Stuffs to read before going to bed

[Charles MacDonald]

This page links to a PDF transcript of a discussion with the Motorola 68000 design team:

http://www.computerhistory.org/collecti ... /102658164

There's a lot of cool stuff in here, like all 680x0 circuitry was tested on breadboards until the 68040, at which point they finally started to simulate the design with FPGAs. And Motorola and Hitachi really worked closely to exchange IP and manufacturing processes for mutual gain, which drastically improved Motorola's fabrication abilities. I guess that makes it no surprise that Hitachi second-sourced nearly everything Motorola made back then.

I wish they'd had discussed the often overlooked 68012 and had included people from the 68060 team, but regardless it's a great read.

[MooZ]

Charles: Cool! They also have it for the Z80, intel 386. And There're a bunch of docs (like schematics for a 6502 tester, assembly books...). You found a gold mine

Anyway, PS/2 stuffs:

• PS/2 Mouse on a PIC with a 9x11 LED Display
• PS/2 Keyboard Interface

Anyone for a PC-Engine PS/2 keyboard?

[Charles MacDonald]

Here's a good article about how voltage at an I/O pin of an unpowered chip can find a path to the supply rail through ESD protection diodes:

http://www.analog.com/library/analogDia ... _amps.html

I have an interesting real-world example to add to this:

The Sega Saturn Action Replay cartridge connects to a Comms Link ISA card using an 8-bit bidirectional data bus. The card has a '573 latch which connects to a '245 transceiver in the cartridge. If the Saturn is turned off and the '573 latch value is non-zero (one or more data lines driven high), current will flow from the '573 Q pins to the '245 input pins, then through the ESD protection diodes in each pin to the Saturn's +5V rail. In this way the PC powers the Saturn.

In fact this is sufficient enough that the number of set bits in the '573 latch corresponds to the brightness of the Saturn's power LED.

One problem is that this low voltage on the +5V rail prevents the reset generator from detecting a power-up event as the system is already powered to some extent, presumably past the trip point. So switching on the Saturn does not cause a reset pulse to be generated and the various components are left in an undefined state. You have to run a program on the PC to clear the latch to make the Saturn boot correctly.

The author's suggestion of adding series resistors to limit current would probably work in this case, though the article focuses on op-amps. For unidirectional signals you could use isolation like an optocoupler, and I could swear TI makes level shifters that deliberately omit the ESD diodes for this purpose, but I can't find the part number. When you are mixing powered and unpowered ICs it can be quite an issue to overcome.

EDIT: The last section in this application note discusses this issue for CMOS-based digital logic:

http://www.fairchildsemi.com/an/AN/AN-376.pdf

In particular tri-stating the output of a powered IC that connects to an unpowered IC seems to be the trick. ;D

[MooZ]

So that may explain some weird behaviour on some electronic equipements?

By the way, here's the LM13600/LM13700 story.

[MooZ]

• Building a computer around a 6502 processor.
• IEEE 754 multiplication and addition.

[MooZ]

Jamis Buck's Maze Generation: Algorithm Recap.

[MooZ]

• EEPROM Programmer using an ATTiny and USBTinyISP.

[MooZ]

LZSS, LZW, Huffman, LZARI, LZHUF documentation and source code by Haruhiko Okumura (don't forget to take a look at History of Data Compression in Japan).

[MooZ]

Akihabara video tour

[MooZ]

This is totally unrelated but it may be useful : How to Make Sriracha from Scratch.