Reset, Clock, and Power Controller
LH79524/LH79525 User’s Guide
13-28
Version 1.0
13.2.2.18 USB Clock Prescaler Register (USBPRE)
The value in this register is used as a divisor for the clock source to derive the USB clock
(USBCLK) frequency. The USB clock source (PLL clock, or HCLK) is selected with the
PCLKSEL1:USB bit (see Section 13.2.2.13). It is important to note that this bit defaults to
select HCLK, and must be reprogrammed to use the USB PLL as the source for most designs.
The on-board USB Device requires a 48 MHz clock for Full Speed (12 Mbp/s) operation.
The reset value of USBDIV is 0x00, resulting in division by 1. If the USB PLL is pro-
grammed for higher-frequency operation to improve jitter, this field must be programmed
to the appropriate divisor following reset (see Section 13.2.2.23).
Table 13-45 shows the valid combinations for USBDIV and the resulting USB clock fre-
quency. All other USBDIV values are invalid.
Table 13-43. USBPRE 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
///
USBDIV
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
RW
RW
RW
RW
RW
RW
RW
RW
ADDR
0xFF 0x4C
Table 13-44. USBPRE Fields
BITS
NAME
DESCRIPTION
31:8
///
Reserved
Reading returns 0. Write the reset value.
7:0
USBDIV
USB Clock Divisor
Program with the clock source divisor for the USB Clock
prescaler (Table 13-45).
Table 13-45. USBPRE Register Values
USBDIV
DIVISOR
ƒ(ADC)
00000000 (default)
1
ƒ(clock source)
00000001
2
ƒ(clock source)/2
00000010
4
ƒ(clock source)/4