Dynamic Shift Registers - Bitsavers - Trailing-Edge

enEQ)...en>en..."CcooJ:l>Q)~r:Q)(,)CO...'t:Q)r:KEYBOARD MATRIX,....---------------,i!(t(~I Lf+.-1J Lff,-U II _ ' IL _____________ ..JNUMERICSALPHA ALARM OUT VGGI "R6A5R4R LINESR3R2TI9DATA BUSTIS:~:~llL~~I----be PARITY1----.,KMr_-bsPARITYI--+-bs PARITYCODN:a~~~ER ~--+-b4 PARITY~_-+_b3PARrTY1--+-b2 PARITY~--+-bl PARITYT LINESR1ROTO 11 T2 T9 TilTIleLKv~CLOCKDM885DSN14tDlCONTROLLOWERCASEFIGURE 7. ASCII Keyboard Encoder Using the MM5704NLnIz«command. This will cause the keyboard to sendits data from the keyboard shift register out ontothe data bus (microinstruction) during time slot 22through 30 immediately following the receipt ofthe transmit command. The data bits are receivedby the T & C chip. The static switch information,from the keyboard, is handled in a similar mannerby the system. It is the intent of this section toonly deal with the interaction of the KI chip withthe rest of the system. Therefore, a more detailedtreatment of the operation beyond this pointbelongs in a description of the T & C chip, asrelated to MAPS.THE KEYBOARD INTERFACE ELEMENTAS A STANDARD ASCII ENCODE SYSTEMThe MM5704 may be used quite conveniently inapplications other than as part of the calculatorchip system for which it was designed. To illustratehow this may be done, a small keyboard systemwas designed to perform the keyboard to ASCIIencoding function. Only one of the MM5704swas required to provide for all 128 characters inthe ASCII 7-bit communications code. This systemis illustrated in Figure 7.A two phase clock must be generated and a mastercounter designed to operate from the clock system.A DM8850 (9601) is used to drive a JKflip-flop in the toggle mode of operation. Thisforms a master clock that has an equal duty cycle.One eight bit shift register and one SN74107 isused to form a twisted ring counter. This type ofcounter was chosen because of the ease withwhich its states may be decoded. One and 1/2 quad2 input NAND gates are used to decode thecounter outputs into the discrete time slots requiredfor the bit serial operation of the KI.Because there is no need to concern ourselveswith anything other than the time slots requiredto operate the KI chips, the time slots have beenreorganized and the word length shortened. This ismore clearly defined and illustrated in Figure 8.The cycle marker signal is generated by setting alatch at time 0 and resetting it at time 1. CM isonly used, in this system, by the KI chip.The keyboard switch entry operation is accomplishedexactly as described in a previous sectionof this application note. The KI chip will interrogatethe keyboard switch matrix. The alpha/numeric control is accomplished externally by a258