Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family of Wireless MCUs

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.
1
2
3
4
5
6
7
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9
10
Contents
Keywords ..................................................................................................................... 3
Acronyms ..................................................................................................................... 3
Oscillator and Crystal Basics ............................................................................................... 3
Overview of CC26xx/CC13xx Crystal Oscillators ....................................................................... 6
Selecting Crystals for the CC26xx and CC13xx ......................................................................... 8
PCB Layout of the Crystal ................................................................................................ 12
Configuring the CC26xx or CC13xx for Different Crystals ............................................................ 13
Measuring the Amplitude of the Oscillations of Your Crystal ........................................................ 16
Crystals for CC26xx and CC13xx ........................................................................................ 18
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
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
..............
7
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.
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1
www.ti.com
List of Tables
2
1
Crystal Parameters .......................................................................................................... 6
2
Using External Capacitor Results in Worse Frequency Stability Over Temperature .............................. 10
3
Impact of SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA on Crystal Load Capacitance
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
of Wireless MCUs
...........
14
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Keywords
www.ti.com
1
Keywords
•
•
•
•
•
•
•
•
2
3
Crystal oscillator
Crystal selection
IEEE 802.15.4
RF4CE
Bluetooth Smart
Frequency tuning
CC2650/40/30/20
CC1310/50
Acronyms
Acronyms
Term
BLE
Bluetooth low energy
EM
Evaluation Module
IC
Integrated Circuit
ISM
Industrial, Scientific, Medical
LPRF
Low-Power RF
ppm
Parts per Million (1 × 10–6)
RF
Radio Frequency
RF4CE
Radio Frequency for Consumer Electronics
SoC
System on Chip
ESR
Equivalent Series Resistance
Oscillator and Crystal Basics
This section explains fundamentals of a quartz crystal and the oscillator operations required to understand
the trade-offs when selecting a crystal for the CC26xx. The complete crystal oscillator circuit includes the
loading capacitance, crystal, and the on-chip circuitry.
3.1
Oscillator Operation
The circuit used as high-accuracy clock source for TI’s low-power RF products is based on a Pierce
oscillator as shown in Figure 1. There is no on-chip damping resistor and none must be added by the
customer. The oscillator circuit consists of an inverting amplifier (shown as an inverter), a feedback
resistor, two capacitors, and a crystal. When operating, the crystal and the capacitors form a pi filter that
provides an 180-degree phase shift to the internal amplifier, keeping the oscillator locked at the specified
frequency.
U1
R1
X1
CL1
CL2
Figure 1. Pierce Oscillator
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Oscillator and Crystal Basics
3.2
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The Quartz Crystal Electrical Model
A quartz crystal is a piezoelectric device that transforms electric energy to mechanical energy. This
transformation occurs at the resonant frequency. Figure 2 shows the simplified electric model that
describes the quartz crystal, where C0 is the shunt capacitance, LM is motional inductance, CM is motional
capacitance, and RM is motional resistance. The model in Figure 2 is a simplified model and includes only
the fundamental oscillation frequency. In reality, crystals can also oscillate at odd harmonics of the
fundamental frequency.
C0
X1
LM
RM
CM
Figure 2. Crystal Symbol and the Electrical Model of a Quartz Crystal
3.2.1
3.2.2
Frequency of Oscillation
A crystal has two resonant frequencies characterized by a zero-phase shift. Equation 1 is the series
resonance.
1
fs =
2p LM ´ CM
(1)
Equation 2 is the antiresonant frequency.
1
fa =
C ´ C0
2p LM ´ M
CM + C0
(2)
As specified in the data sheet of the crystal, the frequency of oscillation is between the resonance
frequencies. See Equation 3.
fs < fXTAL < fa
(3)
Equivalent Series Resistance
The Equivalent Series Resistance (ESR) is the resistance the crystal exhibits at the series resonant
frequency. Equation 4 gives the ESR.
2
æ C ö
ESR = RM ç 1 + 0 ÷
ç C ÷
Lø
è
(4)
Because C0 is typically on the order of 1 pF and CL is 5 – 9 pF, ESR is approximately RM for many
crystals, sometimes ESR is approximated as motional resistance.
