MPR121 datasheet(47/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

AN3895: MPR121 Serial Communication

INTRODUCTION

The MPR121 uses an I2C Serial Interface. The I2C protocol implementation and the specifics of communicating with the Touch

Sensor Controller are detailed in this application note.

SERIAL-ADDRESSING

The MPR121 operates as a slave that sends and receives data through an I2C 2-wire interface. The interface uses a Serial

Data Line (SDA) and a Serial Clock Line (SCL) to achieve bi-directional communication between master(s) and slave(s). A

master (typically a microcontroller) initiates all data transfers to and from the MPR121, and it generates the SCL clock that

synchronizes the data transfer.

The MPR121 SDA line operates as both an input and an open-drain output. A pull-up resistor, typically 4.7 k

Ω, is required on

SDA. The MPR121 SCL line operates only as an input. A pull-up resistor, typically 4.7 k

Ω, is required on SCL if there are multiple

masters on the 2-wire interface, or if the master in a single-master system has an open-drain SCL output.

Each transmission consists of a START condition (Figure 48) sent by a master, followed by the MPR121’s 7-bit slave address

plus R/W bit, a register address byte, one or more data bytes, and finally a STOP condition.

Figure 48. Wire Serial Interface Timing Details

START AND STOP 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. The bus is then free for another

transmission.

Figure 49. Start and Stop Conditions

SCL

SDA

tLOW

tHIGH

tHD STA

tHD DAT

tHD STA

tSU DAT

tSU STA

tBUF

tSU STO

ST ART

CONDIT ION

ST OP

CONDIT ION

REPEAT ED ST ART

CONDIT ION

ST ART

CONDIT ION

DATA LINE STABLE

DATA VALID

CHANGE OF

DATA ALLOWED

SDA

SCL

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