Hardware Reference
In-Depth Information
only with professional-grade cabling. USB 3.0 devices are structurally identical to
earlier USB devices, and they fully implement the USB 2.0 standard. If plugged
into a USB 2.0 socket, they will operate correctly.
A typical small- to medium-sized computer system consists of a CPU chip,
chipset, memory chips, and some I/O devices, all connected by a bus. Sometimes,
all of these devices are integrated into a system-on-a-chip, like the TI OMAP4430
SoC. We have already studied memories, CPUs, and buses in some detail. Now it
is time to look at the last part of the puzzle, the I/O interfaces. It is through these
I/O ports that the computer communicates with the external world.
Numerous I/O interfaces are already available and new ones are being intro-
duced all the time. Common interfaces include UARTs, USARTs, CRT con-
trollers, disk controllers, and PIOs. A
UART
(
Universal Asynchronous Receiver
Transmitter
) is an I/O interface that can read a byte from the data bus and output
it a bit at a time on a serial line for a terminal, or input data from a terminal.
UARTs usually allow various speeds from 50 to 19,200 bps; character widths from
5 to 8 bits; 1, 1.5, or 2 stop bits; and provide even, odd, or no parity, all under pro-
gram control.
USART
s
Universal Synchronous Asynchronous Receiver
Transmitters
) can handle synchronous transmission using a variety of protocols as
well as performing all the UART functions. Since UARTs have become less
important as telephone modems are vanishing, let us now study the parallel inter-
face as an example of an I/O chip.
PIO Interfaces
A typical
PIO
(
Parallel Input/Output
) interface (based on the classic Intel
8255A PIO design) is illustrated in Fig. 3-59. It has a collection of I/O lines (e.g.,
24 I/O lines in the example in the figure) that can interface to any digital logic de-
vice interface, for example, keyboards, switches, lights, or printers. In a nutshell,
the CPU program can writea0or1toanyline, or read the input status of any line,
providing great flexibility. A small CPU-based system using a PIO interface can
control a variety of physical devices, such as a robot, toaster, or electron micro-
scope. Typically, PIO interfaces are found in embedded systems.
The PIO interface is configured with a 3-bit configuration register, which
specifies if the three independent 8-bit ports are to be used for digital signal input
(0) or output (1). Setting the appropriate value in the configuration register will
allow any combination of input and output for the three ports. Associated with
