ÉlanSC310 Microcontroller Evaluation Board User’s Manual
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Appendix C
Board Layout Suggestions
The following suggestions concern the ÉlanSC310 microcontroller evaluation
board layout strategy for the 32-kHz oscillator, the PLLs, and the power supplies.
The goal is to minimize noise and noise coupling associated with the way the board
is laid out. Special care is needed to minimize board leakages which can be fatal
to pins that are sensitive to leakage currents, such as the two crystal oscillator pins,
XTAl1 and XTAL2.
32-kHz Oscillator
Prudent board layout for the 32-kHz oscillator suggests the following precautions:
• Keep the two traces, XTAL1 and XTAL2, as short as possible, especially the
input trace, XTAL1. XTAL1 is extremely sensitive to leakage. Total leakage
from/to XTAL1 to/from all the pins on the board must be kept under 300 nA.
XTAL2 can tolerate a leakage as high as 900 nA.
• Keep all noisy signals (e.g., PLL outputs and other clocking signals) as far away
from XTAL1 and XTAL2 as possible. Again, XTAL1 is much more sensitive
to noise coupling than XTAL2.
• Minimize parasitic capacitance between XTAL1 and XTAL2; even a few
picofarads can potentially cause the oscillation frequency to be off target.
• Do not use a feedback resistor larger than 20 M
Ω
; it may fail to start up if the
leakage at XTAL1 is equivalent to 5M
Ω
or less. The feedback resistor value
can be lowered to counter leakage at XTAL1, but that increases start-up time.
The lower bound for the feedback resistor should be about 10 M
Ω
.
• The capacitors connected between XTAL1, XTAL2, and analog ground should
be between 15 pF and 30 pF, and they should be about equal in value. Increasing
the two capacitor values increases start-up time and power consumption, but it
does reduce noise coupling into XTAL1 and XTAL2.
evalbd.book : appc Page 1 Thursday, August 8, 1996 2:34 PM