MMA7455L accelerometer breakout board
From DP
Accelerometers measure orientation. The MMA7455L is an inexpensive accelerometer ($2 in 1s), with adjustable measurement resolution of 2/4/8g.
We used this chip in the USB POV Toy to sync the POV display to the speed of the user's hand motions. It only comes in a leadless LGA-14 package. It's a challenging chip to solder, so we though a inexpensive breakout board might be useful.
- 2/4/8g selectable
- 10bit or 8bit output
- force and pulse detection with interrupt
- I2C and SPI interface
- 3.3volts
Get the [MMA7455L breakout board for $15].
Read about the design below.
Contents |
Hardware
Click for a full size schematic image. Schematic and PCB were designed with the freeware version of Cadsoft Eagle.
- Download the latest schematic and PCB files.
MMA7455L
PCB
Click the picture for a full size placement image.
- PCB and placement notes
- soldering advise
Partlist
| Part | Quantity | Value | Package |
|---|---|---|---|
| C1, C3 | 2 | 10uF/6v tantalum | SMC_A |
| C2, C4 | 2 | 0.1uF | C805 |
| U1 | 1 | MMA7455L | LGA-14 |
SPI demo
The MMA7455L can operate in I2C or SPI mode. This section demonstrates the SPI mode with the Bus Pirate.
Connections
| Bus Pirate | Direction | MMA7455L | Description |
|---|---|---|---|
| MOSI | → | SDA/SDI | Master Out, Slave In |
| MISO | ← | SDO | Master In, Slave Out |
| CLK | → | SCL/SPC | Clock signal |
| CS | → | CS | Chip Select |
| +3.3V | → | 3.3V | Power supply |
| GND | ↔ | GND | Ground |
Bus Pirate setup
Connect the Bus Pirate to the chip as shown in the table.
HiZ>m<<<mode command
1. HiZ
...
5. SPI
...
(1)>5<<<SPI mode
Select output type:
1. Open drain (H=Hi-Z, L=GND)
2. Normal (H=3.3V, L=GND)
(1)>2<<<normal pinout
Ready
SPI>
Configure the Bus Pirate for SPI mode. Chose all the default settings except output type. Choose normal pin outputs.
SPI>W<<<enable power supplies
Power supplies ON
SPI>
The chip is powered by the Bus Pirate, enable the power supplies (W).
Interfacing
| Command | Description |
|---|---|
| 0x0D | I2C address |
| 0x16 | Mode control |
| 0x06 | 8bit accel X |
| 0x07 | 8bit accel Y |
| 0x08 | 8bit accel Z |
We'll cover these commands. The commands are sent after taking the CS pin to ground ([), transactions end by returning CS high (]).
Check the connection
Register 0x0D contains the I2C address. This address isn't relevant to SPI mode, but we can grab it to make sure the connection to the chip is functioning.
Lower chip select ([), then setup the register to access(0x1a), read a byte (r), and raise chip select (]). The reply is 0x1D, the default I2C address according to page 23 of the datasheet.
Register access format
| 7 | 6 | 5!4 | 3 | 2 | 1 | 0 | |
|---|---|---|---|---|---|---|---|
| r/w | A5 | A4 | A3 | A2 | A1 | A0 | x |
The first byte of SPI commands (0x1a above) chooses the register to work with, and selects read or write access. The table shows how the byte is packed. The first bit selects read (0) or write (1) access, the next 6 bits are the register to access, the last bit doesn't matter.
Setup the mode
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| access mode | x | x | x | x | x | HOLD disable | |
Read accel data
- Read X/Y/Z
- Read all at once
I2C demo
Get one!
You can [get one for $00].
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Links
License
- CC-0, reference boards shouldn't be copyrighted!