AN2294 Freescale Semiconductor / Motorola, AN2294 Datasheet - Page 6

• Manufacturer Part Number: AN2294
• Description: MC68SZ328 USB Configuration Setup Application Note
• Manufacturer: Freescale Semiconductor / Motorola

Software Interface
3.1.2 Clock Configuration
As shown in Figure 2 and Figure 3, there are two different clocks that can be used to drive the USB
module; the 32.768 kHz and the 16 MHz crystals. Clock source selection is controlled by bit 15 in the
CSCR register (see Figure 6). If the 16 MHz clock is selected, no premultiplier is required. If the
32.768 kHz crystal is selected, the signal comes from the output of the premultiplier.
The clock sources control register (CSCR) is used to select the clock source and control the divide ratios
for the USB module. This register is shown in Figure 6.
The PLL registers work in two different modes according to the hardware settings programmed. The
following examples show the configuration for a module to run the PLL with the 32.768 kHz crystal or
using the 16 MHz crystal.
Using Equation 1 on page 5:
To produce an output of 48 MHz, use the values shown in Table 3 and use a USB Divider = 4.
6
CSCR
RESET
TYPE
BIT
USBSEL PLLBYPB
Table 2. Corresponding UPDF to the UPFSR1 Register Input Values (Continued)
15
1
14
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0
Freescale Semiconductor, Inc.
13
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0
USBCDIV
For More Information On This Product,
Figure 6. Clock Sources Control Register
UMFD[9:0]
UPDF[3:0]
Clock Sources Control Register
Variable
12
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0
UMFN
UMFI
Table 3. Programming Values
UPDF[3:0]
MC68SZ328 Application Note
11
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1
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0xE
0xF
...
1
2
10
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0
DMACDIV
32.768 kHz
rw
9
0
361
499
5
0
rw
8
1
0x0001
rw
7
0
UPDF
rw
6
0
15
16
...
2
3
16 MHz
rw
5
0
6
0
0
0
rw
4
0
OSC16EN
rw
3
0
0x(FF)FFF20C
rw
2
0
CLKOSEL
MOTOROLA
rw
1
1
Addr
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0
1