Clock; Digital I/O; Characteristic Of Recommended 32K Xtal; Characteristic Of Recommended 40M Xtal - Texas Instruments CC3100 SimpleLink Wi-Fi User Manual

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5.3

Clock

5.3.1 32K RTC
The 32.768K XTAL should be placed close to the QFN package. Ensure the load capacitance is tuned
based on board parasitic, so that the frequency tolerance is within ±150 ppm.
Tolerance with temperature and aging
5.3.2 40M XTAL
The 40M XTAL should be placed close to the QFN package. Ensure the load capacitance is tuned based
on board parasitic, so that the frequency tolerance is within ±20 ppm. The frequency tolerance for the
XTAL across temperature and with aging should be ±25 ppm. In addition, ensure no high frequency lines
are routed closer to the XTAL routing, to avoid any phase noise degradation.
Tolerance with temperature and aging
5.4

Digital I/O

Route serial peripheral interface (SPI) and universal asynchronous receiver/transmitter (UART) lines away
from any RF traces, as these digital I/O lines are high-frequency lines and can cause interference to the
RF signal.
Keep the length of the high-speed lines as short as possible to avoid transmission line effects. Keep the
line lower than 1/10th of the rise time of the signal to ignore transmission line effects. This is required only
if the traces cannot be kept short. The resistor should be placed at the source end, closer to the device
driving the signal.
Add series-terminating resistors for each high-speed line (for example, SPI_CLK, SPI_DATA) to match the
driver impedance to the line. Typical terminating resistor values range from 27 to 36 Ω for a 50-Ω line
impedance.
Route high-speed lines with a ground reference plane continuously below it to offer good impedance
throughout, and help shield the trace against EMI interference.
Avoid stubs on high speed lines to minimize the reflections. If the line needs to be routed to multiple
locations, use a separate line driver for each line.
If the lines are longer compared to the rise time, add series-terminating resistors near the driver for each
high-speed line (for example, SPI_CLK, SPI_DATA) to match the driver impedance to the line. Typical
terminating resistor values range from 27 to 36 Ω for a 50-Ω line impedance.
SWRU370B – June 2014 – Revised August 2018
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Table 11. Characteristic of Recommended 32K XTAL
Parameter
Nominal frequency
Table 12. Characteristic of Recommended 40M XTAL
Parameter
Nominal frequency
Tolerance
Load capacitance
CC3100 and CC3200 SimpleLink™ Wi-Fi
Copyright © 2014–2018, Texas Instruments Incorporated
Layout Guidelines
Spec
32.768 kHz
±150 ppm
Spec
40 MHz
±20 ppm
8 pF
±25 ppm
®
and Internet of Things Solution
Layout Guidelines
17