AN4077, MMA845xQ Design Checklist and Board Mounting Guidelines - Application Notes

Freescale Semiconductor
Application Note
Document Number: AN4077
Rev. 2, 10/2012
MMA845xQ Design Checklist and
Board Mounting Guidelines
by: Kimberly Tuck
Applications Engineer
1.0
Introduction
This document is intended to assist customers with the
design-in of the MMA845xQ 3-axis low-g consumer grade
accelerometers. This document points out the similarities of
the pinout for these devices with a recommended layout for
implementing one board compatible for all devices. Guidelines
for board mounting to a PCB are given with recommendations
for I2C communication and speeds.
1.1
Key Words
Accelerometer, Board Mounting, Solder Paste, Printed Circuit
Board (PCB), I2C Communication, Pull-up Resistor, Pull-down
Resistor, Sensor, I/O pins, Non Solder Mask Defined, Solder
Mask, Land Pattern, Stencil, Halogen Free Package, RoHS
Compliant, Level translator, QFN, Bypass Capacitor,
MMA8450Q, MMA8451Q, MMA8452Q, MMA8453Q
© 2010-2012 Freescale Semiconductor, Inc. All rights reserved.
TABLE OF CONTENTS
1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.0 MMA845xQ Consumer 3-axis Accelerometer 3 x 3 x 1 mm . . . . . . . . . . . 2
2.1 Brief Product Sensitivity and g-range Descriptions . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 MMA8450Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.2 MMA8451Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.3 MMA8452Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.4 MMA8453Q Note: No HPF Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.0 Pin Connections to the MMA845XQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.0 Board Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Power Supply Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Level Translators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3 I/O Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4 Sensor Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.0 I2C Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.0 Offset Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.0 Mounting Guidelines for the Quad Flat No Lead (QFN) Package . . . . . . 6
7.1 Overview of Soldering Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.2 Halogen Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.3 PCB Mounting Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.0
MMA845xQ Consumer 3-axis Accelerometer 3 x 3 x 1 mm
14
VDDIO
1
13
NC
BYP
2
12
SCL
MMA8450Q
16-Pin QFN
3
11
3 mm by 3 mm by 1 mm
4
10
(Top View)
GND
5
16
15
14
VDDIO
1
13
NC
GND
BYP
2
12
GND
INT1
NC
GND
SCL
MMA845xQ
3
11
16-Pin QFN
3 mm by 3 mm by 1 mm
4
10
(Top View)
INT2
GND
5
6
7
8
SDA
SA0
NC
9
8
NC
7
SA0
9
6
SDA
NC
VDD
VDD
15
NC
NC
16
NC
NC
The MMA8451, 2, 3Q has a selectable dynamic range of ±2g, ±4g, ±8g. The device has 8 different output data rates, selectable
high pass filter cut-off frequencies, and high pass filtered data. The available resolution of the data and the embedded features
is dependant on the specific device.
Note: The MMA8450Q has a different memory map and has a slightly different pin-out configuration.
INT1
GND
INT2
Figure 1. Device Pinouts for the MMA8450Q and the MMA845xQ
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2.1
2.1.1
Brief Product Sensitivity and g-range Descriptions
MMA8450Q
1. 12-bit data
2g (1024 counts/g = 1 mg/LSB) 4g (512 counts/g = 2 mg/LSB) 8g (256 counts/g = 3.9 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g =31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
Embedded 32 sample FIFO (Fill and Circular Modes) Regular Data Only- no HPF in FIFO
2.1.2
MMA8451Q
1. 14-bit data
2g (4096 counts/g = 0.25 mg/LSB) 4g (2048 counts/g = 0.5 mg/LSB) 8g (1024 counts/g = 1 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
Embedded 32 sample FIFO (Fill, Circular and Trigger Modes) Regular Data and HPF Data in FIFO
2.1.3
MMA8452Q
1. 12-bit data
2g (1024 counts/g = 1 mg/LSB) 4g (512 counts/g = 2 mg/LSB) 8g (256 counts/g = 3.9 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.2 5 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
2.1.4
MMA8453Q Note: No HPF Data
1. 10-bit data
2g (256 counts/g 3.9mg/LSB) 4g (128 counts/g = 7.8mg/LSB) 8g (64 counts/g= 15.6 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
3.0
Pin Connections to the MMA845XQ
Table 1 describes each pin of the MMA8450Q, the MMA8451, 2, 3Q with recommended connections.
