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XC2200 Derivatives
System Units (Vol. 1 of 2)
Architectural Overview
User’s Manual
2-16
V2.1, 2008-08
ArchitectureX22, V1.1
Peripheral Interfaces
The on-chip peripherals generally have two different types of interfaces: an interface to
the CPU and an interface to external hardware. Communication between the CPU and
peripherals is performed through Special Function Registers (SFRs) and interrupts. The
SFRs serve as control/status and data registers for the peripherals. Interrupt requests
are generated by the peripherals based on specific events which occur during their
operation, such as operation complete, error, etc.
To interface with external hardware, specific pins of the parallel ports are used, when an
input or output function has been selected for a peripheral. During this time, the port pins
are controlled either by the peripheral (when used as outputs) or by the external
hardware which controls the peripheral (when used as inputs). This is called the
‘alternate (input or output) function’ of a port pin, in contrast to its function as a general
purpose I/O pin.
Peripheral Timing
Internal operation of the CPU and peripherals is based on the system clock (f
SYS
). The
clock generation unit uses external (e.g. a crystal) or internal clock sources to generate
the system clock signal. Peripherals can be disconnected from the clock signal either
temporarily to save energy or permanently if they are not used in a specific application.
Peripheral SFRs may be accessed by the CPU once per state. When an SFR is written
to by software in the same state where it is also to be modified by the peripheral, the
software write operation has priority. Further details on peripheral timing are included in
the specific sections describing each peripheral.
Programming Hints
•
Access to SFRs:
All SFRs reside in data page 3 of the memory space. The following
addressing mechanisms allow access to the SFRs:
– Indirect or direct addressing with
16-bit (mem) addresses
must guarantee that the
used data page pointer (DPP0 … DPP3) selects data page 3.
– Accesses via the Peripheral Event Controller (
PEC
) use the SRCPx and DSTPx
pointers instead of the data page pointers.
–
Short 8-bit (reg) addresses
to the standard SFR area do not use the data page
pointers but directly access the registers within this 512-byte area.
–
Short 8-bit (reg) addresses
to the extended
ESFR
area require switching to the
512-byte Extended SFR area. This is done via the EXTension instructions EXTR,
EXTP(R), EXTS(R).
•
Byte Write Operations
to wordwide SFRs via indirect or direct 16-bit (mem)
addressing or byte transfers via the PEC force zeros in the non-addressed byte. Byte
write operations via short 8-bit (reg) addressing can access only the low byte of an
SFR and force zeros in the high byte. It is therefore recommended, to use the bitfield