MPR081

Sensors

8

Freescale Semiconductor

ACKNOWLEDGE

The acknowledge bit is a clocked 9th bit (Figure 7) which

the recipient uses to handshake receipt of each byte of data.

Thus each byte transferred effectively requires 9 bits. The

down SDA during the acknowledge clock pulse, such that the

SDA line is stable low during the high period of the clock

pulse. When the master is transmitting to the MPR081, the

MPR081 generates the acknowledge bit because the

MPR081 is the recipient. When the MPR081 is transmitting to

the master, the master generates the acknowledge bit

because the master is the recipient.

Figure 7. Acknowledge

THE SLAVE ADDRESS

The MPR081 has a 7-bit long slave address (Figure 5).

The bit following the 7-bit slave address (bit eight) is the R/W

bit, which is low for a write command and high for a read

command. The MPR081 has a factory set I²C slave address

which is normally 1001100 (0x4C). Contact the factory to

range 0001000 to 1110111 (0x08 to 0xEF).

Figure 8. Slave Address

The MPR081 monitors the bus continuously, waiting for a

START condition followed by its slave address. When a

MPR081 recognizes its slave address, it acknowledges and

is then ready for continued communication.

MESSAGE FORMAT FOR WRITING THE MPR081

A write to the MPR081 comprises the transmission of the

MPR081'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 MPR081 is to be written by

the next byte, if received. If a STOP condition is detected after

the command byte is received, then the MPR081 takes no

further action (Figure 9) beyond storing the command byte.

Any bytes received after the command byte are data bytes.

Figure 9. Command Byte Received