I
2
C Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
32-2
Freescale Semiconductor
32.1
I
2
C Features
The following is a list of the I
2
C controller’s main features:
•
Two-signal interface (SDA on PB[27] and SCL on PB[26])
•
Support for master and slave I
2
C operation
•
Multiple-master environment support
•
Continuous transfer mode for autoscanning of a peripheral
•
Supports a maximum clock rate of 520 KHz (with a CPM utilization of 25%), assuming a 25-MHz
system clock.
•
Independent, programmable baud-rate generator
•
Supports 7-bit I
2
C addressing
•
Open-drain output signals allow multiple master configuration
•
Local loopback capability for testing
32.2
I
2
C Controller Clocking and Signal Functions
The I
2
C controller can be configured as a master or slave for the serial channel. As a master, the controller’s
BRG provides the transfer clock. The I
2
C BRG takes its input from the BRG clock (BRGCLK), which is
described in
Section 14.3, “Clock Signals.”
SDA and SCL are bidirectional signals connected to a positive supply voltage through an external pull-up
resistor. When the bus is free, both signals are pulled high. The general I
2
C master/slave configuration is
shown in
Figure 32-2
.
Figure 32-2. I
2
C Master/Slave General Configuration
When the I
2
C controller is the master, the SCL clock output, taken directly from the I
2
C BRG, shifts receive
data in and transmit data out through SDA. The transmitter arbitrates for the bus during transmission and
aborts if it loses arbitration. When the I
2
C controller is a slave, the SCL clock input shifts data in and out
through SDA. The SCL frequency can range from DC to BRGCLK/48.
32.3
I
2
C Controller Transfers
To initiate a transfer, the master I
2
C controller sends a message specifying a read or write request to an I
2
C
slave. The first byte of the message consists of a 7-bit slave port address and a R/W request bit. Note that
V
DD
V
DD
Master
Slave
(EEPROM, for example)
SCL
SCL
SDA
SDA
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