MMA7450LR2 datasheet(8/22 Pages) FREESCALE

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Sensors

Freescale Semiconductor

MMA7450L

DIGITAL INTERFACE

The MMA7450L has both an I2C and SPI digital output

available for a communication interface. When CS pin is used

for Slave Select, SPI communication is selected. When CS is

high, I2C communication is selected and SPI is disabled.

NOTE: It is recommended to disable I2C during SPI

communication to avoid communication

errors between devices using a different SPI

communication protocol. To disable I2C, set

the I2CDIS bit in I2C Device Address register

using SPI.

I2C SLAVE INTERFACE

I2C is a synchronous serial communication between a

master device and one or more slave devices. The master is

typically a microcontroller, which provides the serial clock

signal and addresses the slave device(s) on the bus. The

MMA7450L communicates only in slave operation where the

device address is $1D. Multiple read and write modes are

available. The protocol supports slave only operation. It does

not support Hs mode, “10-bit addressing”, “general call” and

:”START byte”.

SINGLE BYTE READ

The MMA7450L has an 8-bit ADC that can sample,

convert and return sensor data on request. The transmission

of an 8-bit command begins on the falling edge of SCL. After

the eight clock cycles are used to send the command, note

that the data returned is sent with the MSB first once the data

is received. Figure 7 shows the timing diagram for the

accelerometer 8-bit I2C read operation. The Master (or MCU)

transmits a start condition (S) to the MMA7450L, slave

address ($1D), with the R/W bit set to “0” for a write, and the

MMA7450L sends an acknowledgement. Then the Master (or

MCU) transmits the 8-bit address of the register to read and

the MMA7450L sends an acknowledgement. The Master (or

MCU) transmits a repeated start condition (SR) and then

addresses the MMA7450L ($1D) with the R/W bit set to “1” for

a read from the previously selected register. The Slave then

acknowledges and transmits the data from the requested

received the transmitted data, but transmits a stop condition

to end the data transfer.

Figure 7. Single Byte Read - The Master is reading one

address from the MMA7450L

MULTIPLE BYTES READ

The MMA7450L automatically increments the received

received. Therefore, after following the steps of a single byte

read, multiple bytes of data can be read from sequential

registers after each MMA7450L acknowledgment (ACK) is

received until a NACK is received from the Master followed

by a stop condition (SP) signalling an end of transmission.

See Figure 8.

Figure 8. Multiple Bytes Read - The Master is reading

multiple sequential registers from the MMA7450L

SINGLE BYTE WRITE

To start a write command, the Master transmits a start

condition (ST) to the MMA7450L, slave address ($1D) with

the R/W bit set to “0” for a write, the MMA7450L sends an

acknowledgement. Then the Master (MCU) transmits the 8-

bit address of the register to write to, and the MMA7450L

sends an acknowledgement. Then the Master (or MCU)

transmits the 8-bit data to write to the designated register and

the MMA7450L sends an acknowledgement that it has

received the data. Since this transmission is complete, the

Master transmits a stop condition (SP) to the data transfer.

The data sent to the MMA7450L is now stored in the

appropriate register. See Figure 9.

Figure 9. Single Byte Write - The Master (MCU) is writing

to a single register of the MMA7450L

MULTIPLE BYTES WRITE

The MMA7450L automatically increments the received

received. Therefore, after following the steps of a single byte

write, multiple bytes of data can be written to sequential

registers after each MMA7450L acknowledgment (ACK) is

received. See Figure 10.

Figure 10. Multiple Byte Writes - The Master (MCU) is

writing to multiple sequential registers of the MMA7450L

SPI SLAVE INTERFACE

The MMA7450L also uses serial peripheral interface

communication as a digital communication. The SPI

communication is primarily used for synchronous serial

communication between a master device and one or more

slave devices. See Figure 16 for an example of how to

configure one master with two MMA7450L devices. The

MMA7450L is always operated as a slave device. Typically,

the master device would be a microcontroller which would

drive the clock (SPC) and chip select (CS) signals.

The SPI interface consists of two control lines and two data

lines: CS, SPC, SDI, and SDO. The CS, also known as Chip

Select, is the slave device enable which is controlled by the

SPI master. CS is driven low at the start of a transmission. CS

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