As it is important to understand variable types in AVR-GCC lest take a deeper look at this.
Choosing proper variable type sometimes may be crucial in designing embedded software as microcontroller resources are limited. Choice of proper variable type may save lots of time and space of your program.
What is common variable type in 8 bit system? Of course byte or so called char type. Char name comes from word “character” because char type usually is used to store character symbols (ASCII). Char type can have values from -128 to +127 while unsigned char may have values from 0 to 256. Some compilers may use byte type instead of unsigned char.
All AVR ports have Read-modify-write functionality when used as genera I/O ports. Direction of separate port pin can be changed. Each pin buffer has symmetric capability to drive and sink source. Pin driver is strong enough to drive LED directly , but it is not recommended. All port pins have selectable pull-up resistors. All pins have protection diodes to both VCC and GND.
Each port consists of three registers DDRx, PORTx and PINx (where x means port letter). DDRx register selects direction of port pins. If logic one is written to DDRx then port is configured to be as output. Zero means that port is configured as input. If DDRx is written zero and PORTx is written logic “1” then port is configured as input with internal pull-up resistor. Otherwise if PORTx is written to zero, then port is configured as input but pins are set to tri-state and you might need to connect external pull-up resistors.
If PORTx is written to logic “1” and DDRx is set to “1”, then port pin is driven high. And if PORTx=0, then port is driven low.
Sometimes you might like to manipulate the members of structures in more generalized way. This can be done by using pointer reference to structure. Sometimes it is easier to pass pointer to structure than passing entire structure to function.
