THE CONCEPT OF PORTS

A port can be thought of as a data channel that comes into or goes out of the uC. There can be parallel ports (all data bits present on their own lines) and serial ports (data bits passed serially along one or more lines). A number of types of serial ports will be discussed in later chapters of this tutorial, but for now we'll just talk about GPIO ports. A GPIO port is a set of GPIO pins which are organized into a single N-bit (8/16/32 bit) word, and are thus written out and read in as operations on a single word. A uC can have one or more GPIO ports, and given port may be a full N-bits wide, or a given port may be fewer than N-bits. A shortened port (fewer than N-bits wide) usually exists because the chip designers simply ran out of pins for the extra GPIO bits. For example, a 28-pin, 8-bit microcontroller may dedicate 5 pins to power and GND, and all other pins are available as GPIO. That means 23 GPIO pins, which could be organized as two 8-bit ports and one 7-bit port.

A shortened port will still appear as a full N-bit value to the CPU, it is just that the bit positions that are not brought out to pins are unused, since they can never transmit or receive valid data. and must be ignored. Depending on the CPU design these unused bits may always read as 0s or they may read as undetermined values. The datasheet will give the details, and in any case, the unused bits should be masked out before using the port data.

You will also see this phenomenon of missing or unused bits in configuration registers. It is very common for a configuration register to have unused bits - sometimes a configuration register will only use a single bit in the register! Again, these unused bits may read as 0s or they may be indeterminate. When writing to a configuration register that has unused bits, the datasheet will tell you what values to write to those bits. Usually you are required to write 0s to the unused bits. One reason for this is that unused bit positions may become used in a future version of the device, and writing a 1 to a configuration bit is more likely to enable some feature - some feature you didn't intend, since the bit and the feature didn't even exist when you wrote the software. Just be safe and write the unused bits of any configuration register according to the datasheet. To sum up, mask out unused bits when reading ports or configuration registers, and set unused bits as per the datasheet when writing to ports or configuration registers.

Each GPIO port will have a number of data and configuration registers associated with it. These registers will access data read as inputs, hold data written as outputs, and will allow configuration settings (often for each individual GPIO pin) such as (depending on the device) data direction (input or output), built-in pullup/pulldowns, input hysteresis, output drive strength, and so on. All GPIO register information will be found in the datasheet.