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 AN4077 2 Sensors Freescale Semiconductor, Inc. 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 0.1F 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 AN4077 Sensors Freescale Semiconductor, Inc. 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 AN4077 4 Sensors Freescale Semiconductor, Inc. 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. AN4077 Sensors Freescale Semiconductor, Inc. 5 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. AN4077 6 Sensors Freescale Semiconductor, Inc. 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 AN4077 Sensors Freescale Semiconductor, Inc. 7 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. AN4077 8 Sensors Freescale Semiconductor, Inc. How to Reach Us: Information in this document is provided solely to enable system and software Home Page: freescale.com implementers to use Freescale products. There are no express or implied copyright Web Support: freescale.com/support information in this document. licenses granted hereunder to design or fabricate any integrated circuits based on the Freescale reserves the right to make changes without further notice to any products herein. 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U.S. Pat. & Tm. Off. Xtrinsic is a trademark of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © 2012 Freescale Semiconductor, Inc. All rights reserved. AN4077 Rev. 2 10/2012