Re: UDS/LDS

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Posted by Michael van Dam on February 24, 1998 at 10:32:52:

In Reply to: UDS/LDS posted by Jim on February 24, 1998 at 09:26:18:

: Does the UDS get used in part 2 of the lab?
: Because only 8 bits need be transferred, thus you
: only need A18..A1, and it will always be LDS?

: So in the handout, where $1234 was placed at
: location $200000, the LDS was asserted. If UDS was
: asserted there would be garbage there?

: I assume that in Part1 to transfer an upper byte,
: that it occupies D7..D4?

: Thanks
: Jim

Not quite... There are 16 data lines on the 68000 bus
altogether, and to reduce wiring time, you need only
connect 8 of these: D7..D0. (I'm talking about Part 2
here.) These 8 data lines comprise the lower byte.
D15..D8 are the upper byte.

This doesn't mean you can ignore UDS though. You still
need to respond. When the processor reads the entire word,
(by asserting UDS and LDS), your circuit should return
the lower byte from memory. The upper byte is not connected
and the CPU will capture garbage when it reads D15..D8.
When the processor reads just the upper byte, then you
still have to respond with DTACK to avoid a bus error,
but the processor will be reading garbage from the
unconnected lines.

Hope this helps...
Mike

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Comments:
: : Does the UDS get used in part 2 of the lab? : : Because only 8 bits need be transferred, thus you : : only need A18..A1, and it will always be LDS? : : So in the handout, where $1234 was placed at : : location $200000, the LDS was asserted. If UDS was : : asserted there would be garbage there? : : I assume that in Part1 to transfer an upper byte, : : that it occupies D7..D4? : : Thanks : : Jim : : Not quite... There are 16 data lines on the 68000 bus : altogether, and to reduce wiring time, you need only : connect 8 of these: D7..D0. (I'm talking about Part 2 : here.) These 8 data lines comprise the lower byte. : D15..D8 are the upper byte. : This doesn't mean you can ignore UDS though. You still : need to respond. When the processor reads the entire word, : (by asserting UDS and LDS), your circuit should return : the lower byte from memory. The upper byte is not connected : and the CPU will capture garbage when it reads D15..D8. : When the processor reads just the upper byte, then you : still have to respond with DTACK to avoid a bus error, : but the processor will be reading garbage from the : unconnected lines. : Hope this helps... : Mike

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