1 - Al Kossow's Bitsavers

1. Refresh TimeTABLE 1 -STRAP CONFIGURATIONWAITSTRAP INPUT MODESSTATESCLOCKFOR MINIMUM CYCLESMEMORY REFRESH CLK FREQ. REFRESH FOR EACHTWST FS1 FSO ACCESS RATE IMHz) FREQ.lkHz) REFRES~l l Lt 0 EXTERNAL - REFREQ 4l l H 0 elK + 31 1,984 64 - 95t 3l H l 0 elK + 46 2,944 64 - 85t 3l H H 0 elK + 61 3,904 64 - 82§ 4H l l 1 elK + 46 2,944 64 - 85t 3H l H 1 elK + 61 3,904 64 - 80* 4H H l 1 elK + 76 4,864 64 -7H 4H H H 1 elK + 91 5,824 64 ~ 88+."4This strap configuration resets the Refresh TImer circui.try.Upper figure in refresh frequency is the frequency that is prodllced if the minimum elK frequency of the next select state is Ilsed.Refresh frequency if ClK frequency is 5 MHz.+ Refresh frequency if ClK frequency is 8 MHz.From Table 1 of the TMS4500A data sheet, there are two strap selections that would be appropriate for a 3 MHz clockfrequency. One selection inserts a wait state on every memory access while the other does not. Assuming that no waitstates will be necessary, select the strap input: FSO = TWST = 0 and FS1 = 1. This will yield a refresh rate of (3 MHz)/46or approximately 65 kHz. Each refresh will take 3 clock cycles.2. Memory Precharge Time (RAS precharge)The memory precharge time must be calculated for access, refresh, and access grant memory cycles to ensure the minimumRAS precharge time is satisfied.a. Access CyclesThe precharge time on access cycles is given by:wheretRPtRPtc2td1tc2/4+td1 +tAEL-REL* - td7 - tACH-REH - tt(REH)RAS precharge timeCLKOUT periodDelay from ALATCH low from reference line (ry1AX,99000 Spec.)tc2/4 + 10co"';::mE~~.5enco"~CJ=&c.~thustAEL-RELtd7tACH-REHtt(REH)tRPTime delay, ALE low to RAS starting low (MAX, 4500A-20 Spec. *)WE, AD control release delay (MAX, 99000 Spec.)Time delay, ACX high to RAS starting high (MAX, 4500A-20 Spec.)RAS rise time (MAX, 4500A-20 Spec.)(333/4 + 333/4 + 10 + 36 - 30 - 40 - 20) ns123 ns RAs precharge time.This value should otherwise be a minimum value; however as all propagation delays on a given chip will tend to track each other, the maximumvalue is multiplied by a skew factor to reflect variations in same-chip propagation delays. The skew factor used here is 0.9. AU values (followedby an asterisk) are obtained by multiplying the specified maximum value by 0.9.9-57