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3 8250 UART Programming
3.1 Introduction
Finally we are moving away from wires and voltages and hard-core electrical engineering
applications, although we still need to know quite a bit regarding computer chip architectures
at this level. While the primary focus of this section will concentrate on the 8250 UART,
there are really three computer chips that we will be working with here:
• 8250 UART
• 8259 PIC (Programmable Interrupt Controller)
• 8086 CPU (Central Processing Unit)
Keep in mind that these are chip families, not simply the chip part number itself. Computer
designs have evolved quite a bit over the years, and often all three chips are put onto the
same piece of silicon because they are tied together so much, and to reduce overall costs of
the equipment. So when I say 8086, I also mean the successor chips including the 80286,
80386, Pentium, and compatible chips made by manufacturers other than Intel. There are
some subtle differences and things you need to worry about for serial data communication
between the different chips other than the 8086, but in many cases you could in theory write
software for the original IBM PC doing serial communication and it should run just fine on
a modern computer you just bought that is running the latest version of Linux or Windows
XP.
Modern operating systems handle most of the details that we will be covering here through
low-level drivers, so this should be more of a quick understanding for how this works rather
than something you might implement yourself, unless you are writing your own operating
system. For people who are designing small embedded computer devices, it does become
quite a bit more important to understand the 8250 at this level.
Just like the 8086, the 8250 has evolved quite a bit as well, e.g. into the 16550 UART.
Further down I will go into how to detect many of the different UART chips on PCs, and
some quirks or changes that affect each one. The differences really aren't as significant as
the changes to CPU architecture, and the primary reason for updating the UART chip was
to make it work with the considerably faster CPUs that are around right now. The 8250
itself simply can't keep up with a Pentium chip.
Remember as well that this is trying to build a foundation for serial programming on the
software side. While this can be useful for hardware design as well, quite a bit will be
missing from the descriptions here to implement a full system.
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