Completley forget about doing any internal logic with signals with multiple drivers.

FPGAs do not work like that. In fact, modern CMOS does not work like that. If you ask for a multi-driver bus, then the software will synthesize it poorly by building a big multiplexer somewhere for all your outputs, then feeding all the inputs off the output of the mux.

The bus that connects to the outside world can go as input to all your components; thats fine. Now, build a mux, and feed it the outputs of all your devices. However you want the mux to work is up to you; it can either be just an OR of them all (and they each have their own shutoffs), or it can be an actual selector fed with the address decode logic. The output drivers of this mux should be controlled by the read/write signal.

Because you're now designing to the hardware, you'll find things work better. They will also be smaller.

(This is one of the reasons I recommend anyone planning on doing FPGA/CPLD work learn Verilog or VHDL. They make you think much more closely about how the hardware actually works. In the long run, you will also find it much faster to work in.)

@OwenS,

It is nice to finally be able to post here in the 6502 forums as a beginner to CPLD/FPGA design and have someone familiar, with more knowledge offer a more expert advice! I post in the Xilinx forums rarely because I am a proponent of schematic design... I seem to be the only one!

A multiple driver design 'error' implies a design with feedback, thus inputs and outputs are in conflict. Xilinx CPLD's seem to give you more room in this regard. More room than their FPGA's.

How would a FPGA design correctly implement multiple internal I/O ports through a single external bus?

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

In http://www.baltissen.org/zip/rb65-10.zip you'll find RB65.vhd which is my not complete VHDL version of the 6502.
Defenition of the data bus and the use of it:

Where D_out is a signal set by the opcodes like "D_out <= X;" in any of the STX opcodes. RW_i is the Read/Write signal set by the opcode, PHI_i is the synchronised clock.
IntData holds the data read at every clock, Opcode speaks for itself IMHO.

Have fun!

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A more readable version of his VHDL.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

You're absolutely right!

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Clue here, last post...: http://forums.xilinx.com/t5/General-Tec ... 930/page/2

I post for us beginners.

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

If one could fit a core into a CPLD vs. an FPGA, would there be any advantages? Disadvantages?

I was able to fit a 6502 Core into a Coolrunner II XC2C512 208-pin QFP.

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

I am pretty sure I tries something like this in a Xilinx CPLD (9572), to define a bidirectional in/out port. But the pins were always pulled low, so I had to remove the read feature and make that port write-only.

Might have been an error in my code though.

Is there a specific difference between CPLDs and FPGAs in that respect?

Thanks
André

Hallo André,


I had the same problem with Altera CPLDs. That doesn't mean that all CPLDs will behave like this but unfortunately just the ones we picked.

Gideon stated that he is quite sure that future FPGA's won't be bi-directional anymore. I know nowadays FPGAs have a lot of pins. But with a 64-bit processor you'll need at least 192 pins for the address and data bus only!.

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Andre,
I can only speak from my use ot FPGA's and CPLD's using schematic entry as I have not crossed the bridge to HDL yet, but I can say this: That link I had posted has given me insight to problems I've been having.
This what I have learned about implementing bidirectional buses inside CPLD's & FPGA's:

When working with CPLD's, you can have a bidirectional bus that goes to each of your internal ports.
RE, WE are Read Enable & Write Enable signals decoded with Address and R/W.


When working with FPGA's however, you can have only 1 point within the entire IC that is bidirectional (i.e. high impedance "Z" state). So, in that link to the Xilinx forums I mentioned earlier, Gabor stated "... it is best to have this bidirectional entry point at the highest level in your design...".

Below is a module similar to one I'm currently using successfully to interface the 6502 databus to multiple, readable output ports inside the FPGA. The only difference is, I've renamed some assignments and added more ports to express the idea more fully.

Databus(7:0) on the left, is assigned to pins on the FPGA. They connect directly to the 6502.
DataIn(7:0) on the top right is common to all the inputs of all the 8-bit PORTs, present inside other modules.
The PORTxRE (read enable) signals from each PORT are NOR'd, to put the data from the corresponding PORT onto the Databus.

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

So now I have a 7-bit (A14-A20) RAMBANK PORT and a 5-bit (A14-A18) ROMBANK PORT that I can perform INC/DEC on. At least I have proven it on the RAMBANK.

The past few days I've been trying to figure out how to INIT an FD8CE for the ROMBANK as it needs to start out with upper bank selected. I need to INIT the 8-bit port for $FF on powerup.
Sounds easy, but I've been having no luck.

Everything else is working.
I know this because when I disconnect the ROMBANK pins (A14-A18 going to the 512Kx8 EEPROM), inside the FPGA, from the PORT and connect each of these pins individually to VCC, the system works.

Can anyone help with Flip Flop INIT values?

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

Having an FD8CP would help a lot (there seems to be only an FDCP in the xilinx library, but no multi-port versions), then you could connect a reset/powerup input to the preset input of the registers.

If you don't want to place a ton of FDCPs, there's a simple workaround: place an INV8 directly before and after the FD8CP. The register will then store inverted data, and the powerup value will be $FF instead of $00.

so long,

Hias

Hiassoft, thanks for posting, but you only see a FD8CP in your library? What device are you using?...

I think I may have made abit of progress INIT'ing a FD8CE....

I was right clicking on the FD8CE, which is an 8-bit D Flip Flop w/ clear enable controlling the ROMBANK A14-A18 (using 5 of 8 bits) in my PWA project, and selected "Object Properties". Then chose "NEW", then "Attribute Name": INIT. It then prompts you for a string by default. And I tried many selections, and many value entries. This is what has not worked in ISE 10.1, schematic entry.

I posted on the Xilinx forums a couple days ago and got no response. sniff, sniff, heh.
Probably means my question is self evident, which got me searching the ISE10.1's help index.

...This morning (11/8/10), I actually read the Help, and searched for "Initialize flip flops and latches".
It describes how to initialize flip flops and latches using a constraints file.
I successfully made one this morning, but can't test it till tomorrow!

(11/9/10) Making a constraints file worked! It gives you the choice to assign 1 or 0 to each bit of any flip flop or latch in the design on powerup.

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65Org16:https://github.com/ElEctric-EyE/verilog-6502

Ah, no. I'm using ISE 12.3 and checked the "CPLD Libraries Guide" (cpld_all_scm.pdf). It lists, amongst others, FDCE, FD4/8/16CE and FDCP, but no FD4/8/16CPs.

I also have to say that I haven't used the schematic entry feature yet, I prefer VHDL :-)

Some 5 or 6 years ago I had a similar problem, I wanted to init registers on a Lattice Mach4A5, using ispLever 4 or so, but couldn't get it to work. I'm not sure what went wrong, the datasheet explicitly mentioned asynchronous reset/preset plus powerup logic in the macrocell description. Maybe I did something wrong, maybe ispLever had some bug (I wouldn't rule that out, I already ran into several ispLever oddities before).

So I added a powerup/reset input to my logic and implemented it like this:

So the init-logic is contained in the VHDL code and I had no problems synthesizing the code for Lattice Mach4 and Xilinx XC95xx/XC95xxXL CPLDs.

so long,

Hias

Wow, finally I confirmed that this code actually work on the Xilinx XC95108 CPLD :-) I don't know what I did different the other times before, but now I have this code in a CPLD-based device (not a CPU - note it sends data when r/-w is high):


with data being the inout port signal, and datain and dataout the internal input (write) and output (read) data busses.

Maybe just the compiler wasn't intelligent enough on my earlier tries to detect this logic or whatever. I did separate the two internal busses (read bus "datain" and write bus "dataout" this time though)

Thanks Ruud!

André