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Rev. 1.00
85 of 576
January 28, 2022
32-Bit Arm
®
Cortex
®
-M0+ MCU
HT32F54231/HT32F54241/HT32F54243/HT32F54253
6 Clock Control Unit (CKCU)
6 Clock Control Unit (CKCU)
Phase Locked Loop – PLL
This PLL can provide a 4 ~ 60 MHz clock output which is 1 ~ 15 multiples of a fundamental
reference frequency of 4 ~ 16 MHz. The rationale of the clock synthesizer relies on the digital Phase
Locked Loop (PLL) which includes a reference divider, a two-stage feedback divider, a two-stage
output divider, a digital phase frequency detector (PFD), a current-controlled charge pump, a built-in
loop filter and a voltage-controlled oscillator (VCO) to achieve a stable phase-locked state.
PLL
OUT
= 4 ~ 60 MHz
Divider
(NR)
PD
CP
VCO
Loop
Filter
CLK
IN
= 4 ~ 16 MHz
Feedback Divider 2
(NF2)
Output Divider 1
(NO1)
/2
/2
/4
B3 ~ B0
S1 ~ S0
VCO
OUT
= 48 ~ 120 MHz
Output Divider 2
(NO2)
Feedback Divider 1
(NF1)
Ref. Divider
(REFDIV)
Figure 18. PLL Block Diagram
Frequency of the PLL output clock can be determined by the following formula:
PLL
OUT
= CLK
IN
×
NF1 × NF2
REFDIV × NR × NO1 × NO2 = CLK
IN
×
4 × NF2
REFDIV × 2 × 2 × NO2
= CLK
IN
×
NF2
REFDIV × NO2
where REFDIV = 1 or 2, NR = 2, NF1 = Feedback Divider 1 = 4, NF2 = Feedback Divider 2 = 1 ~ 16,
NO1 = Output Divider 1 = 2, NO2 = Output Divider 2 = 1, 2, 4, or 8
Considering the duty cycle with 50%, both input frequency and output frequency is divided by 2.
Assume that a given CLK
IN
frequency as PLL input generates a specific PLL output frequency; a
larger number of NF2 is suggested because it will cause the PLL more stable and less jittered but
enlarges the settling time. The output and feedback of divider 2 value are described in Table 16
and Table
17. All the configuration bits (S1 ~ S0, B3 ~ B0) in Table 16 and
Table
17 as well as the
Bypass mode control are defined in the PLL Configuration Register (PLLCFGR) and PLL Control
Register (PLLCR) in the section of Register Definition. Note that VCO
OUT
is ranged from 48 MHz
to 120 MHz. If VCO
OUT
by user’s configurations exceeds this range, the output frequency of the
PLL will not be promised to match the above PLL
OUT
formula.
The PLL can be switched on or off by using the PLLEN bit in the Global Clock Control Register
(GCCR). The PLLRDY flag in the Global Clock Status Register (GCSR) will indicate if the PLL
clock is stable.
Table 16. Output Divider 2 Value Mapping
Output Divider 2 Setup Bits S1 ~ S0
(POTD Field in the PLLCFGR Register)
NO2
(Output Divider 2 Value)
00
1
01
2
10
4
11
8