RetroBUS(T) - design
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Author:  Pito [ Mon Aug 15, 2011 5:19 pm ]
Post subject:  RetroBUS(T) - design

Hi, as I am thinking to develop a retrobsd pcb board (64pin pic32mx). And a good minicomputer shall possess a BUS, let us discuss the standard for the retroBUS (T).
My idea is to use:
1. signals PB0-PB15 and RD0-RD11 - 28pin
2. 4 pins GND, and two power rails - 1pin 3.3Volt and 1pin 5Volt
3. 34pin connector 2x17pin, 2.54mm (MLW34A)

All other pins will be used for internal purposes (e.g. 2xSPI, 2xUsarts,1xUSB, 4 Leds, 2 buttons/switches).


Author:  [Papa Damien] [ Wed Aug 17, 2011 1:26 pm ]
Post subject:  Reply To: RetroBUS(T) – design

yes !
* can you reserve a place for that and of course a in/out jack
* add a microsd connector
* can you expose a maximum of pin to headers.

SPI can drive more than one peripherial it should be exposed
add a thin connector for power entrance to with diode to avoid polarity inversion.

cms led blue green red white of the ubw32 are just cool !

look at that format its lovely

I not sure but wee need a 32k Xtal if we want RTC, and if possible a thin permanent power to keep it up to date

Could you expose the maximum of pin of the pic to headers ?

Author:  [Papa Damien] [ Wed Aug 17, 2011 1:33 pm ]
Post subject:  Reply To: RetroBUS(T) – design

7805 power driver has overload consumption, if possible avoid themes. You have schematics here for use modern power driver :

If it's your first board :
there is tutorial on sparkfun for eagle good designs

Good luck and thx a lot. I will buy it if you do

Author:  Pito [ Wed Aug 17, 2011 2:38 pm ]
Post subject:  Reply To: RetroBUS(T) – design

Damien, my intention is to define a bus here, with as minimum pins as possible, but usable for all existing pic32mx (current or future) and having some functionality. On such bus you may connect many modules in a standard manner. So the exposition of all pins is a task for dev boards, though.. See for example Maximite - it has a bus, but with 16 signal pins which is too low count, I think. My idea is to have 16bit + 12bit (good throughput), analog/digital and almost all pic peripherals available.

Author:  [Papa Damien] [ Mon Aug 22, 2011 6:15 am ]
Post subject:  Reply To: RetroBUS(T) – design

I was afraid about that :)
A bus make sense when you have a processor, and many manufacturer for peripherials making, who want stable specification in time for stable market of driver making. It's not our case.

A bus eat externals pin and this pins normaly remain to the end user. What if I need 32 relay, what if I need 48 led... I'll need to add a second pic ? It will hardly limit retrobsd hardware.

I consider Can spi dma etc as peripherials built-in by microchip and I have no need for an external bus. It's simply my personnal point of vue, it may differ from the rest of the world. I'm may be wrong.

Author:  Pito [ Mon Aug 22, 2011 7:50 pm ]
Post subject:  Reply To: RetroBUS(T) – design

When having 16+12bits on the bus you can connect 65536 leds or relays to it (4096 x 16bit buffers).. Or you can connect a tft lcd display to it (16bit data and control inclusive touch screen).. Of course to decide which pic32 peripherals shall be there is tricky, however. And yes, I am thinking on a modular design in retro-style, but much smaller size.

Author:  [Papa Damien] [ Wed Aug 24, 2011 6:33 am ]
Post subject:  Reply To: RetroBUS(T) – design

Yes its true a bus can do that, and you can do that without a bus. May be you should produce more precises specifications.

The problem is that sometimes one pin share multiple peripherial. And you cant use both at the same time. For example you can use rb5 as an in/out digital pin, or as an C1IN. You can use RD11 as INT4 or as PMA14, PMCS1/IC3. If you take RB for a digital bus how can I use the capture compare feature ?

When I ear BUS what I see is digital I/O and adress clock and strobe. special frequency definition. And all of that is digital mode I mean strict I/O, PORT exclusively configured as I/O

I buy such kind of chip for such kind of capabilities : have a built-in capture compare, and non need to glue an external comparator, plugin a bus like I did if I used a PC104. Microcontroller is a motherboard on chip, with special feature like CCP.

I had prefere we plan to write driver for each of this special features wich are particulary painfull to use. I had prefered we plan to abstract this layer instead of plan to ignore that. There is a big chance to crush capabitie of the hardware by plugin a bus on. Unless the BUS is optional in the kernel and in the hardware

I don't understand how you want proceed to have a bus plus this specials capabilities. This one are particularely important because they made what is a microcontroller; thats why people buy it. I consider you are more experimented than me, so may be I had not catch something...?

Author:  [JonathanCline] [ Sat Oct 01, 2011 2:26 am ]
Post subject:  Reply To: RetroBUS(T) – design

It would seem the best bet in this direction would be to use the CAN bus as a high speed transport to a peripheral expansion board. The current UBW32's PIC32 has CAN rate of 1Mbit/sec, for heavy peripherals. Otherwise use I2C for large number of LEDs and GPIOs. It would be great to implement a BSD I2C bus driver for retrobsd.

Author:  Pito [ Sat Oct 01, 2011 10:53 pm ]
Post subject:  Reply To: RetroBUS(T) – design

Jonathan, this might be an way too. There is a lot of SPI i/o expanders, etc. Most probably one will use SPI ADC, DAC, and other stuff, CAN and I2C as well. We need a SPI, I2C, CAN driver for sure. And maybe 8bits for addresing those gadgets (chipselects) and PWM. P.

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