68komputer

68komputer is a homebrew computer based around the Motorola 68000 processor, and an ecosystem of peripherals and software to go with it.

68000 CPU Board

The standard 68komputer processor board, featuring 1MB of 0 wait state SRAM and flash, 32K of NVRAM + RTC, a 68681 DUART, I2C controller, and dual SAA1099 PSG’s.

68000 CPU Board

Expansion

Peripherals can be connected to the CPU board through the expansion connector, a 96-pin, 3 row DIN41612 right angle connector.

Pinout

  A B C
1 +5V GND +12V
2 +5V GND +12V
3 I2C_SCL1 /I2C_IRQ1 I2C_SDA1
4 GND GND GND
5 GND D8 D13
6 D9 D11 D15
7 D10 D14 D12
8 D7 D5 GND
9 D6 GND D3

10

GND D4 D1
11 D2 D0 GND
12 GND GND A22
13 A23 GND A19
14 GND A21 A17
15 A20 A15 A18
16 A13 A16 A12
17 A14 A10 GND
18 A11 GND A8
19 GND A9 A6
20 A7 A4 A2
21 A5 A3 A1
22 R/W /LDS /UDS
23 GND /DTACK1 /AS1
24 CLK GND  
25 FC1 FC0 FC2
26      
27     /EXTRST1
28 /EXT5 /BG1 /HALT14
29 /RESET34 /BR1 /BGACK1
30 /IACK1 /IRQ /BERR14
31 GND GND GND
32 SNDR GND SNDL

Note that the data and address bus, as well as most control signals (those in rows 1-25) should be driven through buffers when multiple peripherals are present. These buffers should be direction controlled by the bus request lines, as to allow external bus masters to access CPU board peripherals.

Any empty cells are not currently used and considered reserved for future use, and should be no connect.

Remarks

  1. Pulled up on CPU board
  2. Pulled down on CPU board
  3. Asserted by processor to reset all peripherals. To reset the processor, you must simultaneously assert /RESET and /HALT for at least 1ms.
  4. This signal is open drain.
  5. Asserted when the currently decoded address is valid and is not decoded on the CPU board. Can be used as part of external buffer logic.
68000 CPU Board

Rev1 Assembly Notes

Debug board

D9EAACE4-419B-4D2A-88FB-B2A40D43B003.jpeg

CPU Board

Backplane

Cards are interconnected via a simple backplane. Together with a management card, the backplane provides power, bus control signals, out of band signaling and support for per card audio outputs.

Backplane

Pinout

Using 96 position (3x32) DIN 41612 connectors; right angle male on expansion cards, vertical female on the backplane side. The management card uses a 48 position (3x16) DIN 41612 (C2) right angle male connector, with the corresponding vertical female on the backplane side.

CPU Board

The pinout of the CPU board’s expansion connector is documented on the CPU board expansion page.

Management Card

This provides power input to the rest of the backplane, and provides connectors for the mixed audio from the backplane.

  A B C
1 +5V +5V +5V
2 +5V +5V +5V
3 +5V +5V +5V
4 GND GND GND
5 +12V +12V +12V
6 GND GND GND
7   GND  
8   GND  
9   GND  

10

  GND  
11   GND  
12 /EXTRST GND  
13   GND I2C_IRQ
14 I2C_SCL GND I2C_SDA
15   GND  
16 SNDR GND SNDL

Any empty cells are not filled in are considered reserved for future use, and should be left unconnected.

Remarks

  1. SNDL/SNDR refers to mixed audio from all cards (CPU + expansion) in the system.
  2. EXTRST: when asserted, the CPU board (and with it, all peripherals on the backplane) are reset. This is equivalent to pushing the reset button on the CPU board, if it has one.

Peripheral

These connectors are provided by peripherals; they're 96-position, 3 row type C DIN 41612 connectors. They shall be right angle, male type.

  A B C
1 +5V GND +12V
2 +5V GND +12V
3 I2C_SCL1 /I2C_IRQ1 I2C_SDA1
4 GND GND GND
5 GND D8 D13
6 D9 D11 D15
7 D10 D14 D12
8 D7 D5 GND
9 D6 GND D3

10

GND D4 D1
11 D2 D0 GND
12 GND GND A22
13 A23 GND A19
14 GND A21 A17
15 A20 A15 A18
16 A13 A16 A12
17 A14 A10 GND
18 A11 GND A8
19 GND A9 A6
20 A7 A4 A2
21 A5 A3 A1
22 R/W /LDS /UDS
23 GND /DTACK /AS
24 CLK GND  
25 FC1 FC0 FC2
26      
27 /IRQ_IN /DETECT GND
28 /IRQ_OUT /BG /HALT2
29 /RESET2 /BR2 /BGACK2
30 /IACK /IRQ2 /BERR2
31 GND GND GND
32 SNDR GND SNDL

Remarks

  1. Pulled up to +5V by backplane
  2. These signals should be driven as open drain; they're pulled up by either the backplane or CPU board.
  3. If the card doesn't provide sound, tie SNDL/SNDR to ground through a 100kΩ resistor.
  4. Cards should tie /DETECT to GND so that the backplane can detect which slots are occupied, even if the card doesn't have anything on its I2C bus.