Table 1. Pinout Comparison MMA8450Q, MMA8451,2,3Q
Pin#
MMA8450Q
Recommendations for
MMA8450Q
MMA8451Q
MMA8452Q
MMA8453Q
Recommendation for
MMA8451, 2, 3Q
1
VDDIO
Connect to Pin 14 (1.71 V - 1.89 V)
VDDIO
1.62 V - 3.6 V
2
NC
NC, GND or Bypass Cap to GND
BYPASS
0.1F Bypass Cap to Ground
3
NC
Leave unconnected or GND
NC
MUST leave unconnected
4
SCL
Pull-up Resistor (4.7 k typical) required
on I2C bus
SCL
Pull-up Resistor (4.7 k typical) required
on I2C bus
5
GND
Connect to Ground
GND
Connect to Ground
6
SDA
Pull-up Resistor (4.7 k typical) required
on I2C bus
SDA
Pull-up Resistor (4.7 k typical) required
on I2C bus
7
SA0
Connect to Ground for I2C Address $1C.
Connect to VDD for I2C Address $1D
SA0
Connect to Ground for I2C Address $1C.
Connect to VDD for I2C Address $1D.
8
EN
Connect to MCU
NC
Leave unconnected, connect to VDD or GND
9
INT2
Connect to MCU interrupt
INT2
Connect to MCU interrupt
10
GND
Connect to Ground
GND
Connect to Ground
11
INT1
Connect to MCU interrupt
INT1
Connect to MCU interrupt
12
GND
Connect to Ground
GND
Connect to Ground
13
GND
Connect to Ground
NC
Leave unconnected, connect to VDD or GND
14
VDD
Connect to Pin 1 (1.71 V - 1.89 V)
VDD
Analog Input 1.95 V - 3.6 V
15
NC
Leave unconnected, connect to VDD or GND
NC
Leave unconnected, connect to VDD or GND
16
NC
Leave unconnected, connect to VDD or GND
NC
Leave unconnected, connect to VDD or GND
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3
1.8V - 3.6V
VDDIO
1.95V - 3.6V
VDD
4.7μF
SCL
4.7kΩ
0.1μF
VDDIO
4.7kΩ
3
NC
4
SCL
5
GND
VDD
NC
NC
MMA8451Q
MMA8452Q
MMA8453Q
EN
VDDIO
2
14
SA0
SDA
VDDIO
15
SDA
0.1μF
1
GND
16
6
7
8
GND 13
GND
12
INT1
11
GND
10
INT2
9
INT1
INT2
EN
SA0
Figure 2. Recommended Schematic for the MMA8451, 2, 3Q
1.95V - 3.6V
VDD
1.8V - 3.6V
VDDIO
0Ω
0Ω
4.7μF
4.7kΩ
SCL
VDDIO
4.7kΩ
0.1μF
NC
4
SCL
5
GND
VDD
3
NC
NC
MMA845xQ
EN
VDDIO
2
14
SA0
SDA
VDDIO
15
SDA
0.1μF
1
GND
16
6
7
8
GND 13
GND
12
INT1
11
GND
10
INT2
9
INT1
INT2
EN
SA0
Figure 3. Schematic for either MMA8450Q, or MMA8451, 2, 3Q
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4.0
Board Layout
The following are some important considerations for board layout of the MMA845xQ accelerometers to ensure the best performance. Level translators may be required only if the MCU/processor is operating at a higher voltage. Bypass capacitors are
recommended on the input voltage pins to ensure a clean signal to the sensor for optimal noise performance. Figure 4 is an
example of Freescale’s MMA8450Q evaluation board. This is a four (4) layer board. The board was designed to be able to access
every pin on both the MCU and on the accelerometer for quick debug. The board has 32Mbit Flash which can be used for data
logging and to perform algorithm analysis. The board has an LED and a switch for general purpose functions and a resistor, “R3”,
that can be cut to measure current drain from the accelerometer. This board was designed as small as possible so that it would
be non invasive when mounted on other boards for different applications. The board can plug into an interface board to the PC
or to a battery board. The flat back of the board is great for mounting the board with the battery pack onto moving objects to log
data. The flat back also allows the board to be easily mounted into other designs. All the accelerometer pins are accessible
through the board, bypassing the MC9S08QE8 microcontroller, if another external processor/MCU will be used.
