C517A Datasheet, PDF (30/72 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C517A
Digital I/O Ports
The C517A allows for digital I/O on 56 lines grouped into 7 bidirectional 8-bit ports. Each port bit
consists of a latch, an output driver and an input buffer. Read and write accesses to the I/O ports P0
through P6 are performed via their corresponding special function registers P0 to P6.
The output drivers of port 0 and 2 and the input buffers of port 0 are also used for accessing external
memory. In this application, port 0 outputs the low byte of the external memory address, time-
multiplexed with the byte being written or read. Port 2 outputs the high byte of the external memory
address when the address is 16 bits wide. Otherwise, the port 2 pins continue emitting the P2 SFR
contents.
Analog Input Ports
Ports 7 (8-bit) and 8 (4-bit) are input ports only and provide two functions. When used as digital
inputs, the corresponding SFR P7 and P8 contains the digital value applied to the port 7/8 lines.
When used for analog inputs the desired analog channel is selected by a four-bit field in SFR
ADCON1. Of course, it makes no sense to output a value to these input-only ports by writing to the
SFR P7 or P8. This will have no effect.
lf a digital value is to be read, the voltage levels are to be held within the input voltage specifications
(V_IL/V_IH). Since P7 and P8 are not bit-addressable, all input lines of P7 and P8 are read at the same
time by byte instructions.
Nevertheless, it is possible to use port 7 and 8 simultaneously for analog and digital input. However,
care must be taken that all bits of P7 and P8 that have an undetermined value caused by their
analog function are masked.
Semiconductor Group
28