1 - Al Kossow's Bitsavers

2. Memory precharge timeThe memory precharge time must be calculated for consecutive access, refresh, and access grantcycles to ensure that the minimum RAS precharge time is satisfied.a. Access cyclesThe precharge time for access cycles is given by:tRP tSH - tAEH-REH - tt(REH) + tAEl-REl *where tRP RAS precharge timetSH = AS,OS width high (MIN MC68000l6 Spec.)tAEH-REH = Time delay, ALE high to RAS starting high (MAX 4500A-15 Spec.)tAEl-REl * = Time delay, ALE low to RAS starting low (MAX 4500A-15 Spec.)thus,tRP (180 - 25 - 15 + 27) ns= 167 nsb. Refresh cyclesThe minimum precharge time for refresh cycles is given by:tRP = 1.5(T) - tClSH - tAEH-REH - tt(REH) + tCH-RRl *whereT = Clock cycle timetClSH = Clock low to AS, OS high (MAX MC68000l6 Spec.)tCH-RRl * = Time delay, ClK high to refresh RAS starting low (MAX 4500A-15 Spec. *)thus, tRP = [1.5(166) - 75 - 25 - 15 + 45] ns= 179 nsc. Access grant cyclesThe precharge time for access grant cycles is given by:tRP = T - tCH-RRH - tt(REH) + tCH-REl *where tCH-RRH = Time delay, ClK high to refresh RAS starting high (MAX 4500A-15 Spec.)tCH-REl * = Time delay, ClK high to access RAS starting low (MAX 4500A-15 Spec. *)thus, tRP (166 - 35 - 15 + 54) ns170 ns3. ALE to ClK relationshipc:o'';::;coE ....... o.5(I)c:o'';::;co,~C.c.~The ALE low transition must not occur within 10 ns of the ClK low transition.ALE low to ClK low time is given by:tAEl-Cl = .5(T) -tCHSlxwheretCHSlx = Clock high to AS, OS low (MAX, MC68000l6 Spec.)thus tAEl-Cl [.5(166) - 65] ns18 ns• This value should otherwise be a minimum; however as all propagation delays on a given chip will tend to track each other, the maximum value is multipliedby a skew factor to reflect variations in same chip propagation delays. The skew factor for the TMS4500A is 0.9. All values followed by an asterisk areobtained by multiplying the specified maximum value by 0.9.9-75