RE: [stella] 7800 programs -- potential problems?

Subject: RE: [stella] 7800 programs -- potential problems?
From: Kevin Horton <khorton@xxxxxxxxxx>
Date: Tue, 25 Jan 2000 01:56:32 -0500 (EST)
At 18:25 1/24/00 +0100, you wrote:
>>Not to mention you only get two writes to the control register! After 2
>>writes, it halts the processor!!  There are two chained flipflops that
>>shift a "1" through each bit, and on the third write to the control register,
>>the last output goes high, which runs thru a transistor to the processor's
>>HALT line, stopping the processor.
>
>Are you sure about this? I think Dan Boris once mentioned to me that
>the processor would only be HALTed after two consecutive writes to the

Yes, pretty sure.  The schematic shows two sections of the 74173 connected up
together in series, with the input of the first in the chain tied to VCC.

On reset, both are cleared to 0.  On the first clock, the first one passes the
1 through to the data input of the next.  On the second clock, the second one
passes this through to the HLT line.  I don't see any way around that, unless
my schematic is wrong, so I guess you only get one write to the control reg?



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