Texas Instruments | AN119 - Using an External 32kHz Clock to Drive a CC254x Chip (Rev. A) | Application notes | Texas Instruments AN119 - Using an External 32kHz Clock to Drive a CC254x Chip (Rev. A) Application notes

Texas Instruments AN119 - Using an External 32kHz Clock to Drive a CC254x Chip (Rev. A) Application notes
Application Report
SWRA409A – August 2012 – Revised December 2016
Using an External 32 kHz Clock to Drive a CC254x Chip
Fredrik Kervel
ABSTRACT
The purpose of this application report is to show how an external CMOS clock can be connected to the
crystal pins of the CC2540/41 chip. Note that the results presented in this document are intended as a
guideline only.
1
2
3
4
Contents
Introduction ...................................................................................................................
Application Circuit ............................................................................................................
Clock Signal Requirements .................................................................................................
References ...................................................................................................................
2
2
3
5
List of Figures
1
Application Circuit............................................................................................................ 2
2
32.768 kHz Oscillator Functioning When Driven Using an External Clock
3
32.768 kHz Oscillator Functioning When Driven Using an External 0 – 300 mV Clock Over Various
Operating Conditions ........................................................................................................ 5
..........................................
4
List of Tables
1
Capacitors Used for Driving CC2540/41 With an External Clock ..................................................... 2
2
Clock Signal Requirements ................................................................................................. 3
Bluetooth is a registered trademark of Bluetooth SIG, Inc.
All other trademarks are the property of their respective owners.
SWRA409A – August 2012 – Revised December 2016
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Using an External 32 kHz Clock to Drive a CC254x Chip
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1
Introduction
1
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Introduction
The CC254x family of devices has two types of 32.768 kHz oscillator: an RC oscillator and a crystal
oscillator. The RC oscillator requires less current and no external crystal. However, it has lower accuracy
than the crystal oscillator.
The Bluetooth® low energy timing accuracy requirements means that the RC oscillator cannot be used if
the CC2540/41 chips were to be used with power modes. Hence, a crystal oscillator is used. However,
there is an approach by which an external 32 kHz clock can be used to drive the crystal oscillator.
Thereby, allowing you to get rid of the 32.768kHz crystal.
2
Application Circuit
When driving the CC2540/41 with an external 32.768 kHz clock, make sure to capacitively couple the
external clock signal to the P2_3/XOSC32K_Q2. Ground the P2_4/XOSC32K_Q1 pin via cap C321. For
more details, see Table 1.
Table 1. Capacitors Used for Driving CC2540/41 With an External Clock
Component
Description
Value
C331
32-kHz clock AC couple capacitor
100 nF
C321
Grounding capacitor for XOSC32K_Q1 pin 100 pF
2.0V-3.6V power supply
C331
C401
External 32kHz Clock
2 USB_P
AVDD6 31
P2_3/XOSC32K_Q2 33
1 DGND_USB
P2_4/XOSC32K_Q1 32
P2_1 35
P2_2 34
P2_0 36
P1_6 38
P1_7 37
DVDD1 39
DCOUPL 40
C321
R301
RBIAS 30
AVDD4 29
3 USB_N
4 DVDD_USB
C252
AVDD2 27
CC2540/41
5 P1_5
L252
RF_N 26
6 P1_4
C261
RF_P 25
DIE ATTACH PAD:
7 P1_3
XOSC_Q1 22
18 P0_1
19 P0_0
17 P0_2
16 P0_3
15 P0_4
14 P0_5
13 P0_6
10 DVDD2
20 RESET_N
XOSC_Q2 23
9 P1_1
12 P0_7
L253
L261
AVDD3 24
8 P1_2
11 P1_0
Antenna
(50 Ohm)
L251
C251
AVDD1 28
C262
C253
AVDD5 21
XTAL1
Power supply decoupling capacitors are not shown
Digital I/O not connected
C221
C231
Figure 1. Application Circuit
2
Using an External 32 kHz Clock to Drive a CC254x Chip
SWRA409A – August 2012 – Revised December 2016
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Clock Signal Requirements
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3
Clock Signal Requirements
There are two recommended alternatives for driving the crystal oscillator from an external clock source.
Either a 0 – VDD signal can be used, or alternatively a signal with lower amplitude. The requirements for
these signals are shown in the table below. If generating a lower amplitude signal through a resistive
divider it is recommended to keep the total resistance in the range 50kΩ – 100kΩ.
The clock signal duty cycle should be 50%. While square wave signal is typically used, it is not a
requirement, and it is not a problem using a more sinusoidal clock signal.
The clock requirement applies to the signal before the AC couple capacitor, C331, as shown in Figure 1.
When using a lower amplitude clock signal, the oscillator will apply a DC bias which can be observed by
measuring on the Q2 pin. This bias level will vary with operating conditions, but will typically be around
0.55 V, as can be seen in Figure 3.
From the application circuit shown in Figure 1, the 32.768 kHz crystal oscillator circuit operation is verified
by simulating the conditions where the XOSC32K_Q2 pin is driven with the clock through C331. Figure 2
shows the external and internal clock when the clock signal amplitude is equal to the supply voltage, in
this case 3.0 V. Figure 3 shows the external and internal clock as well as the signal on the Q2 pin. The
input clock signal amplitude is 300 mV, and the simulation is performed over operating conditions
(temperature = -40°C, 25°C and 85°C; supply voltage = 2.0 V, 3.0 V and 3.6 V) and process variations.
Table 2. Clock Signal Requirements
Amplitude
Either
Operating Range: 0 – VDD
Or
Operating Range: 0 – 200/400 mV
Absolute min/max (as defined in data sheet): -0.3 V / VDD + 0.3 V
Duty Cycle
min: 45%, max: 55%
SWRA409A – August 2012 – Revised December 2016
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Using an External 32 kHz Clock to Drive a CC254x Chip
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3
Clock Signal Requirements
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Figure 2 and Figure 3 show the oscillator functioning when the external clock is applied to the
XOSC32K_Q2.
Figure 2. 32.768 kHz Oscillator Functioning When Driven Using an External Clock
4
Using an External 32 kHz Clock to Drive a CC254x Chip
SWRA409A – August 2012 – Revised December 2016
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References
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Figure 3. 32.768 kHz Oscillator Functioning When Driven Using an External 0 – 300 mV Clock Over
Various Operating Conditions
4
References
•
•
•
CC2540F128, CC2540F256 2.4-GHz Bluetooth® low energy System-on-Chip Data Sheet (SWRS084)
CC2541 2.4-GHz Bluetooth™ low energy and Proprietary System-on-Chip Data Sheet (SWRS110)
CC253x System-on-Chip Solution for 2.4-GHz IEEE 802.15.4 and ZigBee® Applications User's Guide
(SWRU191)
SWRA409A – August 2012 – Revised December 2016
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Using an External 32 kHz Clock to Drive a CC254x Chip
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5
Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (August 2012) to A Revision ..................................................................................................... Page
•
6
Updates were made to Section 3........................................................................................................ 3
Revision History
SWRA409A – August 2012 – Revised December 2016
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