Interrupt Arbitration

Peripheral cards arbitrate interrupt priority in slot order. This works by means of a daisy-chained interrupt request line from each slot to the next. The first slot has the daisy chain input tied permanently high. Each peripheral outputs whether it has a pending interrupt on these pins, and then asserts the shared interrupt line. When an interrupt acknowledge cycle takes place, the peripheral with the highest priority – that is, whichever has an /IRQ_IN that's deasserted – responds.

When its interrupt conditions are cleared (from within its ISR,) it will deassert /IRQ_OUT and /IRQ. If there's a lower priority card requiring attention, the interrupt will fire again and it will respond. Otherwise, the processor returns to normal execution.

The /IRQ_IN pin on cards is pulled down to GND with a 1MΩ resistor on the backplane for all slots to prevent erroneous interrupt behavior if cards aren't inserted one after another in the backplane, e.g. the interrupt priority chain is broken. Cards can drive these pins either directly, or open drain, but in the latter case, it must provide the required pull up resistors. For open drain driving, a pull up value of at least 47kΩ is suggested.

Backplane

6 Slot Backplane

A backplane which supports up to six expansion cards, meant to go into a 3U VME crate.

image-1643962680396.png

Features

It’s intended to be used in conjunction with the backplane management card for power, but the required +5V and +12V rails can also be provided through a 3.81mm pitch pluggable terminal block instead.

Assembly Notes

Here are some notes about hardware revisions.

Rev 1

Backplane Management

Provides +5V and +12V power and supervision for backplane, and audio output.

Backplane Management

Overview

Unlike other expansion boards, this one is required to operate the backplane, primarily to power the system. To connect, it uses a smaller half height (48 position, 3 row) DIN 41612 connector with a unique pinout to interface to the backplane. Additionally, it exposes mixed audio from all expansion sources with volume control.

Rev1 backplane management board

Shown above is the Rev1 board backplane management board. This board is two layer, with 2oz copper.

Power

Primarily, the management card exists to provide power to the rest of the system. It accepts any voltage between 14V – 30V through a pluggable 5.08mm pitch terminal block (CUI TBP01R1W-508, but compatible with many other pluggable terminal blocks) as an input. The input features reverse polarity protection and inrush current limiting, and a 10A replaceable fuse. Input power then passes through a common mode choke to filter out noise.

Next, the main +12V power rail is generated via a LM25116 switching controller (U401) with discrete MOSFETs (Q401, Q402.) A maximum of roughly 10A at 12V can be supplied. The +12V rail then has significant bulk decoupling, since it provides both the system’s +12V rail, and the input for the other regulators.

The system’s +5V rail is generated by a PTH08T220 power module; it can provide up to 16A of power. A secondary +5V rail, used exclusively by the management board, is generated using a linear regulator (U301) from the +12V rail. The secondary rail powers the audio amplifier, as well as all active logic on the card.

Lastly, both the +5V and +12V rails pass through 3mΩ current shunts, which is measured by an INA209 current sense device (U303, U304.) These devices expose the voltage/current/power readings over the I2C bus, which can be read out by the host. (There’s also programmable upper/lower bounds and fault outputs, but these are not currently used.)

Both rails then have some further high frequency decoupling on the output, after the current shunt, followed by polyfuses: 13A on 5V, and 5A on 12V.

Audio

Additionally, the card buffers the mixed audio signals provided by the backplane. The audio is filtered by an active low-pass filter with a 16kHz cut-off frequency. A stereo potentiometer is used to control the volume of the output audio, which is provided on a 3.5mm jack.

Miscellaneous

Like all other expansion cards, the management card supports the I2C bus for out-of-band management, and provides a configuration EEPROM (U203; it has an embedded serial number) at addresses 0b1010000 and 0b1011000.

Indicators are provided for all power rails.

Backplane Management

Revisions

This page lists any assembly remarks and issues with each revision of the board.

Rev 1