[Coco] Interfacing 3.3V Logic to the Coco

[RETRO Innovations]

On 10/14/2017 11:31 AM, Michael R. Furman wrote:
>   I’ve got a partial hand-wavy understanding of Push-Pull but Open-Drain?
Instead of repeating something, I thought I'd just link: 
https://electronics.stackexchange.com/questions/28091/push-pull-open-drain-pull-up-pull-down

open drain is used when you need to share a signal without a R/W signal 
to tell folks what you are doing, like you'd do in I2C communication.  
Push Pull is the standard IO pin we all know.
>
> I also took a look at Zippster’s proposed design for the Coco FPGA Experimenter’s Board and he  seems to be using a LCX16245 which is not a level shifter but seems to be a 5V-Tolerant bus transceiver.  I guess this looks appropriate for sticking on the Data or Address Busses where you already know which direction things are going via the R/W line?
Yep.  Though, there are bidirectional versions of the 16245 parts 
without the direction line:


https://www.maximintegrated.com/en/products/interface/level-translators/MAX13103E.html

https://assets.nexperia.com/documents/data-sheet/CBT16211.pdf (12 lines)

But, the latter requires pullups on all lines.

That said, how many lines do you need to level shift?  And, how fast of 
signals do you need to handle (MHz)?  If not very many and not in the 
double digits of MHz, simple voltage dividers will work just as well:

https://learn.sparkfun.com/tutorials/voltage-dividers/all?print=1

1K and 2.2K will form a nice divider, with good speed, no latency, and 
requires only 2 resistors per IO line.  For things like SPI, and such, 
it's ideal (and cheap)

Jim

-- 
RETRO Innovations, Contemporary Gear for Classic Systems
www.go4retro.com
store.go4retro.com