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SWRA495D–December 2015–Revised April 2016
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Copyright © 2015–2016, Texas Instruments Incorporated
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
of Wireless MCUs
Bluetooth is a registered trademark of Bluetooth SIG.
ZigBee is a registered trademark of ZigBee Alliance.
All other trademarks are the property of their respective owners.
Application Report
SWRA495D–December 2015–Revised April 2016
Crystal Oscillator and Crystal Selection for the CC26xx
and CC13xx Family of Wireless MCUs
James Murdock and Danielle Griffith
ABSTRACT
The CC26xx (CC2620/30/40/50) and CC13xx (CC1310/50) family is a low-power wireless MCU platform
supporting multiple standards (that is, BLE, IEEE802.15.4, and proprietary RF protocols). The devices
have integrated 24-MHz and 32.768-kHz crystal oscillators TI designed for use with low-cost quartz
crystals. The 24-MHz oscillator generates the reference clock for the RF blocks and the MCU system. RF
systems are dependent on accurate clocks for correct operation. A deviation in clock frequency is
reflected as a deviation in radio frequency. This deviation can degrade RF performance, violate regulatory
requirements, or lead to a nonfunctioning system. In power-down mode, the high-frequency oscillator is
typically turned off and a low-frequency oscillator is the system clock. For time-synchronized protocols
such as Bluetooth
®
Smart, a tight tolerance on the sleep clock enables longer time in low-power mode and
reduced power consumption important in battery-powered applications. For this low-frequency oscillator,
typically a 32-kHz crystal oscillator is used.
The scope of this application report is to discuss the requirements and trade-offs of the crystal oscillators
for the CC26xx and CC13xx devices and provide information on how to select an appropriate crystal. This
document also presents steps to configure the device to operate with a given crystal. You must configure
the CC26xx and CC13xx based on the crystal used (that is, adjust the internal capacitor array to match
the loading capacitor of the crystal for the 24-MHz oscillator). The application report also discusses some
measurement approaches that may be used to characterize certain performance metrics, including crystal
oscillator amplitude, and start-up time.
Contents
1 Keywords ..................................................................................................................... 3
2 Acronyms ..................................................................................................................... 3
3 Oscillator and Crystal Basics............................................................................................... 3
4 Overview of CC26xx/CC13xx Crystal Oscillators ....................................................................... 6
5 Selecting Crystals for the CC26xx and CC13xx......................................................................... 8
6 PCB Layout of the Crystal ................................................................................................ 12
7 Configuring the CC26xx or CC13xx for Different Crystals............................................................ 13
8 Measuring the Amplitude of the Oscillations of Your Crystal ........................................................ 16
9 Crystals for CC26xx and CC13xx........................................................................................ 18
10 References.................................................................................................................. 18
List of Figures
1 Pierce Oscillator .............................................................................................................. 3
2 Crystal Symbol and the Electrical Model of a Quartz Crystal.......................................................... 4
3 Simplified Block Diagram of the CC26xx/CC13xx High-Frequency Oscillator With Quartz Crystal .............. 7
4 Simplified Block Diagram of the 32.768-kHz Oscillator With Quartz Crystal......................................... 8
5 Typical Frequency vs Temperature Curve for a 32.768-kHz Tuning Fork Crystal ................................. 10
6 Layout of the CC26xx EVM............................................................................................... 13
7 Example Startup Time Plot................................................................................................ 17
