The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Unassigned space remains in the memory map, but there’s nothing wrong with this since it
leaves room for additions. Also, this isn’t the only way to configure an 8052-BASIC system.
For example, if you use a single 8K EPROM at 8000h, you can use the area from A000h to
BFFFh for additional I/O.
Not shown are two areas that BASIC-52 reserves for optional enhancements. If you
customize BASIC-52 by adding your own instructions, commands, or reset routines,
BASIC-52 expects to find parameters relating to these in code memory from 2001h to 2090h.
And, if you want to call assembly-language interrupt routines, BASIC-52 expects to find
vectors for these in code memory from 4003h to 41FFh. For these, you can use EPROM,
EEPROM, or NV RAM, as Chapter 13 shows.
Uses for I/O Ports
Inputs and Outputs
Just about all microcontroller circuits need to be able to do more than just read and write to
memory. Other uses involve sensing and controlling of conditions, events, or devices
external to the basic circuits. For example, a microcontroller-based drilling machine for
printed-circuit boards might have these responsibilities:
Detect when a user presses a switch.
Move the pc board so that the hole to be drilled lies under the drill bit.
Set the speed of rotation for the drill bit.
Lower the bit into the board, then raise it after drilling.
Detect problems, such as a bit that doesn’t lower or a drilling obstruction.
Display messages to prompt the user for input or show progress.
These functions all involve reading and writing, but instead of reading and writing to
memory, the microcontroller reads sensors and switches, and writes to motors and displays.
Reading and writing to devices other than memory is often called input/output, or I/O for
short.
Adding Ports
The 8052-BASIC has a few pins on Port 1 that you can use for I/O. Chapter 3 included
programs for reading and writing to these. But many projects will require more I/O than
these few pins can offer.
Figure 6-2 shows a circuit that allows you to add up to eight 8-bit ports to the main circuit.
You can design the ports as inputs or outputs, in any combination.
The Microcontroller Idea Book 89