[Coco] Max baud rate for 68681 in MM/1?

[Joel Ewy]

On 01/31/2016 09:38 PM, Dave Philipsen wrote:
> I'm not sure if Motorola (Freescale) even produces the 68C681 anymore 
> but the ones I used were the Exar 44PLCC parts.  The datasheet is a 
> little misleading when it says that maximum bit rate with a 16x clock 
> is 125 Kb/s because later the sheet says the maximum clock frequency 
> is 7.372 MHz which would yield a bit rate of 460.75 Kb/s. I would 
> hazard a guess and say that the Exar 68C681 is quite capable of that 
> speed and I can say for sure that none of the parts I bought ever 
> failed to work with a 4 MHz clock and 250 Kb/s bit rate.
>

FWIW, I still have 17 Signetics 68681C1A44 parts that were intended for 
MM/1 I/O boards in the '90s, most of which never got assembled. I 
assembled boards, but never looked too deeply into the specifics of how 
that chip was used there.  In those days, 34,800 would have been plenty 
adequate, since a 14.4 modem would have been the fastest thing I would 
have been able to hook up to it.  I guess I need to delve into the 
schematics and data sheets.  I was hoping someone on the list might just 
have an answer.

JCE

> Dave
>
> //
> On 1/31/2016 9:22 PM, Dave Philipsen wrote:
>> And any form of 68000 CPU should have no problem servicing interrupts 
>> at 1Mbps.  The 68C681 has a receive buffer so you can receive many 
>> bytes while generating only one interrupt and even if it did generate 
>> an interrupt for every byte that's only 100k interrupts per second 
>> which I believe is quite doable with a 68000. Without looking at the 
>> technical info on the 6809 I would guess a 2 MHz 6809 could do it too 
>> although it might tax it a bit.
>>
>> Dave
>>
>>
>> On 1/31/2016 9:16 PM, Dave Philipsen wrote:
>>> I have personally used a 68C681 UART in a 68HC11-based system that 
>>> transmits DMX512 theatrical standard lighting data at 250 kbps 
>>> (RS422/485).  It is tried and proven and currently running in about 
>>> 400 different locations in the US and a few other countries.  It is 
>>> using a 4 MHz clock as the BRG.
>>>
>>> Dave
>>>
>>>
>>> On 1/31/2016 3:33 PM, Joel Ewy wrote:
>>>> I read ambiguous information about the 68681 on the MM/1's I/O 
>>>> board.  Does anybody have a definitive word?  Page 2-5 of the MM/1 
>>>> technical manual claims that it can be run at "1 megabytes per 
>>>> second" (seriously 8M bits/sec?), but the table on page 9-2 and the 
>>>> text on 9-2 state that the maximum baud rate is 34,800.  Is this a 
>>>> pragmatic maximum based on the CPU's ability to service the DUART's 
>>>> on-chip buffers without interrupt-masking trickery, is it the real 
>>>> hard maximum for the 68681, or is it a hard maximum imposed by the 
>>>> MM/1's hardware? Anybody have any insight?
>>>>
>>>> JCE
>>>>
>>>>
>>>
>>>
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