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Slave Port Interface
8.5
8.5.1
I 2 C serial protocol and timing
Baud rates
I 2 C supports several ranges of operation. From the perspective of a slave device such as this module, the
protocol is the same, regardless of speed. Differences in the I 2 C spec arise in terms of noise suppression,
bit times and electrical drivers, but the logical behavior of the slave is consistent across the modes.
The MMA955xL platform utilizes an internal, 8-MHz CPU and peripheral clock, yielding an internal
clock rate of 125 ns. At this speed, a minimum (60 ns) SCL, high signal cannot be reliably sampled in
high-speed mode. Therefore, the communication rate for this module is limited to 2 MHz or less.
The MMA955xL data sheet summarizes timing options and requirements for the slave I 2 C module.
8.5.2
Serial-addressing
The MMA955xL device operates as a slave that sends and receives data through an I 2 C, two-wire
interface. The interface uses a serial data line (SDA) and a serial clock line (SCL) to achieve I-directional
communication between master(s) and slave(s). A master (typically a microcontroller) initiates all data
transfers to and from the MMA955xL platform and generates the SCL clock that synchronizes the data
transfer.
The SDA line operates as both an input and an open-drain output. A pull-up resistor, typically 4.7 k ? , is
required on SDA. The SCL line operates only as an input. A pull-up resistor, typically 4.7 k ? , is required
on SCL if there are multiple masters on the two-wire interface or if the master in a single-master system
has an open-drain SCL output.
8.5.3
Start, Stop and Repeated Start conditions
Each transmission consists of a START condition ( Figure 8-21 ) sent by a master, followed by the device
seven-bit slave address and a read/write bit, a register address byte, one or more data bytes and, finally, a
STOP or REPEATED START bit.
SDA
SCL
S
Start Condition
P
Stop Condition
Figure 8-21. Start (S) and Stop (P) conditions
Both SCL and SDA remain high when the interface is not busy. A master signals the beginning of a
transmission with a START (S) condition by transitioning SDA from high to low while SCL is high. When
the master has finished communicating with the slave, it issues a STOP (P) condition by transitioning SDA
from low to high while SCL is high.
MMA955xL Intelligent, Motion-Sensing Platform Hardware Reference Manual, Rev. 1.0
Freescale Semiconductor, Inc.
141
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