MCU Connections: All Pins
SRL Flash, 32Mbit 2.7-5.5V
Freescale
MC9S08QE8
3.3V 100mA Quasi LowDropout Regulator
1.8V 150mA ULDO Regulator
Dual Bidirectional
I2C Bus & SMBus
Voltage-Level
Translators
MMA8450Q Accelerometer
MMA8450Q Accelerometer Connections
Figure 4. MMA8450Q Example Evaluation Board
4.1
Power Supply Decoupling
A 4.7 F capacitor is recommended to filter noise in case the input supply is noisy, and a 0.1 F bypass capacitor is recommended to filter out high frequency noise. Place these capacitors as close to the part as possible. The caps should be placed
between power and ground as shown in the schematics.
4.2
Level Translators
As shown from the MMA8450Q Evaluation board in Figure 4, level translators are required because the MCU is operating at
3.3 V and the operating voltage of the MMA8450Q is limited to 1.8 V. The SCL, SDA, SA0, EN, INT1 and INT2 signals will shift
through the level translator when communicating back and for to the MCU. Level translators on this board are not required for
the other family members of the MMA845xQ (MMA8451, 2, 3Q) since the operating voltage can be regulated at 3.3 V which
matches the voltage of the MCU on this board.
4.3
I/O Pins
Pin 7 (SA0: Address Pin), Pin 9 (INT2), Pin 11 (INT1), Pin 6 (SDA), Pin 4 (SCL) and Pin 8 (EN) should all use pull-up/down
resistors to ensure they maintain the state of the pin. The interrupt pins can be configured to be push-pull or open drain and can
be either Active High or Active Low. If a pull-up resistor is used on the interrupt pins with the open drain setting, then the accelerometer should be configured to Active Low. If a pull-down resistor is used, then the interrupt pin should be configured to be
Active High. This is done in Register 0x2C (MMA8451, 2, 3Q). Bit 0 sets the configuration of the interrupt pins to be open drain
or push pull-and Bit 1 sets the polarity, either Active High or Active Low. Note the MMA8450Q register map is different and the
interrupt configuration settings are held in Register 0x3A.
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4.4
Sensor Placement
Sensor placement is very important and is often overlooked. The MEMS sensor inside the package is very sensitive to stresses. Small deflections inside the MEMS sensor on the order of 10 nm correspond to a change in acceleration of 1g. Care must be
taken to ensure that the package is not stressed by holes or components on the PCB placed too closely to the accelerometer. It
is important to place the sensor where it is NOT vulnerable to be pushed or otherwise affected directly by the user’s hands. Also
avoid bending the PCB containing the sensor as the PCB stress is transferred to the accelerometer. Temperature can also be an
issue. It is good to avoid placing the sensor near components that may have large temperature variations, or that are constantly
very hot as this will affect the offset of the sensor. For optimal motion detection, place the sensor away from the center of the
device. This will ensure better acceleration readings and make them more significant to detect smaller motions, from a higher
moment of inertia than if placed right on the center of movement.
5.0
I2C Communication
The MMA845xQ, I2C communication protocol follows the Philips Semiconductors (now NXP Semiconductors) standard. In this
interface, only two bus lines are required: a serial data line (SDA) and a serial clock line (SCL). Serial, 8-bit oriented bidirectional
data transfers can be made at up to 100 kbit/s in the standard mode and up to 400 kbit/s in the fast mode. The maximum allowable
bus capacitance is 400 pF. Both SDA and SCL are bidirectional lines, connected to a positive supply voltage via a pull-up resistor.
The recommended value is between 1k - 4.7k. The maximum I2C speed with 1k pull-up resistor is 4.75 MHz and with 4.7k pullup is 2.25 MHz. The accelerometer is always considered the slave and the MCU is always considered the master.
The benefits of the I2C communication interface is that many ICs can be added to this bus. The only limitation is the bus
capacitance. The simple 2-wire serial I2C-bus minimizes interconnections so ICs have fewer pins and there are not as many PCB
traces, resulting in smaller and less expensive PCBs. Each device is recognized by a unique address (whether it is a microcontroller, memory or an accelerometer). The MMA845xQ has an extra address bit to allow for two different addresses available
using Pin 7 (SA0). When Pin 7 is high, the 7-bit I2C address is 0x1D. When Pin 7 is low, the 7-bit I2C address is 0x1C.
