|
|
Serial is an umbrella
word for all that is "Time Division Multiplexed", to use an
expensive term. It means that the data is sent spread over time, most often
one single bit after another. All the protocols you're naming are serial
protocols.
UART, for Universal Asynchronous
Receiver Transmitter, is one of the most used serial protocols. It's almost
as old as I am, and very simple. Most controllers have a hardware UART on
board. It uses a single data line for transmitting and one for receiving
data. Most often 8-bit data is transferred, as follows: 1 start bit(low
level), 8 data bits and 1 stop bit(high level). The low level start bit and
high level stop bit mean that there's always a high to low transition to
start the communication. That's what describes UART. No voltage level, so you
can have it at 3.3 V or 5 V, whichever your microcontroller uses. Note that
the microcontrollers which want to communicate via UART have to agree on the
transmission speed, the bit-rate, as they only have the start bit's falling
edge to synchronize. That's called asynchronous communication.
For
long distance communication (That doesn't have to be hundreds of meters) the
5 V UART is not very reliable, that's why it's converted to a higher voltage,
typically +12 V for a "0" and -12 V for a "1". The data
format remains the same.
RS-232 (which you
actually should call EIA-232, but nobody does.)
The
timing dependency is one of the big drawbacks of UART, and the solution is USART,
for Universal Synchronous/Asynchronous Receiver Transmitter. This can do
UART, but also a synchronous protocol. In synchronous there's not only data,
but also a clock transmitted. With each bit a clock pulse tells the receiver
it should latch that bit. Synchronous protocols either need a higher bandwidth,
like in the case of Manchester encoding, or an extra wire for the clock, like
SPI and I2C.
SPI (Serial
Peripheral Interface) is another very simple serial protocol. A master sends
a clock signal, and upon each clock pulse it shifts one bit out to the slave,
and one bit in, coming from the slave. Signal names are therefore SCK for
clock, MOSI for Master Out Slave In, and MISO for Master In Slave Out. By
using SS (Slave Select) signals the master can control more than 1 slave on
the bus. There are two ways to connect multiple slave devices to one master,
one is mentioned above i.e. using slave select, and other is daisy chaining,
it uses less hardware pins(select lines), but software gets complicated.
I2C (Inter-Integrated
Circuit, pronounced "I squared C") is also a synchronous protocol,
and it's the first we see which has some "intelligence" in it; the
other ones dumbly shifted bits in and out, and that was that. I2C uses only 2
wires, one for the clock (SCL) and one for the data (SDA). That means that
master and slave send data over the same wire, again controlled by the master
who creates the clock signal. I2C doesn't use separate Slave Selects to
select a particular device, but has addressing. The first byte sent by the
master holds a 7 bit address (so that you can use 127 devices on the bus) and
a read/write bit, indicating whether the next byte(s) will also come from the
master of should come from the slave. After each byte receiver must send a
"0" to acknowledge the reception of the byte, which the master
latches with a 9th clock pulse. If the master wants to write a byte the same
process repeats: the master puts bit after bit on the bus and each time gives
a clock pulse to signal that the data is ready to be read. If the master
wants to receive data it only generates the clock pulses. The slave has to
take care that the next bit is ready when the clock pulse is given. This
protocol is patented by NXP(formerly Phillips), to save licensing cost, Atmel
using the word TWI(2-wire interface) which exactly same as I2C, so any AVR
device will not have I2C but it will have TWI.
Two or more signals on
the same wire may cause conflicts, and you would have a problem if one device
sends a "1" while the other sends a "0". Therefore the
bus is wired-OR'd: two resistors pull the bus to a high level, and the
devices only send low levels. If they want to send a high level they simply
release the bus.
TTL (Transistor
Transistor Logic) is not a protocol. It's an older technology for digital
logic, but the name is often used to refer to the 5 V supply voltage, often
incorrectly referring to what should be called UART.
mean Transistor-Transistor-Logic and has its level
for logical zero near 0V and for logical one near 5V. Often any 5V logic is
called TTL, although most circuits nowadays are built as CMOS. Today there
are also many circuits that work at 3.3V, which is no longer TTL.
With respect to the internal levels the levels on the RS-232
line are inverted, +12V corresponds to logical low and -12V corresponds to
logical high, which can be confusing.
For describing the data format one usually shows the logical
signal. When the line is idle it is high. A transmission starts with a low
start bit, the data bits, an optional parity bit and one to two stop bits
(logical 1). This is called asynchronous transmission, because the start bit
synchronizes the data for each byte separately.
|
