Re: Plus/4 RS232 woes

On 05/09/2018 19:40, Jim Brain wrote:
>
> a crystal input is a clock input.  It may be imprecise, but most 
> datasheets use the same term for both input types.
>
No, a clock input (at least a single phase like is used on the 6551) is 
a single pin. A crystal has two pins.

You can use a clock input directly, a crystal needs a circuit to 
generate a clock from. This clock circuit is integrated in the 6551.

It's not clear from the datasheet whether the circuit is bypassed when 
you select the /16 external clock mode, or whether an external clock 
will go through the internal clock circuit.

I'd like to see a decap of the chip to know what clock circuit it uses 
and what it does when you select / 16 external clock mode.


> The 6551 came out years earlier.  And, Commodore and Apple knew about 
> the speed.  Again, the drivers were no doubt the limiting factor.

Except it seems to work reasonably reliably & commodore repeatedly ran 
chips outside spec where they barely work. The A2232 board came out in 
1990, way late enough that 115200 would have been useful.

The people who wrote that driver obviously didn't know that it could do 
it, or they would have used it.


> You're misreading the DS.  There is no requirement that the /16 mode 
> only works with an oscillator.  It will work fine with a crystal.

I think you're misreading it.

In the control register it calls it "16x External clock"

It differentiates between crystal and clock when discussing the XTAL1 & 
XTAL2 inputs.

"XTAL1, XTAL2 (Crystal Pins) These pins are normally directly connected 
to the external crystal (1.8432 M Hz) used to derive the various baud 
rates. Alternatively, an externally generated clock may be used to drive 
the XTAL1 pin, in which case the XTAL2 pin must float. XTAL1 is the 
input pin for the transmit clock."

The person who wrote the datasheet chose to make the distinction between 
clock and crystal and refer to the 15 baud rates with regards to the 
crystal and the /16 when referring to the clock. There is not one place 
I can find that lets me think it's safe to come to a conclusion that it 
was designed to do this.


>
> Still, I can confirm that /16 and all of the regular bps rates work 
> with both oscillators and crystals, so the ACIA does not care.
>
I understand that it works. Just because it works, doesn't mean it was 
designed.