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Thread: EPROM emulation 2K x 8 for an old 8MHz intel MCS-48 microprocessor

  1. #1

    EPROM emulation 2K x 8 for an old 8MHz intel MCS-48 microprocessor

    The original system is from a 1985 Volkswagen "DigiJet" engine control unit. The P80A49H micro is using an external 2K x 8 EPROM (2716?), and while I was able to drive the address bus and dump the data using GPIO of a Teensy 3.2, I'm wondering if it should be feasible to emulate it with a Teensy 3.5 or 3.6?

    My goal is to make the Teensy an "interface" to the original system for monitoring and tuning.

    The original code uses lookup tables for adjusting engine operations based on sensor inputs. Being able to monitor/shadow the code and data fetches, I can better understand the state of the control system. If there are spare CPU cycles and ROM locations, I could add instructions that reveal the state of CPU registers or flags etc.

    The "interface" Teensy would take logs and stats of notable ROM fetches, which another system can visualise or otherwise monitor over a (uart?) serial interface.

    An operator could adjust the lookup table mapping and other data constants to tune engine operation, especially as mechanical components and sensors wear out and operating ranges shift slightly.

    So, my key concern is satisfying the timing constraints of the EPROM address/data bus while still being able to service basic logging/serial needs to make a useful interface. I have the data sheet of the micro and the ROM (see chapter 5 "Automotive"), but generally an 8-bit micro running at 8MHz, and Teensy running at up to 120MHz, is that likely enough bandwidth to cope with writing the data bits in the 360 nano seconds required after the interrupt fires for "program store enable"?

    Has anyone tackled something like this with Teensy. It's just a personal hobby project, and I'm looking for advice or guidance on how to tackle it with the developer-friendly Teensy board!

  2. #2
    Senior Member
    Join Date
    Jan 2013
    Posts
    843
    Teensy 3.6 is probably fast enough, 3.5 probably not.

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