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LH75400/01/10/11 (Preliminary) User's Guide
1.6 Crystal Oscillator Usage
When a chip containing a crystal oscillator is used on a board, the user must make some
resistance and capacitance connections from the chip to the board.
Figure 1-1 shows an application diagram for the crystal oscillator.
Rfb is a feedback resistor, and C1 and C2 are load capacitances. To determine the appro-
priate values for the system, a good starting point is 1 MΩ for Rfb, 10 pF for C1, and 10 pF
for C2. The values of Rfb, C1, and C2 may be further refined to meet the frequency require-
ments of the system.
1.7 Clocking Strategy
The SoCs provide for two crystal oscillators.
• The first oscillator drives an internal Phased Lock Loop (PLL) and the three UARTs. It
supports a frequency range from 10 MHz to 20 MHz and requires a 1.8 V source on its
external inputs.
• The second oscillator is a 32.768 kHz oscillator that generates a 1 Hz clock for the RTC.
The internal PLL multiplies its input frequency by seven. Its input frequency range is from
14 MHz to 40 MHz; however, the crystal oscillator limits its useful range to 14 MHz to
20 MHz. The resulting output frequency range is 98 MHz to 140 MHz.
INTERNAL ENABLE
INTERNAL ENABLE
INTERNAL
INTERNAL TO
CLOCK
THE SoC
EXTERNAL TO
THE SoC
Figure 1-1. Crystal Oscillator
(XTALIN)
C1
7/15/03
Introduction
(XTALOUT)
Rfb
C2
LH754xx-4
1-7
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