Nuts & Volts

loads there are. Each contributes some capacitance
on the line that must be charged so the lower the
pull-up value, the faster the rise time and the greater
the data rate. And don’t forget that the longer the
bus lines, PC board copper, or cable, the greater the
capacitance to charge, so lower pull-ups will give
better performance. While the I 2 C system can accommodate
up to 128 parallel devices on the line, rarely
is even a fraction of this capacity used.
While most I 2 C systems run at low speed from
10 kbps to 100 kbps, there are faster versions
available to run at 400 kbps up to 3.4 Mbps. These
are typically used over shorter distances.
As Figure 1 shows, the I 2 C bus devices are usually
designated as either a master or slave. The master provides
overall control of transmissions and supplies the clock to
the slave devices. Only a master can initiate a data transfer.
Slaves simply respond to read or write commands from the
master. The master can be an IC designed for the purpose
or, in most cases, it is actually built into an embedded controller.
Many single chip microcontrollers include an I 2 C bus
as a standard I/O port. The slave devices are usually other
ICs designed to be set up and controlled over this bus.
The I 2 C bus has a transmission protocol like all other
networking methods. It is a synchronous bus meaning that
the bit transfer is controlled by the clock. But it is also asynchronous
since it can start and stop at any time as initiated
by the master. It uses start and stop bits somewhat like those
used in standard RS-232 UART serial port communications.
The SDA line stays high until a one bit long binary 0
(low) bit occurs. The SCL line goes low shortly thereafter,
and the clock causes the serial data to be transferred. A
stop sequence occurs when the SCL line goes high and
stays high for a one bit period, then the SDA line rises back
the the binary 1 or high condition.
Data is designed to be transmitted in eight-bit bytes.
These are accompanied by some control bits that determine
what happens. Figure 2 shows the protocol. The data
bit D7 is the MSB and is transmitted first. A ninth bit designated
ACK follows the data bits. The ACK bit is transmitted
by the receiving slave device to acknowledge the receipt of
the data and this indicates it is ready for another transfer.
The first transmission is usually the address that identifies
the slave to be involved in a read or write operation. The
format is shown in Figure 3. The address is seven bits long,
giving a maximum of 2 7 = 128 possible slave devices. Address
bit A6 is the MSB and is transmitted first. The I 2 C system
includes a 10-bit addressing option but rarely is it ever
used. The eighth bit transmitted is a read or write (R/W)
bit that tells if the master is requesting the slave to be
read from (binary 1) or written to (binary 0). The ninth
bit is the ACK bit as described before.
After the address is transmitted, the protocol usually
calls for sending a code identifying a register in the
slave that is to be read or written to. Some slave ICs
have multiple registers that are either monitored by the
master or written to. After the register number is sent,
the data transfer is initiated. Then the sequence ends.
■ FIGURE 2
■ FIGURE 3
Most I 2 C procedures are programmed as part of
the embedded controller code in the product. Micros
incorporating this bus have instructions and software to
handle the I 2 C functions.
I 2 C APPS
As you can imagine, with such a simple and flexible
protocol you can control and monitor almost anything.
Changing channels on a TV set via the controller accepting
inputs from a remote control is a classic example. You can
use the bus to also control volume, color hue and balance,
contrast, and other video settings. You can turn LEDs off or
on or feed an LCD display with alphanumerics. You can
also read from and write to RAM, ROM, or Flash memory.
You can use it to feed data to serial analog-to-digital
converters (ADCs) and digital-to-analog converters (DACs).
Some frequency synthesizers in cell phones and radios get
their frequency settings via an I 2 C bus. With an embedded
controller and I 2 C circuits, you can implement almost any
special monitor or control function you want.
I 2 C DERIVATIVES
OPEN COMMUNICATION
The I 2 C bus has proved to be so versatile and successful
that it has spawned a number of offshoots and enhancements.
The most notable are the ACCESS bus, SMBus, and PMBus.
The ACCESS bus uses the I 2 C basic format and protocol, but
goes further in adding a +5 volt line. This creates a four-wire
bus where the +5 volt line can be used to power external
devices such as displays, keyboards, and other peripherals.
The speed is set to 100 kbps and a maximum of 400 pF
is designated for the bus capacitance. The ACCESS
standard also specifies a type of cable up to 10 meters long
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