected as the system clock source. While using internal oscillators, X
IN
automatically switch as one
general purpose I/O P0.0 to expend the number of general purpose I/O. The I/O output mode of P0.0
can be selected by configuring P0M1 and P0M2 registers.
Since HIRC value defined by a 9-bit data, user can adjust SFR to modify HIRC value, each bit of this
value is about modify 40 kHz. Decrease this value will cause HIRC more quick and add one means