3.2.3
Drive Level
The drive level of a crystal refers to the power dissipated in the crystal. The maximum drive level of a
crystal is often specified in the data sheet of the crystal in µW. Exceeding this value can damage or
reduce the life of the crystal. Equation 5 gives the drive level in W.
2
DL = ESR pf CL + CM Vpp
(
4
(
)
)
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
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Where Vpp is the peak-to-peak voltage across the crystal. Calculating the DL with Equation 5 and
comparing this value to the maximum specified DL in the crystal data sheet may reveal if the crystal is
likely to have reliability issues during operation. Section 8 describes how to measure the value of Vpp.
3.2.4
Crystal Pulling
The crystal pullability is a measure of the frequency change of the crystal given by a change in its load
capacitance. The pulling is given by Equation 6 around the specified (parallel) resonance frequency of the
crystal.
F ´ CM
CLMAX - CLMIN
ΔF =
2
(C0 + CLMAX ) C0 + CLMIN
(6)
(
)
CLMAX and CLMIN are the maximum and minimum load capacitance that can be presented to the crystal. For
more information, see Section 7. Table 3 shows how to change the internal load capacitance on the
crystal using software
3.3
Negative Resistance
Negative resistance (RN) is a parameter of the complete oscillator circuit, including capacitor values,
crystal parameters, and the on-chip circuit. To ensure robust start-up of the crystal oscillator, the
magnitude of the negative resistance must be at least 5 times greater than ESR during the initial start-up
of the crystal but can be 2 to 3 times greater than ESR after start-up and during steady state operation.
The following section shows an increasing negative resistance magnitude reduces the start-up time of the
oscillator (see Equation 7).
R
ESR < N
5
(7)
Equation 8 approximates the negative resistance and shows that a low CL gives a larger negative
resistance.
-gm
RN »
2
(2pf )2 2CL
(
)
(8)
Where:
gm— the transconductance of the active element in the oscillator
CL— the load capacitance
You can also find the negative resistance of the circuit by introducing a resistor in series with the crystal.
To avoid parasitic effects, TI recommends using a 0201 resistor for this task. The threshold of the sum of
the extra 0201 external resistance and ESR or the crystal where the oscillator is unable to start up is
approximately the same as the circuit negative resistance.
3.4
Time Constant of the Oscillator
The start-up time of a crystal oscillator is determined by transient conditions at turn-on, small-signal
envelope expansion due to negative resistance, and large-signal amplitude limiting. The envelope
expansion is a function of the total negative resistance and the motional inductance of the crystal. The
time constant of the envelope expansion is proportional to the start-up time of the oscillator given by
Equation 9.
-2LM
-2LM
t=
»
, Rn ? Rm
RN
RM + RN
(9)
(
)
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A crystal with a low LM gives a shorter start-up time and so does a high-magnitude RN (low CL). A trade-off
exists between pullability due to low-motional capacitance (CM) and fast start-up time due to low-motional
inductance (LM), because the frequency of the crystal is dependent on the both CM and LM. Crystals in
smaller package sizes have larger LM, and start more slowly than those in larger package sizes (see
Section 3.2.1). Lowering CL also reduces the drive level, allowing for an increase in oscillation amplitude
(see Equation 5).
Table 1 summarizes crystal parameters and their values for the reference crystals recommended by TI for
use with CC26xx and CC13xx.
Table 1. Crystal Parameters
4
Parameters
Description
24-MHz Crystal
Used in TI Characterization
TI-Assumed Default
32.768-kHz Crystal
5.0 kH
Motional Inductance (LM)
Partly determines crystal speed 12.6 mH
(how quickly the crystal
responds to a change from the
oscillator). Lower Lm → crystal
responds more quickly to
changes from the oscillator.
Along with CM, a major
determiner of the crystal quality
factor
3.4 fF
4.718 fF
Motional Capacitance (CM)
Partly determines crystal
speed. Lower CM → crystal
responds more slowly to
changes from the oscillator.
