A PICmicro chip contains an area of Programmable Read Only Memory (PROM). This is non-volatile in that it holds its contents when the PICmicro is not powered on. When the PICmicro is switched on it executes the program in the PROM.Another part of the PICmicro chip holds Random Access Memory (RAM) which is where the variables in our programs are held. The PICmicro RAM is more properly called Static RAM (SRAM) in that the values are held statically, with no need for a dynamic refresh circuit.

Some PICmicros are "One Time Programmable" OTP in that the program they run is stored into the PROM once at manufacture. If you need to change the program, you have to replace the PICmicro. Other PICmicros are erasable, in that the program in them can be wiped out and replaced with a new one.

There are two ways that this can be done. Some PICmicros have a window in the top of the package. This allows you to erase the program on the PICmicro by shining ultra-violet light on the chip. This is called Erasable Programmable Read Only Memory (EPROM).

Other PICmicros are electrically erasable. These are easier to use, in that the contents are wiped by sending signals into the chip. While it can take up to 20 minutes to erase a chip with ultra-violet light the Electrical Erasable Programmable Read Only Memory (EEPROM - or E squared PROM) PICmicros can be wiped in a few seconds. The PICmicro we are going to use is electrically erasable.

The PICmicro is actually programmed by sending it a programming signal. This is done using a "PICmicro Programmer". The PICmicro development board will function as a PICmicro programmer as well as allowing you to run programs on the PICmicro. Sometimes you program your PICmicro "in circuit" in that the program is loaded into the PICmicro after it is fitted into the product. Other times you program the PICmicro and then insert it into the target device. We will use a program to take the raw PICmicro instructions which come out of the assembler and program the PICmicro in the PICmicro development board.