Reset, Clock, and Power Controller
LH79524/LH79525 User’s Guide
13-34
Version 1.0
13.2.2.23 USB PLL Control Register (USBPLLCTL)
This register controls the USB PLL frequency and power down. The USB PLL frequency
is calculated by:
The maximum USB PLL frequency is 304.819 MHz
.
Since the jitter is better with the PLL running above 100 MHz, it is best to program the PLL
to a frequency greater than 100 MHz and divide it by four using the USBPRE:USBDIV bit.
Here is a programming example using the typical crystal frequency of 11.2896 MHz:
Target frequency (48 MHz)
×
4 = 192 MHz
Program USBLOOPDIV = 17 and USBPREDIV = 1
47.9808 ÷ 48.000 MHz = 0.04%
Well within USB 2.0 Specification of ±0.25%
192 MHz = (11.2896 MHz × USLOOPDIV) ÷ USBPREDIV
192 MHz ÷ 11.2896 MHz = USBLOOPDIV ÷ USBPREDIV = 17.0068
(11.2896 MHz ÷ 17)/1 = 191.9232 MHz
191.9232 MHz/(USBDIV = 4) = 47.9808 MHz
Table 13-54. USBPLLCTL Register
BIT
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
FIELD
///
RESET
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
RW
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
BIT
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
FIELD
///
USBPDN
U
S
B
F
RA
NG
E
USBPREDIV
USBLOOPDIV
RESET
0
0
1
0
0
0
0
0
0
1
0
0
0
1
0
1
RW
RO
RO
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
ADDR
0xFF 0xC4
Table 13-55. USBPLLCTL Fields
BITS
NAME
DESCRIPTION
31:14
///
Reserved
Writing to these bits has no effect. Reading returns 0.
13
USBPDN
USB PLL Power Down
1 = USB PLL Active
0 = USB PLL Powered Down
12
USBFRANGE
USB PLL Output Frequency Range Select
1 = 100 MHz - 304.819 MHz (best jitter performance achieved)
0 = 20 MHz - 100 MHz
11:6
USBPREDIV
USB PLL Pre-Divider
Prescales the USB PLL Reference clock. The divisor
chosen must satisfy the equation:
(System Clock Oscillator frequency)
÷
(USBPREDIV)
≥
5 MHz
5:0
USBLOOPDIV
USB PLL Loop-Divider
Prescales the USB PLL Feedback clock. The divisor can
be programmed from 1 to 63.
USBPLLFrequency
SystemClockOscillatorFrequency
(
)
USBLOOPDIV
×
USBPREDIV
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