Dynamic Shift Registers - Bitsavers - Trailing-Edge

......en...ocaQ)c:Q)~...Q)....CJ...caca.r:.Ug.r:.CJcao...Q.Q.«enEQ)....en>UJQ).r:.I-oqoIZ«to generate a 5 x 7 horizontal scan, so all stages ofthe registers must operate at the full characterrate. To form several rows with a single·loopmemory requires an interlaced scan rather than anordinary raster scan. The first series of 5·dot linesare generated by the first N character outputs asbefore, but the next set of N inputs to the ROMwill generate the first group of 5-dot lines in thesecond row of characters on the display. Therefore,the beam must jump to the new line position.To display four rows of 5 x 7 characters, forinstance, would requ ire a staircase generator thatwould step the beam by the height of nine scanlines (seven dot lines, plus two blank spacing linesbetween rows) three times after the initial scan.Then, as the second of the seven recirculationsbegins, the beam would have to be shifted anadditional line to start the second series of linescans-and so forth.The M-N-N technique does not require any moreregister stages than the M-Ioop technique andsignificantly reduces control and drive circuitrequirements-again producing a lower cost perfunction.REFRESH MEMORY MODULATIONThe technique employed in the M-N-N refreshmemory is called "clock modulation". In otherapplications, it has already been found to significantlyreduce total storage costs. 3 It helps minimizepower dissipation-in most terminals, theamount of power consumed is unimportant initself since line power is used, but registers arepowered by clock drivers and the cost and complexityof the drive network is certainly important.Furthermore, the technique allows long, veryhigh-density MaS circuits, produced by relativelyinexpensive low threshold (bipolar compatible)processes to operate at very high effective characterrates.As shown in Figure 7, the raster scan system usesnine clock intervals to generate a row of characterson the display. Seven are for the high-speed recirculations.During the other two intervals, thefirst N characters are fed back from the output ofthe N register to the input of the M-N registerwhile the N register is loaded from the M-N registerwith a new row's worth of characters. Sincetwo intervals are used for this operation, the registersoperate at only half the character rate. Therest of the time, the M-N register is chargequiescent.Its average clock frequency is onlyabout 11 % of the character rate.In other words, most of the refresh memory(perhaps 90% in a large display system) operates atonly half the character rate (say 1 MHz instead of2 MHz) only two-ninths of the time. The savings inthe drive network alone can be judged from thepower-frequency plot for a typical MOS dynamicregister (Figure 8)3. In addition, the designer canincrease the number of characters generated perrefresh cycle, for a larger display, or increase thenumber of dot lines, for a larger font, or both.Remember, though, that dynamic registers mustbe clocked to retain data. How long can the M-Nregister be turned off? Long enough for practicalapplications. The guaranteed minimum frequencyis temperature dependent, since temperatureaffects charge-storage time. The minimum forNational Semiconductor's MM-series registers is500 Hz at 25°e, rising to 3 kHz at 70 0 e (maximumoperating temperature is 125°e, but that isnot a display environment). At room temperature,EizC>E 0.010iiica: 0.00 I~10=TA'" 25°C-v DD" -tOVI-V q.,=-15V0.000 I10 100 1.000MAXIMUM OPERATING FREQUENCY (KHz)Figure 8. Power vs Frequency Plot of Typical MOSDynamic Registerthe registers can safely be quiescent for as long as2 msec. (The typical MM register will actually holddata for 10 msec.) Suppose the N register stores 40characters and operates at 2 MHz. The quiescentperiod can be as short as 40 x 7 x 0.5 = 140 /lS. Ifstandard TV raster timing is maintained then thequiescent period will be 7 x 63 /lS = 441 /lS.Obviously, the designer has great leeway in characterrates, operating temperatures, and registercapacities.Other applications in displays for clock modulationinclude input-output buffering of data duringdata reception and transmission,2 or during displayediting and formatting through the consolekeyboard. The register rates can be adjusted viacontrol logic to accommodate differences betweenI/O and recirculation rates .. Note that the gating inFigure 7 permits data entry under TTL controlinto either register section.CHARACTER GENERATIONThe first generally available MaS character generatorswere kits such as those in Figure 2, usingthree 1024-bit ROMs (MM521). Although singlechipgenerators were being developed in 1969,they were in very short supply. The kits cost abouthalf as much as diode generators and thus allowedterminal manufacturers to start the changeover toMOS.The kits are also a good place to begin describingcharacter generator operation in this applicationnote, because they provide an "exploded view" ofmulti-ROM generator operation. Similar techniqueswill be needed to build larger fonts with the234