Purely as an experiment, an academic exercise, I'm trying to interface an MC68882 to my 65C265 board (http://forum.6502.org/viewtopic.php?f=4&t=7866) in "peripheral processor" mode. Unfortunately, I came across a few signals that I'm unfamiliar with:

Address Strobe AS - address is valid
Data Strobe DS - data is valid

As far as I know, we have no equivalents on the 65Cxx's.

Has anyone ever interoperated in a situation requiring these Motorola signals? Is there any way to fake them or cobble them together using the 65C265's available signals? I only came across one post in the archive here, regarding the 68030, but it was exactly the opposite of what I need to do, i.e., using the signals output by the '030. Here, I need to generate them for input into the 68882.

Any suggestions would be appreciated!

Jonathan

the 65816 does have an AS equivalent, VPA and VDA, OR them together and you have a single signal that indicates a valid Address and data being on the bus (only when PHI2 is high of course).

the 65C265 has a similar pin, "BA" or "Bus Available" which is only asserted when it's activly driving the external bus. you should be able to use that signal plus RW and PHI2 to generate the AS and DS signals.

DS = !(BA & !RW & PHI2)
AS = !(BA & PHI2)

just keep in mind that these would only be valid for half a clock cycle, and the MC68882 is likely not fast enough to handle that.

Could I just use the BA signal, directly, for both AS and DS, then? Would that work?

not really. according to the MC68882 datasheet DS is only supposed to be active during a write cycle, and while the 65C265 datasheet says that the Address bus is valid ~90ns after PHI2 falls, it's still a better idea to only use it after PHI2's rising edge, same with the data bus.

so to properly generate AS and DS you do need all 3 signals: BA, RW, and PHI2

Proxy,

The clock running the MC68882 is 20mhz. The 65C265 is running at 8mhz. Do you think that makes a difference, i.e., makes it more likely that the timing with PHI2, RW, and BA will work?

Jonathan