20 Ω (60-Ω maximum)
37 kΩ (70-kΩ maximum)
Motional Resistance (RM)
At resonance, Lm and CM
cancel and RM is presented to
the oscillator. RM ~ ESR
assuming CL >> CO.
9 pF
7 pF
Load Capacitance (CL)
The amount of load capacitor
to tune the crystal to the
correct frequency. This load
capacitance also helps
determine drive level.
Shunt Capacitance (CO)
This is a parasitic capacitance 1.2 pF
due to crystal packaging. It
helps determine the acceptable
drive level.
ESR
Equivalent Series Resistance.
If CL >> CO, then ESR ~ RM
Drive Level
The maximum level of power in 200 µW
the crystal for reliable longterm operation. 2 × ESR (ω (CL
+ CO)) × Vosc where Vosc is the
amplitude of the crystal
oscillations.
1 pF
20 Ω (60-Ω maximum)
37 kΩ
<500 nW
Overview of CC26xx/CC13xx Crystal Oscillators
The CC26xx and CC13xx have integrated 24-MHz and 32.768-kHz crystal oscillators that TI designed for
use with low-cost quartz crystals. High-frequency (48 MHz) and low-frequency (32 kHz) RC oscillators are
available on the CC26xx/CC13xx (beyond the scope of this application report).
6
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
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4.1
24-MHz Crystal Oscillator
Figure 3 shows a simplified block diagram of the 24-MHz crystal oscillator. The oscillator circuit consists of
an inverting amplifier, a feedback net, capacitors, and a crystal. The CC26xx and CC13xx have an internal
capacitor array that can be adjusted and eliminates the requirement for external loading capacitors. The
default setting of the internal capacitance is 9 pF, but this setting can be adjusted by register configuration
within a range of 2 pF to 11 pF. For reliable operation, TI recommends operating the crystal with CL from 5
to 9 pF. Section 7 shows how to set this value. If no external capacitors are used then the value of CL is
determined by the internal loading capacitors plus board parasitic capacitance
Feedback
INV AMP
CL_INT x 2
CL_INT x 2
X24M
CC26xx/CC13xx
CP
CL1
X24P
CL2
X1
Figure 3. Simplified Block Diagram of the CC26xx/CC13xx High-Frequency Oscillator With Quartz Crystal
The 24-MHz crystal is controlled with a complex control loop described in Section 4.2 and Section 4.3.
4.2
24-MHz Crystal Control Loop
TI intends the amplitude control loop to regulate the amplitude of the oscillations of the crystal for optimal
performance. The following are the two primary portions of the control loop:
• Start-up: The control loop injects as much current as possible into the oscillator that drives the crystal
resonator.
• Steady state regulation: The amplitude of the crystal oscillator can be regulated in a steady state
manner if required.
To turn on the crystal so that the radio can operate, start-up is required. Steady state amplitude regulation
is not required for the crystal or radio to function.
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Overview of CC26xx/CC13xx Crystal Oscillators
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32.768-kHz Crystal Oscillator
Figure 4 shows a simplified block diagram of the 32.768-kHz crystal oscillator. The oscillator circuit
consists of an inverting amplifier, a feedback net, capacitors, and a crystal. The 32-kHz crystal lacks
internal capacitors and requires external loading capacitors.
Feedback
INV AMP
CC26xx/CC13xx
X32K_Q2
X32K_Q1
CP
CL1
X1
CL2
Figure 4. Simplified Block Diagram of the 32.768-kHz Oscillator With Quartz Crystal
5
Selecting Crystals for the CC26xx and CC13xx
This section presents some important considerations when selecting crystals for the CC26xx and CC13xx.
