MMA7450L データシートの表示（PDF） - Freescale Semiconductor
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MMA7450L Datasheet PDF : 22 Pages
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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
register. The Master does not acknowledge (NACK) it
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
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 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.
MMA7450L
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
register address commands after a write command is
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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