1802 problems

J.C. Wren jcwren at jcwren.com
Fri Dec 30 23:22:03 CST 2005

Oh, also the SC0/SC1 states are only valid on TPA.  Does your datasheet 
call that out?


Tony Duell wrote:

>Does anyone really know the RCA 1802 processor hardware?
>I am still debugging the microwriter, and have not got very far. I have 
>an RCA user manual on the processor which seems quite comprehensive, I am 
>asusming it's accurate.
>Now, according to that manual, the processor can be in one of 4 states 
>(fetch, execute, interrupt responze, DMA), and which state it's in is 
>output on 2 lines called SC0 and SC1. 
>The INT/ line is high, both DMA IN/ and DMA OUT/ are also high. I can 
>detect no gltiches on any of these signals. The clock is running, the bus 
>looks active, etc.
>The problem is that I am getting active-high pulses on SC1. This, 
>according to the data sheet means it's doing a DMA or interrupt response 
>cycle. But why the heck should it?
>I thought it was a defective 1802 chip, but another one pulled from an 
>old board shows identical behaviour (right down to locking up after 
>typing 8 characters on the microwriter keyboard). I can't believe 2 have 
>failed in exactly the same way. Unless of course this is a well-known 
>Any thoughts?

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