CC26xx Crystals TI Wiki lists the crystals tested with the CC26xx and CC13xx. Selecting a crystal for a
specific application depends on the following three factors:
• Size (footprint area and height)
• Performance (accuracy over temperature, lifetime, power consumption, and start-up time)
• Cost
Consider the following when selecting a crystal:
• Crystals must be selected to meet CC26xx and CC13xx data sheet or specification requirements
– ESR must not be greater than can be driven by
– CC26xx and CC13xx (60 Ω for the 24-MHz crystal). Capacitive loading and frequency tolerance
must meet the specifications of the standard used (for example, Bluetooth)
– Motional inductance must also meet specifications. Many crystal manufactures provide only
motional inductance data upon customer request. TI has tested crystals with motional inductances
up to the value specified in the CC2650 SimpleLink ™ Multistandard Wireless MCU data sheet
(SWRS158).
• Configuring the device
– The required tuning capacitance of the 24-MHz crystal cannot lie outside of 2 – 11 pF unless
external tuning capacitors are used.
NOTE: To achieve reliable results, keep the required tuning capacitance from 5 to 9 pF though a
wider tuning range can be set.
– The frequency accuracy of the 32.768-kHz crystal determines the accuracy of the sleep clock. The
BLE (or other standard) stack must be updated to match the accuracy of the selected crystal
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•
5.1
Some other considerations when selecting a crystal include the following:
– To improve start-up time and reduce power consumption, the crystal must have the following:
• Low-capacitive loading, at the expense of greater susceptability to frequency variation caused
by the environment
• Low-motional inductance
• Low-motional resistance
Mode of Operation
Quartz crystals are used at the fundamental resonance frequency for frequencies relevant to the
CC26xx/CC13xx, but there are crystals that operate at an odd overtone of the fundamental frequency. TI
recommends using a crystal that operates at the fundamental mode for the CC26xx/CC13xx.
5.2
Frequency Accuracy
The total tolerance of the frequency accuracy of a crystal is dependent on several factors:
• Production tolerance
• Temperature tolerance
• Aging effects
• Frequency pulling of the crystal due to mismatched loading capacitance
When selecting the crystal, consider these parameters. Equation 10 gives the total crystal tolerance.
Toltot = Tol prod + Toltemp + Tolage + Tol pull (ppm )
(10)
These values are given in [ppm] (parts per million) and can be found in the data sheet of the crystal
manufacturer, except pullability which can calculated by the formula in Section 3.
5.2.1
24-MHz Crystal
Because the 24-MHz crystal oscillator is used as a reference to generate the RF signal, any crystal
frequency deviation is directly transferred to deviation of the RF signal. For example, 10 ppm leads a
deviation in RF carrier frequency of 10 ppm. Select a crystal with performance within the limits of the RF
specifications.
• For 802.15.4 (RF4CE/ZigBee®), the maximum deviation in carrier frequency is limited to ±40 ppm (see
Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for LowRate Wireless Personal Area Networks (WPANs)).
• For Bluetooth core specifications 4.2, the limit is ±50 ppm (see Bluetooth Core Specifications, Version
4.2).
TI recommends using a 24-MHz crystal with a maximum 40-ppm total tolerance (including aging,
temperature, and so forth).
5.2.2
32.768-kHz Crystal
The 32.768-kHz crystal oscillator is typically used as the system clock when the CC26xx/CC13xx is in a
standby mode. Because Bluetooth low energy is a time-synchronized protocol, an accurate clock also
enables longer periods of time in a low-power mode. If a lower-accuracy crystal is used, the device must
wake up early to accommodate for the lower accuracy of the clock. If the 32.768-kHz crystal oscillator is
used as the low-frequency clock in Bluetooth low energy, the clock must have a maximum of ±500 ppm of
inaccuracy. For more information, see Bluetooth Core Specifications, Version 4.2. TI recommends using a
tighter tolerance 32.768-kHz crystal to reduce the average power consumption in a typical Bluetooth low
energy connection. In the SimpleLink CC2650 EVM Kit 4XD (CC2650EM-4XD) v1.0.3 Design Files
(SWRC302), TI uses the Epson FC-135 crystal. If a crystal with different specifications is used, this setting
must be adjusted for in the Bluetooth low energy stack. For more details, see Section 7.
The CC26xx device must be used with a 32.768-kHz crystal that has at least 500-ppm accuracy. TI
recommends a crystal of 40-ppm accuracy.
