Looking to build my first SBC, advice welcome!

[BulletMagnet83]

I thought something like this would make a good starting point:

http://searle.hostei.com/grant/6809/Simple6809.html

I'm not too fussy on exactly which version I use, but the 6809 and its variants seem to be quite widely used in older electronic musical instruments, which is one of my main areas of interest at the moment.

If I go through with it and find that I'm capable of assembling a working one, I'd like to alter the design to make it expandable so when I get to writing my own programs for it I could plug in different peripheral components to experiment with, for example DAC, ADC, simple display controllers (LEDs etc, not CRTs), that would be mapped into free address space.

Would I be able to do this just by buffering all the A/D lines from the CPU and run them out to some 40-pin IDC sockets or something? What about the other control lines from the CPU? They're shown connected directly to Vcc but I may one day need to use them for something. Could these be connected to Vcc via weak pull-ups instead and broken out like the A/D lines in case another device needs them? If this approach is correct, what value resistors would be appropriate? If not, what do you veterans suggest?

[legacy]

from the Simple6809 schematic it appears to me there is something wrong with the ACIA around TXclock and RXclock. I think the schematic is not correct, 6850 should have external baud rate generator ... mmm

[BulletMagnet83]

from the Simple6809 schematic it appears to me there is something wrong with the ACIA around TXclock and RXclock. I think the schematic is not correct, 6850 should have external baud rate generator ... mmm

I thought it looked odd actually... but then, it looks like it's fed from the E output of the 6809. Might it be divided down internally somehow, set by some register or another? The page owner seems pretty confident it works. But I'm sure I've seen other designs that needed their very own crystal for that.

[legacy]

I suggest you 8051

i realized this board



8051 has internal uart, like 8085, it only needs 11Mhz clock (precise value in order to have the correct baud rate in the uart section), ram, rom (rom is fetched by PSEN, ram by glue logic), latch (data/address are multiplexed), a pretty glue logic for address-decoding (in order to access the right ram chip or device, e.g. a memory mapped AD, DA) and the proper reset circuit!

it's easy circuit, and you can find "basic" interpreter for intel, or you could put a "monitor" (e.g. paulmon if i remember correctly) inside the eprom and start programming with sdcc, uploading your code and executing it.

[legacy]

I thought it looked odd actually... but then, it looks like it's fed from the E output of the 6809. Might it be divided down internally somehow, set by some register or another? The page owner seems pretty confident it works. But I'm sure I've seen other designs that needed their very own crystal for that.

yes it may be, it should be investigated ... or you could easily build a fix baud rate generator, i have a scheme around in case.

[edavid]

from the Simple6809 schematic it appears to me there is something wrong with the ACIA around TXclock and RXclock. I think the schematic is not correct, 6850 should have external baud rate generator ... mmm

There is nothing wrong with it, it just gives a fixed baud rate of 115.2Kb (or 28.8Kb).

(Actually I think it's out of spec for a 6850, you might want to use a 68B50.)

[BulletMagnet83]

Thanks for the input so far guys Any thoughts on the expansion idea?

[edavid]

Thanks for the input so far guys Any thoughts on the expansion idea?

Sure, you can do it.  You wouldn't want to run long ribbon cables though.

Of the control lines, you would probably only care about the interrupts.  4.7K or 10K pullups would be good.

[legacy]

There is nothing wrong with it, it just gives a fixed baud rate of 115.2Kb (or 28.8Kb).

could you explain more details please ? 6809.E should be related to 6809.CLK
the crystal frequency is 7.3728Mhz, and the bus frequency should be 1/4 of the crystal frequency
so the bus frequency propagated through 6809.E should be 7.3728Mhz / 4 = 1.8432Mhz

am i right ?

we are supposing ACIA clocked at 115200Hz

ACIA is able to divide {RX,TX}clock by {1,16,64} through counter divider select bits registers {CR0, CR1}

1.8432Mhz / 1 = 1.8432Mhz
1.8432Mhz / 16 =  115200Hz
1.8432Mhz / 64 = 28800Hz

6809 datasheet
6850 datasheet

[legacy]

It think this tiny 6809 project looks better, it uses modern FT245 USB-uart

[dannyf]

With most of today's mcu, you don't need the rom nor the rs232 chips. With a bigger chip (like stm32f4), you don't even need the ram -> making it a single chip computer.

[westfw]

I'd love to have some "modern" SBC layouts for 8051, 6809, 6502, etc.  You know - narrow or SOIC *big* RAM chips, ISP-capable cpus (if applicable)  EEPROM/Flash instead of EPROM.  FTDI chip or connector instead of max232.  Serial-bootloadable.

Except I'm not quite sure what that means.  SMT or TH?  Let people populate it with those "junk" parts they've collected, or require new (but cheap!) purchases?  Tiny board or big board?  protoboard friendly, Shield-friendly, or just "whatever" ?  GALs were all the rage for decoding "last time around", but these days no one has programmers and chips are hard to get.  Do you aim for low power (there goes the GAL), or do you not care?    And why struggle with all that when you can go out an get an Arduino for $20?

[Crazy Ape]

I'd love to have some "modern" SBC layouts for 8051, 6809, 6502, etc.  You know - narrow or SOIC *big* RAM chips, ISP-capable cpus (if applicable)  EEPROM/Flash instead of EPROM.  FTDI chip or connector instead of max232.  Serial-bootloadable.

