Sensors

10

Freescale Semiconductor

MMA7455L

DIGITAL INTERFACE

available for a communication interface. When CS pin is used

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

communication to avoid communication

errors between devices using a different SPI

using SPI.

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

MMA7455L 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 MMA7455L 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

transmits a start condition (ST) to the MMA7455L, slave

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

MMA7455L sends an acknowledgement. Then the Master (or

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

the MMA7455L sends an acknowledgement. The Master (or

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

addresses the MMA7455L ($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

register. The Master does not acknowledge (NACK) it

received the transmitted data, but transmits a stop condition

to end the data transfer.

MULTIPLE BYTES READ

The MMA7455L automatically increments the received

register address commands after a read command is

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

read, multiple bytes of data can be read from sequential

registers after each MMA7455L 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.

SINGLE BYTE WRITE

To start a write command, the Master transmits a start

condition (ST) to the MMA7455L, slave address ($1D) with

the R/W bit set to “0” for a write, the MMA7455L sends an

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

bit address of the register to write to, and the MMA7455L

sends an acknowledgement. Then the Master (or MCU)

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

the MMA7455L 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 MMA7455L is now stored in the

appropriate register. See Figure 9.

Figure 7. Single Byte Read - The Master is reading one address from the MMA7455L

Figure 8. Multiple Bytes Read - The Master is reading multiple sequential registers from the MMA7455L

Figure 9. Single Byte Write - The Master (MCU) is writing to a single register of the MMA7455L