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CMS82C55AZ Datasheet(PDF) 5 Page - Intersil Corporation |
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CMS82C55AZ Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 26 page 5 FN6140.2 June 15, 2006 Operational Description Mode Selection There are three basic modes of operation than can be selected by the system software: Mode 0 - Basic Input/Output Mode 1 - Strobed Input/Output Mode 2 - Bidirectional Bus When the reset input goes “high”, all ports will be set to the input mode. After the reset is removed, the 82C55A can remain in the input mode with no additional initialization required. The control word register will contain 9Bh. During the execution of the system program, any of the other modes may be selected using a single output instruction. This allows a single 82C55A to service a variety of peripheral devices with a simple software maintenance routine. Any port programmed as an output port is initialized to all zeros when the control word is written. The modes for Port A and Port B can be separately defined, while Port C is divided into two portions as required by the Port A and Port B definitions. All of the output registers, including the status flip-flops, will be reset whenever the mode is changed. Modes may be combined so that their functional definition can be “tailored” to almost any I/O structure. For instance: Group B can be programmed in Mode 0 to monitor simple switch closings or display computational results, Group A could be programmed in Mode 1 to monitor a keyboard or tape reader on an interrupt- driven basis. The mode definitions and possible mode combinations may seem confusing at first, but after a cursory review of the complete device operation a simple, logical I/O approach will surface. The design of the 82C55A has taken into account things such as efficient PC board layout, control signal definition vs. PC layout and complete functional flexibility to support almost any peripheral device with no external logic. Such design represents the maximum use of the available pins. Single Bit Set/Reset Feature (Figure 5) Any of the eight bits of Port C can be Set or Reset using a single Output instruction. This feature reduces software requirements in control-based applications. When Port C is being used as status/control for Port A or B, these bits can be set or reset by using the Bit Set/Reset operation just as if they were output ports. FIGURE 3. BASIC MODE DEFINITIONS AND BUS INTERFACE DATA BUS 8I/O B PB7-PB0 4I/O PC3-PC0 4I/O C PC7-PC4 8I/O A PA7-PA0 CONTROL BUS ADDRESS BUS RD, WR 82C55A D7-D0 A0-A1 CS MODE 0 8I/O B PB7-PB0 CONTROL C 8I/O A PA7-PA0 MODE 1 OR I/O CONTROL OR I/O 8I/O B PB7-PB0 C BI- A PA7-PA0 MODE 2 CONTROL DIRECTIONAL FIGURE 4. MODE DEFINITION FORMAT D7 D6 D5 D4 D3 D2 D1 D0 PORT C (LOWER) 1 = INPUT 0 = OUTPUT PORT B 1 = INPUT 0 = OUTPUT MODE SELECTION 0 = MODE 0 1 = MODE 1 GROUP B PORT C (UPPER) 1 = INPUT 0 = OUTPUT PORT A 1 = INPUT 0 = OUTPUT MODE SELECTION 00 = MODE 0 01 = MODE 1 GROUP A 1X = MODE 2 MODE SET FLAG 1 = ACTIVE CONTROL WORD MS82C55A, MQ82C55A, MP82C55A |
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