Z84C15 - using it without enabling CPU ??

Marten Feldtmann

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Sep 17, 2019, 2:51:59 AM9/17/19

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Strange idea, but would this be possible? Just to get access to PIO, CTC and SIO ? Perhaps setting BUSRQ to HIGH permanently ?

Bill Shen

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Sep 17, 2019, 7:10:15 AM9/17/19

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Z84C15 has an "evaluator" mode where the internal Z80's bus will tri-state so an external CPU can take over the on-board I/O. The pin to control that is "EV" (pin 67 of 100QFP). I accidentally connect EV to VCC on my MicroZ design which totally disable the CPU and I was sweating for a while thinking my part is dead on arrival. Once I disconnected the EV pin and ground it, it worked just fine.
Bill

Marten Feldtmann

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Sep 17, 2019, 4:17:10 PM9/17/19

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That's pretty amazing:

"For the Z84013/015, together with /BUSREQ, the RV signal puts the IPC into the evaluation mode. When this signal ecomes actve, the status of /M1, /HALT and /RFSH change to input. When using Z84013/015 as an evaluator chip, the CPU is electrically disconnected after one machine cycle is executed with the EV signal "1" and the /BUSREQ signal "0". It follows the instruction from the other CPU (of ICE ???). Upon receiving /BUSREQ: A15-A0, /MREQ, /IORQ, /RD and /WR are changed to input ad D7-D0 changes its direction, /BUSACK is NOT 3-stated so it should be disconnece by an externally connected circuit"

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