C515C_9711 Datasheet, PDF (64/268 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515C
6 On-Chip Peripheral Components
This chapter gives detailed information about all on-chip peripherals of the C515C except for the
integrated interrupt controller, which is described separately in chapter 7.
6.1 Parallel I/O
6.1.1 Port Structures
Digital I/O Ports
The C515C allows for digital I/O on 49 lines grouped into 6 bidirectional 8-bit ports and one 1-bit
port. 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 P7 are performed via their corresponding special function registers P0
to P7. The port structure of port 5 of the C515C is especially designed to operate either as a quasi-
bidirectional port structure, compatible to the standard 8051-Family, or as a genuine bidirectional
port structure. This port operating mode can be selected by software (setting or clearing the bit
PMOD in the SFR SYSCON).
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 6 is available as input port only and provides two functions. When used as digital inputs, the
corresponding SFR P6 contains the digital value applied to the port 6 lines. When used for analog
inputs the desired analog channel is selected by a three-bit field in SFR ADCON0 or SFR ADCON1.
Of course, it makes no sense to output a value to these input-only ports by writing to the SFR P6.
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 P6 is not bit-addressable, all input lines of P6 are read at the same time by byte
instructions.
Nevertheless, it is possible to use port 6 simultaneously for analog and digital input. However, care
must be taken that all bits of P6 that have an undetermined value caused by their analog function
are masked.
In order to guarantee a high-quality A/D conversion, digital input lines of port 6 should not toggle
while a neighbouring port pin is executing an A/D conversion. This could produce crosstalk to the
analog signal.
Semiconductor Group
6-1
1997-11-01