CAT5259 Datasheet, PDF (6/16 Pages) Catalyst Semiconductor – Quad Digitally Programmable Potentiometers (DPP) with 256 Taps and 2-wire Interface

CAT5259
SERIAL BUS PROTOCOL
The following defines the features of the 2-wire bus
protocol:
(1) Data transfer may be initiated only when the bus
is not busy.
(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.
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
CAT5259 will be considered a slave device in all
applications.
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 CAT5259 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
significant bits of the 8-bit slave address are fixed as
0101 for the CAT5259 (see Figure 5). 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, +5V and ground are hard-wired to
these pins to establish the device's address.
After the Master sends a START condition and the slave
address byte, the CAT5259 monitors the bus and
responds with an acknowledge (on the SDA line) when
its address matches the transmitted slave address.
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 CAT5259 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 CAT5259 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
CAT5259 will continue to transmit data. If no
acknowledge is sent by the Master, the device terminates
data transmission and waits for a STOP condition.
WRITE OPERATIONS
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 CAT5259. 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 CAT5259 acknowledges once more and the Master
generates the STOP condition, at which time if a non-
volatile 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 CAT5259 initiates the internal write cycle. ACK
polling can be initiated immediately. This involves issuing
the start condition followed by the slave address. If the
CAT5259 is still busy with the write operation, no ACK
will be returned. If the CAT5259 has completed the write
operation, an ACK will be returned and the host can then
proceed with the next instruction operation.
WRITE PROTECTION
The Write Protection feature allows the user to protect
against inadvertent programming of the non-volatile
data registers. If the WP pin is tied to LOW, the data
registers are protected and become read only. Similarly,
the WP pin is going low after start will interrupt non-
volatile write to data registers, while WP pin going low
after an internal write cycle has started will have no effect
on any write operation. The CAT5259 will accept both
slave addresses and instructions, but the data registers
are protected from programming by the device’s failure
to send an acknowledge after data is received.
Document No. 2000, Rev. F
6