Diamond Systems RUBY-MM-1612 Special Mode Combination Considerations, Current Drive Capability

82C55A

MODE DEFINITION SUMMARY

	MODE 0		MODE 1		MODE 2
	IN	OUT	IN	OUT	GROUP A ONLY
PA0	In	Out	In	Out
PA1	In	Out	In	Out
PA2	In	Out	In	Out
PA3	In	Out	In	Out
PA4	In	Out	In	Out
PA5	In	Out	In	Out
PA6	In	Out	In	Out
PA7	In	Out	In	Out
PB0	In	Out	In	Out
PB1	In	Out	In	Out
PB2	In	Out	In	Out
PB3	In	Out	In	Out
PB4	In	Out	In	Out
PB5	In	Out	In	Out
PB6	In	Out	In	Out
PB7	In	Out	In	Out
PC0	In	Out	INTRB	INTRB	I/O
PC1	In	Out	IBFB	OBFB	I/O
PC2	In	Out	STBB	ACKB	I/O
PC3	In	Out	INTRA	INTRA	INTRA
PC4	In	Out	STBA	I/O	STBA
PC5	In	Out	IBFA	I/O	IBFA
PC6	In	Out	I/O	ACKA	ACKA
PC7	In	Out	I/O	OBFA	OBFA

Mode 0 or Mode 1 Only

Special Mode Combination Considerations

There are several combinations of modes possible. For any combination, some or all of Port C lines are used for control or status. The remaining bits are either inputs or outputs as defined by a “Set Mode” command.

		INPUT CONFIGURATION
D7	D6	D5	D4	D3	D2	D1	D0
I/O	I/O	IBFA	INTEA	INTRA	INTEB	IBFB	INTRB
		GROUP A				GROUP B

During a read of Port C, the state of all the Port C lines, except the ACK and STB lines, will be placed on the data bus. In place of the ACK and STB line states, flag status will appear on the data bus in the PC2, PC4, and PC6 bit positions as illustrated by Figure 17.

Through a “Write Port C” command, only the Port C pins programmed as outputs in a Mode 0 group can be written. No other pins can be affected by a “Write Port C” command, nor can the interrupt enable flags be accessed. To write to any Port C output programmed as an output in Mode 1 group or to change an interrupt enable flag, the “Set/Reset Port C Bit” command must be used.

With a “Set/Reset Port Cea Bit” command, any Port C line programmed as an output (including IBF and OBF) can be written, or an interrupt enable flag can be either set or reset. Port C lines programmed as inputs, including ACK and STB lines, associated with Port C fare not affected by a “Set/Reset Port C Bit” command. Writing to the correspond- ing Port C bit positions of the ACK and STB lines with the “Set Reset Port C Bit” command will affect the Group A and Group B interrupt enable flags, as illustrated in Figure 17.

		OUTPUT CONFIGURATION
D7	D6	D5	D4	D3	D2	D1	D0
OBFA	INTEA	I/O	I/O	INTRA	INTEB	OBFB	INTRB
		GROUP A				GROUP B
	FIGURE 15. MODE 1 STATUS WORD FORMAT
D7	D6	D5	D4	D3	D2	D1	D0
OBFA	INTE1	IBFA	INTE2	INTRA	X	X	X
		GROUP A				GROUP B
	(Defined by Mode 0 or Mode 1 Selection)

FIGURE 16. MODE 2 STATUS WORD FORMAT

Current Drive Capability

Any output on Port A, B or C can sink or source 2.5mA. This feature allows the 82C55A to directly drive Darlington type drivers and high-voltage displays that require such sink or source current.

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