Except I'm not quite sure what that means.  SMT or TH?  Let people populate it with those "junk" parts they've collected, or require new (but cheap!) purchases?  Tiny board or big board?  protoboard friendly, Shield-friendly, or just "whatever" ?  GALs were all the rage for decoding "last time around", but these days no one has programmers and chips are hard to get.  Do you aim for low power (there goes the GAL), or do you not care?    And why struggle with all that when you can go out an get an Arduino for $20?

SMT or TH?

Surface Mount Technology or Through Hole.

But I think you know that, so I'm puzzled by

Except I'm not quite sure what that means.  SMT or TH?

[legacy]

I'd love to have some "modern" SBC layouts for 8051, 6809, 6502, etc.  You know - narrow or SOIC *big* RAM chips, ISP-capable cpus (if applicable)  EEPROM/Flash instead of EPROM.  FTDI chip or connector instead of max232.  Serial-bootloadable.

Except I'm not quite sure what that means.  SMT or TH?

means to be practical ! You should prefer DIP packages because they don't require to be soldered, they needs sockets which need TH, but the PCB looks easier in TH way. Unfortunately modern PC are missing the RS232 port, so you can have still fun using USB-2-UART, and this is the reason of the FT245 chip! Is it provided in smd package ? You have to choose if to solder the chip to a smd-dip adapter or to solder the chip directly on the PCB.


about Arduino ... you can choose how to have fun, or to blink a led just writing a sketch in C++ on a system on chip, so called Avr8, ram, rom, uart all inside the chip, or to build an SBC in where you have to provide ram, rom, uart, and glue logic to handle them, and toy with assembly and C in order to put a monitor, then a firmware on the board. Which is the most amazing ? it depends on you !

[legacy]

personally i like boards like this one



it is a 32bit with nvram, i have realized in my spare time, all in DIP packages

[marshallh]

6800-based smart VFD display

[edavid]

6800-based smart VFD display

Hey, that's a 6803!  Much fancier than a 6800.

[westfw]

I meant: what would people WANT on a "modern" x51 SBC?
I have a (paper, untested, unbuilt) 50x50mm (cheap to have fabbed) smt PCB.
I have a similar single-sided TH design (80x100mm) (theoretically homebrewable.)
Each has generic 40pin x51, 32k (or more) RAM, latch, GAL, and Serial EEPROM for holding a serial-downloaded program for copying to RAM.

I don't know whether either one is worthwhile, or whether I should just pony up $100 for a board from silabs...

[GK]

Meh. Upgrade the front shocks and shoehorn in a BBC.

[BulletMagnet83]

Legacy, that 68k board looks niiice I'd love to work my way up to something like that one day...  but the thought of soooo many A/D lines to wire up makes me die a little bit inside, and saps my enthusiasm! Maybe one day. I have some old books on that CPU after all.

Why struggle when I could get an Arduino for $20? Well... I have some of those already! And while there's still plenty to be learned on the software side of things using that platform, I find the hardware a bit boring. A project that requires good assembly, test and debugging skills on the hardware side will give me training that's more applicable to my current job. Recreating, adapting and expanding upon those simple designs will, in my opinion, eventually make me a better engineer

[legacy]

Legacy, that 68k board looks niiice I'd love to work my way up to something like that one day

thank you, i am trying to build a plastic case in order to let her to looks more prettier. Also the bad side of the story is firmware ... well i have written an easy bootloader able to upload srec file. The solution is easy: you upload an exec, you put it into non-volatile-ram. You can also see the pretty toy-language called "little" i have developed in C++ and opened this this thread. The idea is to provide something to be run in this kind of SBC, but you may prefer Basic, forth, or whatever you want. "little" is written in C++ and the binary may be too heavy for a tiny SBC, but you could port "little" to C, reducing and optimizing (guess if it is really possible, but it may be), or you could rewrite it from the beginning! 6809 SBC has a basic rom, it is pretty from what i can see from the emulator. Check it out!

[westfw]

I guess that part of my personal problem is that I like SBCs to be "minimal" (like, right now, I'm thinking "if I put a 256k NVRAM chip on there connected to the 8051, for both program and data memory, then using a GAL or even the traditional 7400+74138 logic for memory interfaces is probably overkill.")  But if you're going to build a minimal system, you might as well skip the more complex SBC and just use a more modern single-chip solution.  Sigh.

[legacy]

I think the real fun comes combining an old CPU with CPLD or FPGA, in case i am using spartan_II, which are 5V tolerant and PLCC packaged!

[westfw]

You know, working on a DIP-based SBC-type system, with those big parallel address and data buses going for inches or (remembering things like S100 and MultiBus) feet, diving between and around pins, makes me feel like the time I've spent carefully minimizing and isolating traces on arduino-sized boards (mm instead of cm) has been pretty silly.

Of course, perhaps that old technology really is Just That Slow.

[BulletMagnet83]

Thanks for the input Wilfred! 6809 dev tools do seem to be a bit thin on the ground compared to the others so I am starting to question the wisdom of my choice there Trying to do as much research as possible before wasting money on parts I can't use, without also using that as an excuse to avoid building anything! Seems like the Z80 or 6502 versions have more potential as there's no shortage of software I suppose for my intended purpose either would do equally well. Hopefully doing some bits-buying after payday... but we'll see how that works out.