Legacy Pinball Classics Controller

Specifications:

• Enables communication between Mission Pinball Frameworks (MPF) and 6808/6802 based Williams MPU Boards. Designed for System 6, with System 3-7 support coming soon.
• Compatible with existing original or aftermarket MPU and Driver boards
• Drop in replacement for MC6808 CPU IC1
• USB Interface makes MPF computer connection easy, with a high retention USB-B connector

Planned:

• Onboard 6808 socket enables true original code execution, switchable via Mission Pinball control
• Enhanced hardware debugging via Classics Controller helps identify bad ROMs, RAM, PIAs, and more

Future:

• MPU/Driver Emulation mode enables virtual memory space for 6808 which can be modified in real time, via the Bus Interconnect Matrix
• 6808 Emulation mode eliminates the need for a real 6808 CPU

Hardware overview:

• Classics Controller is comprised of a custom PCB hosting an onboard RP2040 with 2+MB flash, socketed 6808 with crystal, and bus interfacing ICs.
• Optional USB isolation IC ADUM3160 and 5V-5V isolated module adds $20-$25 to BOM.
• RP2040 communicates with the MPU bus using 2 Microchip MCP23S17 SPI GPIO expanders, adding an additional 32 pins for bus data, addressing, and bus logic (such as VMA, IRQ, NMI, Reset monitoring, and Φ2).
• RP2040 interfaces with MCP23S17 via an 8 channel 3.3V to 5V bidirectional logic level translating buffer.
• The RP2040 expander bus is 2:1 multiplexed with the real 6808 bus using 2 Texas Instruments SN74CBT16233DGGR, 16 channel bidirectional general purpose bus multiplexer (mux), providing a total of 32 channels of bus control.
• The output of the 2:1 bus multiplexer is connected to 2 Texas Instruments SN74CBT16245C, a 16 channel bidirectional 1:1 bus switch, enabling the disconnect of the Classics Controller from the MPU bus. In conjunction with the "Test" inputs of the 2:1 bus mux, this allows the RP2040 to communicate directly with the 6808 without any interference from devices on MPU or Driver board. In the future, this can allow the RP2040 to emulate the MPU board and control the state of a live 6808 CPU.
• Classics Controller connects to System 6 MPU via ribbon cable and 40 position 0.600" DIP IDC connector. The Classics Controller hosts a keyed, standard 2x20 2.54mm male header connector.

Simplified Hardware Diagrams

Overview:

graph LR
PC([Computer Running MPF]) <--> |USB Cable| CC[Classics Controller]
CC --> MPU[System 6 MPU Board]
MPU --> DRV[Driver Board]

Address and Data Flow:

graph TB
USB(USB) --> RP[RP2040]
RP --> BLT[Bidirectional Level Translator]
BLT --> MCP1[SPI GPIO Expander, Addr0x20]
BLT --> MCP2[SPI GPIO Expander, Addr0x22]
MCP1 --> MUX1[2:1 Multiplexer, 16CH]
6808 -->|Address 0:15| MUX1
6808 -->|Data 0:7 + Bus Control| MUX2
MCP2 --> MUX2[2:1 Multiplexer, 16CH]
MUX1 --> SW1[Bus Switch, 16CH]
MUX2 --> SW2[Bus Switch, 16CH]
SW1 --> RC[Ribbon Cable to MPU Board]
SW2 --> RC

Buffer Control:

graph TB
RP2040 --> BLT[Bidirectional Level Translator]
BLT -->|A/B Switch + 'test'| MUX1[2:1 Multiplexer, 16CH]
BLT -->|En| SW1[Bus Switch, 16CH]
BLT -->|HALT| 6808
BLT -->|En| SW2[Bus Switch, 16CH]
BLT -->|A/B Switch + 'test'| MUX2[2:1 Multiplexer, 16CH]

Firmware Overview

The Firmware for the Legacy Pinball Classics Controller is written in MicroPython.

Both cores of the RP2040 are utilized:

Core 0:

• Initializes USB Serial and SPI Component
• Waits for a connection with MPF
• Disables 6808
• Implements serial protocol to communicate with MPU board
• Manages locks and mailboxes
• Schedules core 1

Core 1:

• Abstracts address translation layer
• Polls and writes to PIA on MPU and Driver Board
• Handles bus control