I'm building a circuit with a Rockwell R65C02P4. When the RDY line is pulled low, the manual specifies that the address lines will reflect the current address. It doesn't specify what the R/W state will be, nor what the data lines will be. I am assuming that the data lines will be high impedance (undriven) and that the R/W line will be high. Is this assumption true? Here is the clip from the datasheet: 
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- \$\begingroup\$ If it allows you to single-cycle any instruction it would suggest that it freezes all of the CPU outputs in whatever state they're in. \$\endgroup\$Finbarr– Finbarr2020-04-13 18:32:50 +00:00Commented Apr 13, 2020 at 18:32
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4 The main purpose of RDY is to 'stretch' the bus access time for slow memories. During a write cycle that slow memory needs to see the data and active Write signal telling it to write the data.
The description in the datasheet is incomplete. The entire bus, including data and control lines (not just the address lines) is halted. When RDY is detected during φ2 the data bus and /W line are already active. This state will be held through subsequent φ2's until RDY is released.
- \$\begingroup\$ Thank you. That makes sense for slow memory access, that sometimes it would be driving the the data bus and sometimes it would be reading the data bus.However, for DMA, it seems that it would be desirable for the data bus to be not driven - high impedance. I suppose I have to do that on my own with an external buffer/driver? \$\endgroup\$dustin– dustin2020-04-13 19:45:37 +00:00Commented Apr 13, 2020 at 19:45
- \$\begingroup\$ The 65C02 has a BE input which makes the bus high impedance. This works on read and write cycles, unlike the NMOS 6502 which could only do it by using RDY to halt the cycle before PHI2 (which did not work on write cycles). \$\endgroup\$Bruce Abbott– Bruce Abbott2020-04-13 20:29:50 +00:00Commented Apr 13, 2020 at 20:29
- \$\begingroup\$ According to the Rockwell datasheet, the 65C02 does not have the BE signal. Only the 65C102 and 65C112 have it. \$\endgroup\$dustin– dustin2020-04-13 20:33:04 +00:00Commented Apr 13, 2020 at 20:33
- \$\begingroup\$ There are several different versions of 65C02 made by different manufacturers. WDC 65C02 (W65C02S) has it, but most (all?) others don't. There are other differences too. acornatom.nl/sites/fpga/www.howell1964.freeserve.co.uk/parts/… \$\endgroup\$Bruce Abbott– Bruce Abbott2020-04-13 21:19:25 +00:00Commented Apr 13, 2020 at 21:19