6.0
Offset Calibration
After the accelerometer has been mounted onto a PCB there may be a small offset shift. If additional calibration is required
there are three registers to zero out the offset.
7.0
Mounting Guidelines for the Quad Flat No Lead (QFN) Package
Surface mount board layout is a critical portion of the total design. The footprint for the surface mount packages must be the
correct size to ensure proper solder connection interface between the board and the package. With the correct footprint, the packages will self-align when subjected to a solder reflow process. It is always recommended to design boards with a solder mask
layer to avoid bridging and shorting between solder pads.
These guidelines are for soldering and mounting the Quad Flat No-Lead (QFN) package inertial sensors to Printed Circuit
Boards (PCBs). The purpose is to minimize the stress on the package after board mounting. The MMA845xQ digital output
accelerometers use the QFN package platform. This section describes suggested methods of soldering these devices to the PC
board for consumer applications.
7.1
Overview of Soldering Considerations
Information provided here is based on experiments executed on QFN devices. They do not represent exact conditions present
at a customer site. Hence, information herein should be used as guidance only and process and design optimizations are
recommended to develop an application specific solution. It should be noted that with the proper PCB footprint and solder stencil
designs, the package will self-align during the solder reflow process.
7.2
Halogen Content
This package is designed to be Halogen Free, exceeding most industry and customer standards. Halogen Free means that
no homogeneous material within the assembly package shall contain chlorine (Cl) in excess of 700 ppm or 0.07% weight/weight
or bromine (Br) in excess of 900 ppm or 0.09% weight/weight.
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7.3
PCB Mounting Recommendations
1.
2.
3.
4.
5.
6.
7.
The PCB land should be designed with Non Solder Mask Defined (NSMD) as shown in Figure 5 and Figure 6
No additional via pattern underneath package.
PCB land pad is 0.8 mm x 0.3 mm as shown in Figure 5 and Figure 6.
Solder mask opening = PCB land pad edge + 0.113 mm larger all around.
Stencil opening = PCB land pad -0.015 mm smaller all around = 0.77 mm x 0.27 mm.
Stencil thickness is 100 to 125 m.
Do not place any components or vias at a distance less than 5 mm from the package land area. This may cause
additional package stress if it is too close to the package land area.
8. Signal traces connected to pads are as symmetric as possible. Put dummy traces on NC pads in order to have same
length of exposed trace for all pads.
9. Use a standard pick and place process and equipment. Do not use a hand soldering process.
10. It is recommended to use a no clean solder paste.
11. Do not use a screw down or stacking to fix the PCB into an enclosure because this could bend the PCB putting stress
on the package.
12. The PCB should be rated for the multiple lead-free reflow condition with max 260°C temperature.
13. No copper traces on top layer of PCB under the package. This will cause planarity issues with board mount.
Freescale QFN sensors are compliant with Restrictions on Hazardous Substances (RoHS), having halide free
molding compound (green) and lead-free terminations. These terminations are compatible with tin-lead (Sn-Pb) as
well as tin-silver-copper (Sn-Ag-Cu) solder paste soldering processes. Reflow profiles applicable to those processes
can be used successfully for soldering the devices.
Package Footprint
0.467mm x 0.25mm
PCB Land Pattern & Stencil
Package
footprint
Solder mask opening
= PCB land pad edge
+ 0.113mm larger all
around
PCB land pad =
0.8mm x 0.3mm
Stencil opening = PCB land
pad -0.015mm smaller all
around
= 0.77mm x 0.27mm
Figure 5. Recommended PCB Land Pad, Solder Mask and Stencil Opening Near Package Footprint
0.5mm
0.175mm
Note: 0.5 mm separation is recommended on both the horizontal and vertical fields.
Figure 6. Detailed Dimensions
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Table 2. Revision History
Revision
number
Revision
date
2
10/2012
Description of changes
• In Figure 6. Detailed Dimensions on page 7, changed dimension from 1.75mm to 0.175.
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AN4077
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10/2012
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