---
title: Software Design
description: "TommyPROM Arduino 28C256 programmer software design"
has_children: false
nav_order: 3
---

# Software Design

The software is designed around several major blocks and classes.  The code has been
broken out into individual files to make it easy to extend the hardware support to
additional EPROM and EEPROM families. See the
[Adding Chip Families](../extending) page for details on suggested hardware and software
changes needed.

## CommandStatus class

The CommandStatus class stores the execution status of the previous command.  It allows
the status to be saved (and recalled using the / command) instead of just printing the
status at the completion of the command.  This was important for debugging XMODEM
problems, because the error messages would get eaten as part of the transfer.  The class
has utility beyond XMODEM because it includes formatting that relieves each command from
having to build parameterized error messages with multiple print calls.

## Xmodem class

The Xmodem class implements the communications protocols needed to do XMODEM or XMODEM-CRC
transmit and receive.  It calls directly into the PROM read and write code, so the
complete files are never stored during the transfer.

## CLI code and command implementation

This code parses input commands and parameters and executes the commands.

A compile-time switch in Configure.h enables additional debug commands that are not needed
in normal operation, but are very useful to verify proper operation of the hardware.

## PromDevice class

The PromDevice class and its subclasses encapsulate all of the communication between the
Arduino and the target PROM device.

To meet the timing requirements for block writes and 28C chip unlocking, the PromDevice
class accesses the shift registers and data bus using direct port writes instead of 8
individual pin accesses.  This greatly increases performance, but it makes the code
dependent on the particular flavor of Arduino being used.  The code supports the Uno,
Nano, and Boarduino versions of Arduino hardware or any other variant that uses that same
mapping of ATMega ports to I/O pins.  To support a different Arduino board, either change
the pins used to match the mapping in the software, or change the hardware-specific code
in PromDevice.cpp and PromAddressDriver.cpp.

The PromDevice class contains common code used by all devices, including the block write
code that will break a large write request into a set of properly-aligned smaller blocks
for devices that support block writing, or a sequence of individual byte writes for
devices that do not.

Subclasses of the PromDevice class provide the device-specific code. To accommodate the
limited memory footprint of the Arduino, only a single subclass will be compiled into the
final code. This is controlled via switches in the Configure.h file.

The device-specific subclasses are described below.  Each class provides pin definitions,
byte read, byte burn, and optional block burn code that is specific to the timing of the
chip. They also supply a SetAddress method that either calls the built-in code for the
shift register hardware or provides device-specific address code.

### PromDevice28C class

Although the existing code is specific to the 28C256, the constructor has parameters that
easily support other chips.  The current design has been used to read other chips,
including 2764 and 29c040 EPROMs.

The 28C code supports fast block writes, allowing a 32K byte chip to be programmed in just
a few seconds.

### PromDevice8755A class

The PromDevice8755A class provides the byte read and byte write code, as well as
SetAddress code that accommodates the ALE used on this chip.  The byte write code controls
the Vdd switching hardware that toggles the Vdd pin between the normal 5V value and the
25V programming pulse.

### PromAddressDriver class

Although not actually a subclass of PromDevice, this contains the code to drive the shift
registers that provide the address lines. If this hardware is used, the PromDeviceXX class
can call this code to implement the SetAddress methods.