---
title: "Adding New Chip Families"
permalink: /docs/extending
exerpt: "Extending TommyPROM to support new EEPROM, EPROM, and Flash chip families"
---

## Hardware

There are currently two hardware flavors - one for 28C series EEPROMs and one specifically
for the Intel 8255A UV EPROM. There is also a work-in-progress version for 27C series
chips.

The 8755A chip uses a multiplexed set of address and data lines that are directly driven
by the Arduino. Most other chips will not use this addressing method. The 8755A design
also includes a circuit to switch the programming voltage under software control.  This
may be useful for other chips that use non-5V programming voltages, although many of these
chips, like the 27 series EPROMS, have a programming voltage that remains on for the
entire write and verify cycle. It may be easier to simply switch the programming voltage
manually for these chips.

The basic hardware design, used for the 28C EEPROMs, is much more adaptable to additional
chip families. This design uses two shift registers to create 16 dedicated address lines
from only 3 Arduino pins. This design, plus manual switching of the program voltage, would
be very adaptable to EPROMs like the 2716, 2764, 27040, and 272001. The hardware has
already been used with these chips for read-only operations.

The current design can directly address chips as large as 512K bytes, like the 29C040. Use
the shift registers for A<sub>0</sub>..A<sub>15</sub> and wire Arduino pins D10..D12 to
A<sub>16</sub>..A<sub>18</sub> on the target chip.

# Software

The software design is modular, allowing easy extension for chips with different
programming algorithms.  A new class can be added for each new chip family. This class
will include code for byte reads, byte writes, and optional block writes if the chip
supports it.  All of the chip-specific code will be in this single class.

The basic steps to support a new chip are as follows:
* Create or copy an existing PromDeviceXX class to create a new .cpp and .h file for the
chip-specific code.
* Define the pin assignments and implement the chip-specific setAddress, readByte, and
burnByte code in the new files. The setAddress code can call the PromAddressDriver code if
the shift register address hardware is used.
* If supported, also add chip-specific burnBlock code.
* Be sure that the new files have unique #if defined code that matches the class name.
* Edit Configure.h to add the conditional code includes the new PromDevice subclass.
* Edit TommyPROM.ino to add the conditional code that declares the new device.

Note that after files are added or renamed in the TommyPROM directory, the project needs
to be reopened in Arduino IDE to get the changes.