1 - Al Kossow's Bitsavers

An external refresh counter has been added to the drive circuit for the Z-80 since the Z-80 internal refresh counter doesnot support 256 cycle refresh. (Application Brief DR-7 shows a circuit to add the extra refresh address bits similar to theimplementation used here.) As the Z-8000 provides 9-refresh-address bits, its internal refresh counter was used.A description of the signals used in both circuits previously illustrated is given in Table'. Due to slight differences in thesignals available from the Z-80 and Z-8000 processors, a slight modification of the interface between the processor and theTTL drive circuits shown will be required. The differences in the interface are shown in Figure 3. The DS signal is generatedfrom the Z-80's RD and WR lines. The B/Vii input should be tied to a 5-' 0 kilohm pullup resistor. The RFSH signal can bedecoded from the status lines of the Z-8000 as shown with the 74S138; however, it could also be done with other typesof logic if desired. The address of the Z-8000A is only guaranteed valid for 35 ns so the address latches are necessarywhen using DRAMs with this microprocessor. MREQ is used to enable the 74LS373 transparent latches for both memoryaccesses and refresh cycles.TABLE 1 -SIGNAL DESCRIPTIONSIGNAL NAMEMREOBAO,BA'BSRFSHDSRO, R1READB/WAO-A15DESCRIPTIONFrom the Z-80 or Z-8000, indicates address validAddress for RAS selection, decoded from high order addresses.Board select to designate DRAM access, decoded from high order addresses.Z-80 output or decoded from the Z-8000 status outputs. Signals a refresh cycle.Z-8000 output indicating data valid on the multiplexed address/data lines.Address for read or write selection, decoded from high order addresses.If low, indicates a memory read and if high, indicates a memory write.Indicates if the Z-8000 is doing a 8-or , 6-bit memory access.Z-BO address outputsc;'D)...O·::Jen5'.......o3Q)...•o·::JADO-AD15Z-80»"C"2-ii5==Q-Wii74S08Z-BOOO multiplexed address/data linesosBANKSElO ------ ROBANKSEL1 ------ RlRo ------ READVee ----'I/II'Ir-- BtWSKUBANKSH2 ------ BAaBANKSEl3 ------ BAIZ-8000os------osAO------ROBANKSElO ------ RlBtW ------ BtWBANKSHI ------ BAaBANKSEl2 ------ BAI+5SKUGlG2AST3STOG2B74S138ASTlST2FIGURE 3 - INTERFACE CIRCUITRY DIFFERENCES9-28