Reset, Clock, and Power Controller
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:
USBPLLFrequency
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)
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
BIT
FIELD
RESET
RW
BIT
FIELD
RESET
RW
ADDR
BITS
31:14
13
12
11:6
5:0
13-34
(
⎛
SystemClockOscillatorFrequency
-------------------------------------------------------------------------------------------------------------------------------------------------------- -
=
⎝
×
Table 13-54. USBPLLCTL Register
31
30
29
28
0
0
0
0
RO
RO
RO
RO
RO
15
14
13
12
///
0
0
1
0
RO
RO
RW
RW
RW
Table 13-55. USBPLLCTL Fields
NAME
///
Reserved Writing to these bits has no effect. Reading returns 0.
USB PLL Power Down
USBPDN
1 = USB PLL Active
0 = USB PLL Powered Down
USB PLL Output Frequency Range Select
USBFRANGE
1 = 100 MHz - 304.819 MHz (best jitter performance achieved)
0 = 20 MHz - 100 MHz
USB PLL Pre-Divider Prescales the USB PLL Reference clock. The divisor
USBPREDIV
chosen must satisfy the equation:
(System Clock Oscillator frequency)
USB PLL Loop-Divider Prescales the USB PLL Feedback clock. The divisor can
USBLOOPDIV
be programmed from 1 to 63.
USBPREDIV
.
4 = 192 MHz
27
26
25
24
23
///
0
0
0
0
0
RO
RO
RO
RO
11
10
9
8
7
USBPREDIV
0
0
0
0
0
RW
RW
RW
RW
0xFFFE2000 + 0xC4
DESCRIPTION
÷
Version 1.0
LH79524/LH79525 User's Guide
)
×
USBLOOPDIV
⎞
⎠
22
21
20
19
0
0
0
0
RO
RO
RO
RO
6
5
4
3
USBLOOPDIV
1
0
0
0
RW
RW
RW
RW
RW
(USBPREDIV) ≥ 5 MHz
18
17
16
0
0
0
RO
RO
RO
2
1
0
1
0
1
RW
RW