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See the FC-135 crystal at http://www.epsondevice.com/docs/qd/en/DownloadServlet?id=ID000805. The
crystal has ±20-ppm frequency accuracy at room temperature but varies by 100 s of ppm overtemperature
like other 32.768-kHz crystals. When specifying accuracy of the 32.768-kHz crystal, the accuracy over the
entire temperature range, not just at room temperature, must be specified in the software stack.
Use the CC26xx/CC13xx device only with a 32.768-kHz crystal that has at least 500-ppm accuracy over
the desired operating temperature range. The Bluetooth low energy stack is by default set to 40-ppm
accuracy. If a customer product is designed to operate over large temperature ranges, the customer must
adjust this accuracy. Low-frequency tuning fork crystals have a resonance frequency that changes with
temperature with a parabolic coefficient of (–0.04 × 10e – 6) / °C2 typically. Figure 5 shows an example of
this . In Figure 5, 40-ppm accuracy is maintained from –10°C to 50°C. Operating over wider temperature
ranges requires the customer to adjust the Bluetooth low energy stack.
50
0
-50
'F (ppm)
-100
-150
-200
-250
-300
-350
-400
-450
-50
0
50
Temperature (qC)
100
150
D003
Figure 5. Typical Frequency vs Temperature Curve for a 32.768-kHz Tuning Fork Crystal
5.3
Load Capacitance
The crystal oscillator frequency is dependent on the values of the capacitive loading of the crystal. These
capacitors with any parasitic capacitance in the PCB and the crystal terminals compose the total load
capacitance that helps set the crystal resonance frequency. The crystal data sheet provides the optimum
load capacitance for the crystal, CL. This total CL typically consists of both the loading capacitors and the
parasitic capacitance of the layout and packaging.
Using external capacitors to get the correct frequency means that the internal caps must be set to
minimum. For example, an application could use near minimum on-chip capacitance of approximately 2
pF and 7 pF of off-chip capacitor to get 9 pF. Table 2 shows using external caps this way gives slightly
worse frequency stability with temperature than using internal capacitors.
Table 2. Using External Capacitor Results in Worse Frequency Stability Over Temperature
9-pF internal CL
Minimum internal CL
Frequency variation –40°C to 90°C
Set by crystal
Set by crystal + 5 ppm
Voltage accuracy, ppm/V
6.9
9
The following presents the relative advantages of crystals with different CL values.
The disadvantages of lower CL are as follows:
• Crystals with < 7-pF CL are more difficult to source with short lead times.
• Frequency becomes more sensitive to changes in board capacitance as CL decreases. It is possible to
meet frequency stability specifications with a CL as low as 3 pF.
• Lowering CL results in degraded RF phase noise.
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Advantages of lower CL are as follows:
•
•
•
5.4
æ 1 ö
÷
¥ç
ç C2 ÷
è L ø)
Lower CL causes a much faster start-up time. (Start-up time goes as
Lower CL causes a faster amplitude control loop response time.
Lower CL makes it easier to use small size crystals (2.0 × 1.6 and so on) and maintain a start-up time
at or less than 400 µs. Start-up time worsens with smaller crystals due to an increase in LM.
ESR and Start-up Time
ESR (equivalent series resistance) is a parameter of the crystal in the data sheet of the crystal. Negative
resistance is a parameter of the complete oscillator circuit, including capacitor values, crystal parameters,
and an on-chip circuit. To ensure best start-up of the crystal oscillator, the negative resistance magnitude
must be at least 5 times greater than RN (see Equation 11 and Equation 12) during initial start up but can
be 2 to 3 times greater when the crystal has reached steady state.
R
ESR < N
5
(11)
-gm
RN =
2
(2pf )2 2CL
(12)
(
)
If the negative resistance magnitude is not 5× greater than RN during initial start-up, the oscillator might
not operate optimally or might fail to start. An increasing negative resistance magnitude leads to a faster
the start-up time of the oscillator.
NOTE: Crystals with higher ESR typically result in longer start-up times than crystals with lower
ESR. An higher-load capacitance decreases the negative resistance of the oscillator and
increases the start-up time.
