Soft 6502 Second Processor Guide

Use - General

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Overview

The Soft 6502 Second Processor provides the logic to turn your hardware development board into an Acorn 6502 Second Processor.

The currently supported development boards are :

Board	Clock	Bench	I/O
(click link for info)	(MHz)	(MHz)	(Volt)
Terasic DE1	40	40	3.3 (*1)
Terasic DE0 Nano	80	156	3.3
Terasic Cyclone V GX Starter Kit	100	198	3.3
Terasic DE0 Nano SOC	100	198	3.3
Altera Stratix III FPGA Development Kit	110	226	2.5
Terasic DE10 Lite	80	156	3.3
Altera Stratix V Advanced Systems Development Kit	180	384	2.5
Terasic DE10 Nano	90	186	3.3
Terasic Cyclone 10 LP Evaluation Kit	80	156	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 ones (3 or 4 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 roughly two times the clock speed, if the board uses my own 65C02 core (which executes instructions in far less cycles than a real 65C02).

'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 6502 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 (4 MHz).

There are two ways to switch :

By star command. Enter e.g. '*Speed 50'.
By memory writes. First write 10 to location 0xFEF1, then write e.g. 50 to location 0xFEF0 (under BASIC, that would be '?&FEF1 = 10:?&FEF0 = 50').

Acknowledgements

All other logic design, including the 'fast' 65C02 core and the Tube ULA, is my own.