2.3.5 Peripheral Control Functions

2.3.5.1 I/O Ports

Ports A, B and D each consist of an 8-bit Peripheral Data Register (PR) and an 8-bit Data Direction Register (DDR). Port E consists of a 2-bit PR and DDR registers. If a bit in the DDR is set to one, the corresponding bit in the PR is an output, if a DDR bit is set to a zero, the corresponding PR bit is defined as an input. On a READ, the PR bit reflects the information present on the actual port pins (PRA0-PRA7,PRB0-PRB7,PRC2,PRD0-PRD7, PRE0-PRE1) for both input and output bits. All ports have passive pull-up devices as well as active pull-ups, providing both CMOS and TTL compatibility. in addition to normal I/O operation, PRB6,PRB7,PRD6 and PRD7 also provide timer output functions (refer to Control Register section, 2.5.8).

Only bit PC2 and DPC2 of PORT C meet the above description. The other bits function as described in the following.

2.3.5.2 Handshaking

Handshaking on data transfers can be accomplished using the PC/ output pin and either the FLAGA/ or FLAGB/ input pin. The PC/ line will go low and stay low for two cycles, two cycles after a read or write to PORT D. This is required to meet Centronics Parallel Interface specs. The PC/ line can be used to indicate "data ready" at PORT D or "data accepted" from PORT D. Handshaking on 16-bit data transfers (using either PORT A or B and then PORT D) is possible by always reading or writing PORT A or PORT B first. The FLAG/ lines are negative edge sensitive inputs which can be used for receiving the PC/ output from other 4510 devices, or as general purpose interrupt inputs. A negative transition on FLAGA/ or FLAGB/ will set the FLAGA or FLAGB interrupt bits, respectively.