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5.5
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Drive Level and Power Consumption
The maximum drive level of a crystal is often specified in the data sheet of the crystal in µW. Exceeding
this value can damage or reduce the lifetime the crystal. The CC26xx/CC13xx drives the crystal with a
maximum 1.6 Vpp for the 24-MHz crystal and 600 mVpp for the 32.768-kHz crystal. As Section 3.2.3
explains, Equation 13 gives the drive level in W.
2
DL = ESR pf CL + CM Vpp
(13)
(
(
)
)
As in Equation 13 of the drive level, a higher total capacitance load and ESR require more power to drive
the crystal, increasing the power consumption of the oscillator. Because the 32.768-kHz crystal is on for
an extended period of time, this increase is important. Selecting a low ESR and low-CL 32.768-kHz crystal
is important to achieve low-power consumption in a low-power mode.
NOTE: Do not use the internal DC/DC when applying a probe to the 24-MHz crystal oscillator pins.
Applying the probe can lead to the oscillator stopping and may lead to the internal DC-DCproducing a high-output voltage that may damage the device.
5.6
Crystal Package Size
There are several different packages for crystals. The available board space and cost determines the
package size used. Crystals with smaller packages have a higher ESR and motional inductance. These
smaller packages cause a longer start-up time of the crystal oscillator. By choosing a crystal with a low CL
if a smaller package is required, this start-up time increase can be compensated.
6
PCB Layout of the Crystal
The layout of the crystal can reduce the parasitic capacitance and, more importantly, reduce noise from
coupling on the input of the oscillators. Noise on the input of the oscillator can lead to severe side effects
such as clock glitches, flash corruption, or system crashes because the CC26xx and CC13xx relies on the
crystal oscillators as the high- and low-frequency system clock.
The following are a few recommendations for the layout of the crystals:
• Place the crystal as close as possible to the device to minimize the length of the PCB traces. (This
placement reduces crosstalk and minimizes EMI.)
• TI recommends a solid ground plane under the crystal.
• Ensure no high-speed digital signals are close to the crystal to minimize cross-coupling of noise into
the oscillator.
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Figure 6 shows the top layer of the layout of the CC26xx reference design. The bottom layer is a solid
ground plane. For more details, see SimpleLink CC2650 EVM Kit 4XD (CC2650EM-4XD) v1.0.3 Design
Files (SWRC302). The same crystal layout can be used with CC13xx.
Figure 6. Layout of the CC26xx EVM
7
Configuring the CC26xx or CC13xx for Different Crystals
You must set the internal load capacitor to tune the frequency of the 24-MHz oscillator. The capacitor
array can also be disabled and in this case the external loading capacitors are required. If using the TI
BLE stack, you must also specify the tolerance of the 32.768-kHz clock to the BLE stack.
7.1
Internal Capacitor Array
The internal capacitor of the 24-MHz crystal oscillator can be adjusted. Equation 14 gives the total internal
loading capacitance.
CL _ int = 9 + Cdelta (pF )
(14)
The value of Cdelta is set in customer configuration area in the flash memory of the CC26xx/CC13xx. For
more details, see CC26xx Technical Reference Manual (SWRU117). The capacitance of the array can
vary up to 8% from device to device and over operating conditions. Modify the oscillator CCFG.c file by
performing the following steps:
1. Enable the cap-array delta (Cdelta) (see the following code).
//**************************************************
// Enable XOSC cap-array delta
//**************************************************
#define SET_CCFG_MODE_CONF_XOSC_CAP_MOD
0x0
// #define SET_CCFG_MODE_CONF_XOSC_CAP_MOD
0x1
// Apply cap-array delta
// Don't apply cap-array delta
2. Set the value of the Cdelta.
//**************************************************
// Value of XOSC cap-array delta
//**************************************************
#define SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA
directly modifying trimmed XOSC cap-array value
SWRA495D – December 2015 – Revised April 2016
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0xFF
// Signed 8-bit value,
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13
Configuring the CC26xx or CC13xx for Different Crystals
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Table 3 is a table with CL_int for different settings of the
SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA measure in the SimpleLink CC2650 EVM Kit
4XD (CC2650EM-4XD) v1.0.3 Design Files (SWRC302).
