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Altera MAX 7000S - MTXPlus+ Test Bed

Dave's Max 7000 Development Board

 

Hardware Test Bed

Rather than trying to go straight to development of the glue logic for MTXPlus, I think it is a better idea to experiment by developing some test programs that will execute on a simple hardware test device. Altera produce a range of "Development" boards, including the DE1 that I use to run a copy of Andy Key's REMEMOTECH (an emulation of the Memotech MTX computer running on a Cyclone II FPGA (for details on the Cyclone II, see this page on the Altera website), as well as CPLD development boards. These boards come in a range of prices, but a cheaper option is to build my own test bed, which will require a bit more effort but should be able to facilitate testing of logic designs on real hardware.

To be able to exercise the programmed logic and display the resultant outputs, a board with basic inputs and outputs from switches and LEDs will do the job. Here is my design for Dave's Development System . . . . . 

[When I was half way through wiring up my board, I discovered that Altera had produced EPM7128S based Development boards for its University Program - the UP1 and UP2. These boards have two CPLDs installed, a MAX7128S and the more powerful FLEX10K20 which has 20,000 gates. As it turns out, these boards have remarkably similar I/O functions as my board.]

Basic Features :

  • A single board containing and EPM7128S CPLD with input and output hardware

  • Logic Inputs : 2 x 8 bit DIP switch packs for logic inputs (16, configurable as normally high or low, inputs)

  • Logic Outputs : 2 x 7 Segment displays (DP not used), 3 sets of 8 LEDs (38 total outputs)

  • Unallocated I/O : 6 Channels (wired to a SIP header socket and available as required)

  • External power, 5VDC

  • JTAG programming port

  • 16MHz oscillator wired to clock input (as will be used in MTXPlus), not required for combinatorial logic

The basic design was done using KiCad, this is the schematic drawn before the board was built.

While building the board, I made a small modification - each set of DIP switches and pull-up/down resistor sets can be individually set high or low as required using separate jumper blocks.

There are two, 8-bit switch packs for the simulation of logic inputs. Jumpers allow each switch pack can be configured to be high or low when the switch is closed.

The CPLD input is connected via pull/up down resistors to 5v or 0v, this is also selectable by jumper and should be set to the opposite position from the switch input selection.

There are three sets of 8 LEDs for the display of logic outputs. The LEDs are in three colours, red, yellow and green, each set has current limiting resistors with appropriate values for the LED forward voltage and acceptable brightness (see below).

Colour

Vf (i=20mA)

Rc

Ra

Red

2.1

145

220

Yellow

2.1

145

220

Green

2.4

80

130

(Vf = Diode forward voltage, Rc = calculated, Ra = actual)

There are two, 7-segment LED displays (the decimal point is not connected), these will allow 2 Hex digits to be displayed on the board (with suitable VHDL programming inside the CPLD).

The 7-segment displays use red LEDs and have the same forward voltage as the single LEDs and also use 220 ohm current limiting resistors.

The CPLD will be programmed on the board, i.e., using "in system programming" through a JTAG interface.

Power will be supplied from a standard 2.5mm DC power adapter with an LED to indicate power is present.

(The Altera Operating Requirements datasheet advises that 0.2uF decoupling capacitors should be connected across each VCC/GND pair, these have not been included at this point.)

The development board is based on this type of prototyping board - having just plated through holes, rather than strip-board.

The board used is the same size as the MTXPlus PCBs will be, Eurocard size, 160mm x 100mm.

The form factor of the CPLD that I am using is for mounting in a PLCC-84, you can see that the socket takes up a large area of the board - but significantly less space than would be required for discrete logic ships performing the same functions.

Placing the CPLD in the centre of the board allows the most flexibility for wiring the connections on the base of the socket.

Exposing my rather dubious soldering "skills", the PLCC socket mounted on the prototype board.

Adjacent pins on a typical DIP IC have a pitch of 0.1", with pins on the opposite side separated by 0.3". Adjacent pins on a PLCC socket are also 0.1", but a second row is also separated by 0.1". These tightly spaced connections are going to present a bit of a challenge, but should be manageable.

Some time later . . . . . . . . .

(OK then, much, much later)

All non-socketed components mounted, i.e., resistors, LEDs, option jumper headers, power and JTAG connectors and the sockets themselves

Solder side of the board, showing my hopefully functional, but not too pretty, attempt at wiring it all up.

Whilst wiring the board, I found it easier to reroute a few of the inputs to different switch positions. I have not modified the schematic as it has no material impact on the design - Quartus II makes pin assignment & reassignment very easy.

Testing

The terminations on the PLCC socket are tightly packed and many are obscured as later connections are made. To reduce the potential need for rework, I did as much testing during assembly as possible, checking for continuity, short circuits and correct routing from the PLCC socket contacts.

At appropriate stages, I was able to check the I/O - the switches, LEDs and 7-segment displays.

Close up of the 84 connection points under the CPLD socket.

Production of the development board has been good practice for building the MTXPlus CPU board. When I build the CPU board, rather than connecting the external components to the PLCC socket in turn, I think that making all terminations at the PLCC end first will result in a neater build, even though keeping track of the wires may be a bit of work!

Everything but the CPLD installed, "all" that remains is software testing.

Although not shown in the original schematic, the silver can at the bottom left of the i-phone photo is a 16MHz oscillator - kindly donated by Martin.

Somewhat optimistically, I hoped that the oscillator output would be a square wave, however, as you can see from the 'scope trace, it is sinusoidal.

I will either need to condition it on the board or try doing it in the CPLD.

Success - more or less . . .

Before I built the board, I did a couple of tests with the LEDs to choose resistor values to give what I thought to be reasonable levels of brightness while limiting the current appropriately.

As it turns out, the values that I chose results in the LEDs being a bit too bright - this makes photographing them a bit of a problem, at least on my iPhone. On the other hand, should I want to start running a disco - I already have the lighting rig!

However, while it's not quite the ubiquitous "Hello World", at least you get the idea. The switch packs, LEDs and 7-segment displays all do what they are supposed to do - which, I have to confess, was something of a surprise - no errors with the assembly or programming at the first attempt!

More to the point, I have been able to test an early version of the CPU board I/O port decode logic that I loaded as part of the first test program - again, this works as expected. 

Design Development - I very quickly discovered that it would be an advantage to be able to configure the unallocated I/O on an "as needed" basis. I have now wired the unallocated I/O pins to a SIP socket where they are available for use as required.

I don't imagine that anyone will want to build a board the same as mine, but for completeness, I need to do an "as-built" schematic of the board, I'll get to that shortly.

 

 

 

<-----     To do

Comparison With Altera UP1/2 Development Boards

Layout of the Altera UP1 and UP2 boards, you can see that the left hand side of the board has very similar features to mine, including DIP switches, LEDs and 2 x 7-segment displays.

A photo of a UP2 board, obviously looking far better than mine, but these boards are obsolete, not readily available and would in any event, be far more expensive than my version.

 

 

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