Rainbow Electronics T48C862-R4 - Manual (Page 91)

91
T48C862-R4
4551B–4BMCU–02/03
Serial Interface The EEPROM uses a two-wire serial interface (TWI) to the microcontroller for read and
write accesses to the data. It is considered to be a slave in all these applications. That
means, the controller has to be the master that initiates the data transfer and provides
the clock for transmit and receive operations.
The serial interface is controlled by the microcontroller which generates the serial clock
and controls the access via the SCL-line and SDA-line. SCL is used to clock the data
into and out of the device. SDA is a bi-directional line that is used to transfer data into
and out of the device. The following protocol is used for the data transfers.
Serial Protocol • Data states on the SDA-line changing only while SCL is low.
• Changes on the SDA-line while SCL is high are interpreted as START or STOP
condition.
• A START condition is defined as high to low transition on the SDA-line while the
SCL-line is high.
• A STOP condition is defined as low to high transition on the SDA-line while the SCL-
line is high.
• Each data transfer must be initialized with a START condition and terminated with a
STOP condition. The START condition wakes the device from standby mode and the
STOP condition returns the device to standby mode.
• A receiving device generates an acknowledge (A) after the reception of each byte.
This requires an additional clock pulse, generated by the master. If the reception
was successful the receiving master or slave device pulls down the SDA-line during
that clock cycle. If an acknowledge is not detected (N) by the interface in transmit
mode, it will terminate further data transmissions and go into receive mode. A
master device must finish its read operation by a non-acknowledge and then send a
stop condition to bring the device into a known state.
Figure 92. MCL Protocol
• Before the START condition and after the STOP condition the device is in standby
mode and the SDA line is switched as input with pull-up resistor.
• The control byte that follows the START condition determines the following
operation. It consists of the 5-bit row address, 2 mode control bits and the
READ/NWRITE bit that is used to control the direction of the following transfer. A "0"
defines a write access and a "1" a read access.
Start
condition
Data
valid
Data
change
Data/
acknowledge
valid
Stop
condition
SCL
SDA
Stand
by
Stand-
by