Table 3. Impact of SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA on Crystal Load
Capacitance
SET_CCFG_
MODE_CONF
_XOSC_CAP
ARRAY_DEL
TA Value
CL_int
SET_CCFG_
MODE_CONF
_XOSC_CAP
ARRAY_DEL
TA Value
CL_int
SET_CCFG_
MODE_CONF
_XOSC_CAP
ARRAY_DEL
TA Value
CL_int
SET_CCFG_
MODE_CONF
_XOSC_CAP
ARRAY_DEL
TA Value
CL_int
–27
5
–17
6.2
–7
7.7
3
–26
5.1
–16
6.4
–6
7.7
4
9.6
9.8
–25
5.2
–15
6.5
–5
7.9
5
10.1
–24
5.3
–14
6.7
–4
8.2
6
10.3
–23
5.3
–13
6.8
–3
8.4
7
10.5
–22
5.5
–12
7
–2
8.6
8
10.7
–21
5.6
–11
7.1
–1
8.8
9
10.9
–20
5.8
–10
7.3
0
9
10
11.1
–19
5.9
–9
7.4
1
9.2
–18
6.1
–8
7.6
2
9.4
After configuring Cdelta, the value of the DDI_0_OSC ANABYPASSVALUE1 register should match the
required capacitance. To use a 6-pF crystal, set the value of
SET_CCFG_MODE_CONF_XOSC_CAPARRAY_DELTA to –18.
7.2
Set the Sleep Clock Accuracy in the BLE Stack
The 32.768-kHz clock is typically used as the low-frequency system clock in low-power mode, especially
when it is used in an application that relies on a time-synchronized network (for example, Bluetooth low
energy). In a time-synchronized network, a more accurate clock enables a later wake up of the device to
start listening or sending radio packets and not to miss radio events.
14
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
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In the BLE stack from TI, you can set the accuracy of the 32.768-kHz clock to optimize the power
consumption. By default, the accuracy is set to 40 ppm; you must adjust the accuracy to avoid missing
radio events or waking up the device from a low-power mode earlier than necessary. The sleep clock
accuracy is set in the BLE stack with the following HCI command:
HCI_EXT_SetSCACmd(40); //Default 40ppm
/*******************************************************************************
* @fn
HCI_EXT_SetSCACmd
*
* @brief
This API is used to set this device's Sleep Clock Accuracy.
*
*
Note: For a slave device, this value is directly used, but only
*
if power management is enabled. For a master device, this
*
value is converted into one of eight ordinal values
*
representing a SCA range, as specified in Table 2.2,
*
Vol. 6, Part B, Section 2.3.3.1 of the Core specification.
*
*
Note: This command is only allowed when the device is not in a
*
connection.
*
*
Note: The device's SCA value remains unaffected by a HCI_Reset.
*
* input parameters
*
* @param
scaInPPM - A SCA value in PPM from 0..500.
*
* output parameters
*
* @param
None.
*
* @return
hciStatus_t
*/
extern hciStatus_t HCI_EXT_SetSCACmd( uint16 scaInPPM );
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Measuring the Amplitude of the Oscillations of Your Crystal
8
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Measuring the Amplitude of the Oscillations of Your Crystal
Two functions exist in CC26XX/13XXWARE for measuring the amplitude of the oscillations of the crystal,
and comparing this amplitude to the expected amplitude. These functions are as follows:
• uint32_t OSCHF_DebugGetCrystalAmplitude( void );
• uint32_t OSCHF_DebugGetExpectedAvarageCrystalAmplitude( void );
The first function inserted into a piece of code returns the amplitude of the crystal in mV. The second
function returns the expected oscillation amplitude, also in mV. These are debug functions only. The first
function uses an on-chip ADC to measure the amplitude of the crystal. If these functions return greatly
different values, the crystal may have a problem. The uncertainty of the first function is ± 50 mV; a 50 mV
deviation from the expected value is not cause for concern.
