Soft Z80 Second Processor Guide

Use - General

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Overview

The Soft Z80 Second Processor provides the logic to turn your hardware development board into an Acorn Z80 Second Processor.

The currently supported development boards are :

Board	Clock	Bench	I/O
(click link for info)	(MHz)	(MHz)	(Volt)
Terasic DE1	30	120	3.3 (*1)
Terasic DE0 Nano	50	200	3.3
Terasic Cyclone V GX Starter Kit	60	240	3.3
Terasic DE0 Nano SOC	70	280	3.3
Altera Stratix III FPGA Development Kit	90	360	2.5
Terasic DE10 Lite	50	200	3.3
Altera Stratix V Advanced Systems Development Kit	100	400	2.5
Terasic DE10 Nano	60	240	3.3
Terasic Cyclone 10 LP Evaluation Kit	50	200	3.3

'Clock (MHz)' indicates the highest clock speed that the soft Second Processor will run at. It will also run at any lower, integer clock speed, including the original one (6 MHz).

'Bench (MHz)' indicates the 'benchmark' speed at the highest clock speed. On an original Second Processor, this would equal the clock speed. On the soft one, it is four times the clock speed (because the Z80 core that is used executes nearly all instructions in a quarter of the cycles of a real Z80).

'I/O (Volt)' indicates the I/O voltage of the development board. Since the Tube I/O voltage is 5, a converter like TubeSilencer is required to avoid damaging the development board.

Note (*1) : The Terasic DE1 can also be connected directly to the Tube, because its I/O pins are 5V tolerant. But it is nevertheless recommended to use a converter like TubeSilencer.

Before you continue, please note that doing anything described here is at your own risk !

Connecting

Your Acorn machine's Tube interface may be connected to your development board in two different ways, designated as '20-pin' and '40-pin', both involving a TubeSilencer.

Note that the 'in' and 'out' connectors on TubeSilencer are marked by '5V' and 'LV' respectively, at the pin 1 side. Your machine connects to the 'in', your development board to the 'out'.

The 20-pin way

The 20-pin way allows connection via a 20-pin TubeSilencer. The TubeSilencer is plugged directly into the exact middle of the relevant 40-pin development board socket. I.e. this works out as : from 40-pin Tube socket, via 40-wire straight cable, via 20-pin TubeSilencer, to 40-pin development board socket.

So, a 20-pin interface looks like this :

interface 20-pin

The 40-pin way

The 40-pin way allows connection via a 40-pin TubeSilencer. The TubeSilencer is connected to the relevant 40-pin development board socket via a specially cut-up 40-wire cable. I.e. this works out as : from 40-pin Tube socket, via 40-wire straight cable, via 40-pin TubeSilencer, via 40-wire specially cut-up cable, to 40-pin development board socket. The Terasic DE1 (only) may also be connected like this : from 40-pin Tube socket, via 40-wire specially cut-up cable, to 40-pin development board socket (although using a TubeSilencer is still recommended).

So, a 40-pin interface looks like this :

interface 40-pin

The specially cut-up cable is wired as follows (TS = TubeSilencer, DB = development board) :

TS side pins	DB side pins
1 through 10	1 through 10
11 through 26	12 through 27
27 through 32	30 through 35
33 through 40	not connected

The cable can be fabricated quite easily from two 40-pin IDC connectors and a short piece (10 cm or so) of 40-wire flatcable, as follows :

Squeeze the TubeSilencer side connector to the cable 'as usual' (matching the pin 1 marking on the connector with the coloured wire).
Before squeezing the development board side connector to the cable, split the cable (using e.g. scissors) : wires 1 through 10 (i.e. 10 wires, part A), 11 through 26 (i.e. 16 wires, part B), and 27 through 32 (i.e. 6 wires, part C). Feel free to trim 33 through 40, because they won't be used.
Now squeeze the development board side connector, roughly 'as usual', but skipping one pin between part A and B, and two between part B and C.

Programming

Proceed as follows :

Power down your Acorn machine.
Program the development board using the provided .sof file (or the .pof or .jic file generated from the .sof file, refer to the documentation of your development board for instructions). The difference between the file types is that, on powering down the development board, .sof programming will be lost, whereas .pof or .jic programming will remain.
Power up your Acorn machine (after configuring it for using the Tube if needed, e.g., on a BBC Master, issue '*Configure Tube' and '*Configure ExTube').

Using

Using the soft Z80 Second Processor is, in general, no different than using the real one, so please refer to the relevant manuals (if you don't have them, try Internet).

However, on the soft one you can change the processor clock frequency to any whole number of MHz (50 MHz in the examples below). The transition to the new frequency is seamless, and survives (CTRL-)BREAK. Note that the maximum frequency (see table above) is automatically selected if you attempt to exceed it, and that 0 selects the default frequency (6 MHz).

There are two ways to switch :

From the 'monitor' supervisor prompt. Enter e.g. '*Speed 50'.
By IO port writes. First write 10 to IO port 0x9, then write e.g. 50 to IO port 0x8.

Acknowledgements

All other logic design, including the Tube ULA, is my own.