MPR121 datasheet(49/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

MESSAGE FORMAT FOR WRITING THE MPR121

A write to the MPR121 comprises the transmission of the MPR121’s keyscan slave address with the R/W bit set to 0, followed

by at least one byte of information. The first byte of information is the command byte. The command byte determines which

register of the MPR121 is to be written by the next byte, if received. If a STOP condition is detected after the command byte is

received, the MPR121 takes no further action (Figure 53) beyond storing the command byte. Any bytes received after the

command byte are data bytes.

Figure 53. Command Byte Received

Any bytes received after the command byte are data bytes. The first data byte goes into the internal register of the MPR121

selected by the command byte (Figure 54).

Figure 54. Command and Single Data Byte Received

If multiple data bytes are transmitted before a STOP condition is detected, these bytes are generally stored in subsequent

MPR121 internal registers because the command byte address generally auto-increments.

MESSAGE FORMAT FOR READING THE MPR121

MPR121 is read using MPR121's internally stored register address as address pointer, the same way the stored register address

is used as address pointer for a write. The pointer generally auto-increments after each data byte is read using the same rules

as for a write. Thus, a read is initiated by first configuring MPR121's register address by performing a write (Figure 53) followed

by a repeated start. The master can now read 'n' consecutive bytes from MPR121, with first data byte being read from the register

addressed by the initialized register address.

Figure 55. Reading MPR121

OPERATION WITH MULTIPLE MASTER

The application should use repeated starts to address the MPR121 to avoid bus confusion between I2C masters.On a I2C bus,

once a master issues a start/repeated start condition, that master owns the bus until a stop condition occurs. If a master that does

not own the bus attempts to take control of that bus, then improper addressing may occur. An address may always be rewritten

to fix this problem. Follow I2C protocol for multiple master configurations.

SAA

SLAVE ADDRESS

COMMAND BYTE

Acknowledge from MPR121

R/W

Acknowledge from MPR121

D15

D14

D13

D12

D11

D10

Commandbyte is storedonreceipt of STOP condition

SLAVE ADDRESS

COMMAND BYTE

DATA BYTE

Acknowledge from MPR121

R/W

1byte

Auto-increment memory

word address

D15 D14 D13 D12 D11 D10

How command byte and data byte

map into MPR121's registers

Acknowledge from

MPR121

Acknowledge from

MPR121

SLAVE ADDRESS

DATA BYTE

R/W

nbytes

auto-increment memory

word address

Acknowledge from MPR121

Acknowledge from master

Acknowledge from MPR121

R/W

Acknowledge from MPR121

D15 D14 D13 D12 D11 D10 D9

Command byte is stored on receipt of STOP condition

SLAVE ADDRESS

COMMAND BYTE

Repeated Start

Stop

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