16
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
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Measuring the Amplitude of the Oscillations of Your Crystal
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8.1
Measuring Start-Up Time to Determine HPMRAMP1_TH and XOSC_HF_FAST_START
The following code can be used to estimate the start-up time of the 24-MHz crystal. The code counts the
edges of the 32.768-kHz crystal before the 24-MHz crystal is operational. For more accurate results,
external measurement equipment is required.
uint32 Max
uint32 Count = 0;
= 0;
// etc, initialize all variables
// Route SCLK_LF to DIO24, with DIO24 configured as input.
HWREG(AON_IOC + CLK32KCTL) = 0x0; //
HWREG(IOC + IOC24) = 0x20006307; // enable IOC input, connect DIO 24 to SCLK_LF
// Set RCOSC_HF as SCLK_HF source
SafeHFClockSwitch(RCOSC_HF); // use ROM function
// Set XOSC_LF as SCLK_LF source
DDI16BitfieldWrite(AUX_DDI0_OSC_BASE, DDI_0_OSC_O_CTL0,
DDI_0_OSC_CTL0_SCLK_LF_SRC_SEL_M,
DDI_0_OSC_CTL0_SCLK_LF_SRC_SEL_S, 3);
// wait for SCLK_LF to be sourced from XOSC_LF
while((HWREG(DDI_0_OSC + STAT0) & SCLK_LF_SRC_M) != SCLK_LF_IS_XOSC_LF_M) {};
// Measure startup time in 1/2 LF clock periods
State = HWREG(GPIO + DIN31_0); // assume only DIO24 is toggling
PreviousState = HWREG(GPIO + DIN31_0);
while (State == 0x1000000) {State = HWREG(GPIO + DIN31_0); // get to a known starting
spot
EnableXOSC_HF_NoClockSwitch();
while ((HWREG(DDI_0_OSC + STAT0) & PENDING_M) != 1) { //wait for pending bit
State = HWREG(GPIO + DIN31_0);
Max = Max+1;
if (State != PreviousState) {
Count = Count + 1;
PrevMax = Max;
Max = 0;
}
PreviousState = State;
}
StartUpTimePrecharge = Count*100+100*Max/PrevMax;
Figure 7 is an example plot generated with this code across temperature.
80
70
Startup Time (us)
60
50
40
30
20
10
0
-60
-40
-20
0
20
40
Temperature (q C)
60
80
100
D002
Figure 7. Example Startup Time Plot
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17
Crystals for CC26xx and CC13xx
9
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Crystals for CC26xx and CC13xx
Tables giving appropriate crystals for use with the CC26xx/CC13xx can be found at this website:
http://processors.wiki.ti.com/index.php/CC26xx_Crystals.
10
References
1. Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for LowRate Wireless Personal Area Networks (WPANs), 802.15.4-2006,
http://standards.ieee.org/getieee802/download/802.15.4-2006.pdf
2. Bluetooth Core Specifications, Version 4.2, https://www.bluetooth.org/en-us/specification/adoptedspecifications
3. CC2650 SimpleLink™ Multistandard Wireless MCU (SWRS158)
4. CC26xx Crystals TI Wiki, http://processors.wiki.ti.com/index.php/CC26xx_Crystals
5. SimpleLink CC2650 EVM Kit 4XD (CC2650EM-4XD) v1.0.3 Design Files (SWRC302)
6. CC26xx Technical Reference Manual (SWRU117)
18
Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family
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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 C Revision (March 2016) to D Revision .................................................................................................. Page
•
•
Removed 24-MHz Crystals Usable With CC26xx/CC13xx table. ................................................................. 18
Removed 32.768-kHz Crystals Usable With CC26xx/CC13xx table. ............................................................. 18
Revision History
Changes from B Revision (February 2016) to C Revision ............................................................................................. Page
•
•
Updated Crystal Parameters Table. .................................................................................................... 6
Updated 32.768-kHz Crystals Usable With CC26xx/CC12xx table. .............................................................. 17
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