C515A_9708 Datasheet, PDF (52/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6
On-Chip Peripheral Components
This chapter gives detailed information about all on-chip peripherals of the C515A except for the
integrated interrupt controller, which is described separately in chapter 7.
6.1
Parallel I/O
The C515A has six 8-bit digital I/O ports and one 8-bit input port for analog/digital input. Port 0 is an
open-drain bidirectional I/O port, while ports 1 to 5 are quasi-bidirectional I/O ports with internal
pullup resistors. That means, when configured as inputs, ports 1 to 5 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 C515A generally allows digital I/O on 48 lines grouped into 6 bidirectional C501 compatible 8-
bit ports and one 8-bit analog/digital input port. Each port bit (except port 6) consists of a latch, an
output driver and an input buffer. Read and write accesses to the I/O ports P0 to P5 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 6 is used as analog input, an analog channel is switched to the A/D converter through a
3-bit multiplexer, which is controlled by three bits in SFR ADCON (see chapter 6.4). Port 6 lines
may also be used as digital inputs. In this case they are addressed as an input port via SFR P6.
Since port 6 has no internal latch, the contents of SFR P6 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 P6.
This will have no effect.
Semiconductor Group
6-1
1997-08-01