SECTION II-3: PIC micro-controller Program ROM, with UV-EPROM and Flash

SECTION  II-3: PIC micro-controller Program ROM, with UV-EPROM and Flash

PIC Micro-controller Program ROM:

In micro-controllers, the ROM is used to store programs. It is called program or code ROM. The PIC18 has 2MB of program code ROM space. Not all family members come with that much ROM installed. The program ROM size can vary from 4K to 128K at the time of this writing, depending on the family member. The PIC18 program ROM is available in different memory types, such as flash, One time programmable OTP, and masked, all of which have different part numbers. Different flavors of the PIC18 exist in terms of speed and amount of on-chip RAM/ROM, they are all compatible with each other as far as the instructions are concerned. This means that if you write your program for one, it will run on any of them regardless of the chip number.

PIC Micro-controller with UV-EPROM:

Some of the PIC micro-controllers use UV-EPROM, for on-chip program ROM. These kinds of chips for development requires access to a PROM burner, as well as a UV-EPROM eraser ro erase the contents of ROM. The problem with the UV-EPROM is that it takes around 20 minutes to erase the chip before it can be programmed again. This has led Microchip to introduce a flash version of the PIC family. At this time, flash is replacing the UV-EPROM altogether.

Some Members of the PIC18 Family

members-of-pic-family.jpg

PIC18Fxxx with Flash:

Many PIC18 chips have on-chip program ROM in the form of flash memory. The flash version used the letter F in the part number to indicate that the on -chip ROM is flash. PIC18F458 is an example of PIC18 with flash ROM. The flash version is ideal for fast development because flash memory can be erased in seconds compared to 20 minutes or more needed for the UV-EPROM version. The PIC 18F has been used in place of the UV-EPROM to eliminate the waiting time needed to erase the chip, thereby speeding up the development time. To use the PIC18F to develop a micro-controller based system requires a ROM burner that supports flash memory. A ROM eraser is not needed because flash is an EEPROM, electrically erasable PROM. In flash memory, you must erase the entire contents of ROM programmer itself, and so a separate eraser is not needed. You can also program the PIC18F via the PICkit 2 from MicroChip using the USB port of an IBM PC.

One Time Programmable OTP Version of PIC:

OTP versions of the PIC are also available from Microchip. PIC16C432 chip uses OTP for program ROM. Contrast the PIC16C432 and PIC18F252. The letter 'C' indicates the OTP ROM, while the letter 'F' is for flash. The flash version is typically used for product development when a product is designed and absolutely finalized, the OTP version of the PIC is used for mass production because it is cheaper than flash in terms of price per unit. The problem with the OTP is that you cannot reprogram it if you want to modify your program.

Masked Version of PIC:

Microchip Corporation provides a service in which you can send in your program and they will burn the program into the PIC chip during the fabrication process of the chip. This chip is commonly referred to as masked PIC, which is one of the stages of IC fabrication. Masked PIC is the cheapest of all types, if the unit number are high enough. This is because there is a minimum order for the masked version of the PIC micro-controllers.

 

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