C517A_99 Datasheet, PDF (67/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C517A
6 On-Chip Peripheral Components
This chapter gives detailed information about all on-chip peripherals of the C517A except for the
integrated interrupt controller, which is described separately in chapter 7.
6.1 Parallel I/O
The C517A has seven 8-bit digital I/O ports and one 8-bit and one 4-bit input port for analog/digital
input. Port 0 is an open-drain bidirectional I/O port, while ports 1 to 6 are quasi-bidirectional I/O ports
with internal pullup resistors. That means, when configured as inputs, ports 1 to 6 will be pulled high
and will source current when externally pulled low. Port 0 will float when configured as input.
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. In this function, port 0 is not an open-drain port, but uses a strong internal pullup FET .
6.1.1 Port Structures
The C517A generally allows digital I/O on 56 lines grouped into 7 bidirectional C501 compatible 8-
bit ports and one 8-bit and one 4-bit analog/digital input port. Each port bit (except port 7, 8) consists
of a latch, an output driver and an input buffer. Read and write accesses to the I/O ports P0 to P6
are performed via their corresponding special function registers. Depending on the specific ports,
multiple functions are assigned to the port pins. These alternate functions of the port pins are listed
in table 6-1.
When port 7 or 8 is used as analog input, an analog channel is switched to the A/D converter
through a 4-bit multiplexer, which is controlled by three bits in SFR ADCON1. Port 6 lines may also
be used as digital inputs. In this case they are addressed as an input port via SFR P7 or P8. Since
ports 7 and 8 have no internal latch, the contents of SFR P7 or P8 only depends on the levels
applied to the input lines. It makes no sense to output a value to these input-only port by writing to
the SFR P7 or P8. This will have no effect.
Semiconductor Group
6-1