CAT5221
SERIAL BUS PROTOCOL
2
The following defines the features of the I C bus protocol:
1. Data transfer may be initiated only when the bus is
not busy.
significant bits of the 8-bit slave address are fixed as 0101
for the CAT5221 (see Figure 6). The next four significant
bits (A3, A2, A1, A0) are the device address bits and define
which device the Master is accessing. Up to sixteen devices
may be individually addressed by the system. Typically,
+5 V and ground are hard-wired to these pins to establish the
device’s address.
2. During a data transfer, the data line must remain
stable whenever the clock line is high. Any
changes in the data line while the clock is high
will be interpreted as a START or STOP condition.
After the Master sends a START condition and the slave
address byte, the CAT5221 monitors the bus and responds
with an acknowledge (on the SDA line) when its address
matches the transmitted slave address.
The device controlling the transfer is a master, typically a
processor or controller, and the device being controlled is the
slave. The master will always initiate data transfers and
provide the clock for both transmit and receive operations.
Therefore, the CAT5221 will be considered a slave device
in all applications.
Acknowledge
After a successful data transfer, each receiving device is
required to generate an acknowledge. The Acknowledging
device pulls down the SDA line during the ninth clock cycle,
signaling that it received the 8 bits of data.
The CAT5221 responds with an acknowledge after
receiving a START condition and its slave address. If the
device has been selected along with a write operation, it
responds with an acknowledge after receiving each 8-bit
byte.
When the CAT5221 is in a READ mode it transmits 8 bits
of data, releases the SDA line, and monitors the line for an
acknowledge. Once it receives this acknowledge, the
CAT5221 will continue to transmit data. If no acknowledge
is sent by the Master, the device terminates data transmission
and waits for a STOP condition.
START Condition
The START Condition precedes all commands to the
device, and is defined as a HIGH to LOW transition of SDA
when SCL is HIGH. The CAT5221 monitors the SDA and
SCL lines and will not respond until this condition is met.
STOP Condition
A LOW to HIGH transition of SDA when SCL is HIGH
determines the STOP condition. All operations must end
with a STOP condition.
Device Addressing
The bus Master begins a transmission by sending a
START condition. The Master then sends the address of the
particular slave device it is requesting. The four most
SCL FROM
MASTER
1
8
9
DATA OUTPUT
FROM TRANSMITTER
DATA OUTPUT
FROM RECEIVER
START
ACKNOWLEDGE
Figure 5. Acknowledge Timing
WRITE OPERATION
In the Write mode, the Master device sends the START
condition and the slave address information to the Slave
device. After the Slave generates an acknowledge, the
Master sends the instruction byte that defines the requested
operation of CAT5221. The instruction byte consist of a
four-bit opcode followed by two register selection bits and
two pot selection bits. After receiving another acknowledge
from the Slave, the Master device transmits the data to be
written into the selected register. The CAT5221
acknowledges once more and the Master generates the
STOP condition, at which time if a nonvolatile data register
is being selected, the device begins an internal programming
cycle to non-volatile memory. While this internal cycle is in
progress, the device will not respond to any request from the
Master device.
Acknowledge Polling
The disabling of the inputs can be used to take advantage
of the typical write cycle time. Once the stop condition is
issued to indicate the end of the host’s write operation, the
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