M68HC11-EPROM-Programmer

Part 1: EPROM Reader

Overview

This project implements an EPROM reader using the M68HC11 microcontroller and a 2764 EPROM chip. The system reads the first 25 bytes of data stored in the EPROM and transmits it to a host computer via serial communication for display in both hexadecimal and ASCII formats.

Hardware Configuration

The hardware setup maps the M68HC11 ports to the 2764 EPROM pins as follows:

• Port B (PB0-PB7): Connected to EPROM address inputs A0-A7
• Port C (PC0-PC7): Connected to EPROM data outputs Q0-Q7
• Port A (PA5, PA6): Control signals for Chip Enable (CE) and Output Enable (OE)
• Upper Address Lines (A8-A12): Grounded to restrict addressable space to the first 256 bytes

The EPROM operates with a 5V supply and has an access time of 180-250ns depending on the speed grade.

Software Implementation

The system consists of two main components:

Assembly Program (programmer.asm)

The M68HC11 assembly program performs the following steps:

1. Initialization: Sets up the stack pointer and forces the device into single-chip mode
2. Port Configuration: Configures Port C as input and sets control pins to inactive (HIGH)
3. Serial Setup: Initializes the SCI (Serial Communication Interface) to 9600 baud
4. Startup Delay: Implements a LONG_DELAY routine to allow the bootloader time to switch baud rates
5. Read Loop: Iterates 25 times to:
   • Assert the address on Port B
   • Activate control signals (pull CE and OE LOW)
   • Read the data byte from Port C
   • Send the byte to the PC via SEND_SERIAL subroutine
   • Deactivate control signals (pull CE and OE HIGH)

Python Bootloader (HC11_BL-ASCII.py)

The modified bootloader script:

• Loads the assembly program into the HC11's RAM via serial at 1200 baud
• Switches the serial port to 9600 baud after successful echo-back
• Calls read_eprom_data() to receive and display the 25 bytes
• Displays output in hexadecimal format (8 bytes per line)
• Displays output in ASCII format (replaces non-printable characters with ".")

Key Challenge: Baud Rate Synchronization

During initial testing, a race condition occurred where the HC11 began transmitting data before the bootloader could switch from 1200 to 9600 baud. This resulted in corrupted or "ghost" byte values.

Solution: Implemented the LONG_DELAY subroutine after baud rate configuration to waste CPU cycles, giving the bootloader sufficient time to reinitialize the serial connection at 9600 baud before the read loop begins.

Results

The system successfully:

• Reads the first 25 bytes from the 2764 EPROM
• Transmits data error-free via serial communication
• Displays data in both hexadecimal and ASCII formats
• Handles the 0xFF value gracefully by displaying it as "." in ASCII output (since ASCII is a 7-bit standard supporting only values 0-127)

Materials

• 1x M68HC11 Interface Board
• 1x 2764 EPROM
• 1x Breadboard
• 2x Power Supplies
• 5x 10Kohm Resistors
• 1x Oscilloscope
• 1x VOM (Voltmeter/Ohmmeter)
• 1x Computer System

Usage

1. Assemble the assembly code into an S19 file
2. Run the bootloader script: python HC11_BL-ASCII.py -c /dev/ttyUSB0 -i programmer.s19
3. Press RESET on the HC11 board when prompted
4. The program will load, switch baud rates, and display the EPROM data

Recommendations for Future Work

While the current implementation successfully resolves the baud rate race condition, it uses a fixed delay that is not deterministic. A more robust approach would implement a software handshake using interrupts, similar to TCP's three-way handshake. This would ensure 100% data synchronization without relying on timing assumptions that may fail on slower systems or under heavy CPU load.