Max7315 – Rainbow Electronics MAX7315 User Manual

Page 8

MAX7315

only as an input. A pullup resistor, typically 4.7k

Ω, is

required on SCL if there are multiple masters on the 2-
wire interface, or if the master in a single-master system
has an open-drain SCL output.

Each transmission consists of a START condition
(Figure 3) sent by a master, followed by the MAX7315
7-bit slave address plus R/W bit, a register address
byte, one or more data bytes, and finally a STOP condi-
tion (Figure 3).

Start and Stop Conditions

Both SCL and SDA remain high when the interface is
not busy. A master signals the beginning of a transmis-
sion with a START (S) condition by transitioning SDA

from high to low while SCL is high. When the master
has finished communicating with the slave, it issues a
STOP (P) condition by transitioning SDA from low to
high while SCL is high. The bus is then free for another
transmission (Figure 3).

Bit Transfer

One data bit is transferred during each clock pulse.
The data on SDA must remain stable while SCL is high
(Figure 4).

Acknowledge

The acknowledge bit is a clocked 9th bit that the recipi-
ent uses to handshake receipt of each byte of data
(Figure 5). Thus, each byte transferred effectively
requires 9 bits. The master generates the 9th clock
pulse, and the recipient pulls down SDA during the
acknowledge clock pulse so the SDA line is stable low
during the high period of the clock pulse. When the
master is transmitting to the MAX7315, the device gen-
erates the acknowledge bit because the MAX7315 is
the recipient. When the MAX7315 is transmitting to the
master, the master generates the acknowledge bit
because the master is the recipient.

Slave Address

The MAX7315 has a 7-bit long slave address (Figure 6).
The eighth bit following the 7-bit slave address is the
R/W bit. The R/W bit is low for a write command, high
for a read command.

The slave address bits A6 through A0 are selected by
the address inputs AD0, AD1, and AD2. These pins can
be connected to GND, V+, SDA, or SCL. The MAX7315
has 64 possible slave addresses (Table 1) and, there-
fore, a maximum of 64 MAX7315 devices can be con-
trolled independently from the same interface.

Message Format for Writing the MAX7315

A write to the MAX7315 comprises the transmission of
the MAX7315’s slave address with the R/W bit set to
zero, followed by at least 1 byte of information. The first
byte of information is the command byte. The command
byte determines which register of the MAX7315 is to be
written to by the next byte, if received (Table 2). If a
STOP condition is detected after the command byte is
received, then the MAX7315 takes no further action
beyond storing the command byte.

8-Port I/O Expander with LED Intensity
Control and Interrupt

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Figure 3. Start and Stop Conditions

SDA

SCL

START

CONDITION

STOP

CONDITION

Figure 4. Bit Transfer

SDA

SCL

DATA LINE STABLE;

DATA VALID

CHANGE OF DATA

ALLOWED

Figure 5. Acknowledge

SCL

SDA BY

TRANSMITTER

CLOCK PULSE

FOR ACKNOWLEDGE

START

CONDITION

SDA BY

RECEIVER

Figure 6. Slave Address

SDA

SCL

MSB

LSB

ACK

R/W