
MOTOROLA
MPC823 USER’S MANUAL
12-1
SYSTEM
INTERFACE
UNIT
12
SECTION 12
SYSTEM INTERFACE UNIT
The system interface unit controls system startup, initialization and operation, protection, as
well as the external system bus. The system configuration and protection function controls
the overall system and provides various monitors and timers, including the bus monitor,
software watchdog timer, periodic interrupt timer, PowerPC decrementer, timebase, and
real-time clock. The clock synthesizer generates the clock signals for other modules and
external devices that the system interface unit uses. This circuitry generates the system
clock from an inexpensive 32kHz crystal or an oscillator with a maximum frequency of
5MHz. The system interface unit supports various low-power modes that supply different
ranges of power consumption, functionality, and wake-up time. The clock scheme supports
low-power modes for applications that use baud rate generators and/or serial ports in
standby mode. The main system clock can be changed dynamically, but the baud rate
generators and serial ports work with a fixed frequency. For more information on clocks,
The external bus interface handles the transfer of information between internal buses and
the memory or peripherals in the external address space. The MPC823 is designed to allow
external bus masters to request and obtain mastership of the system bus. For additional
controller module provides a glueless interface to many types of memory devices and
peripherals and it supports a maximum of eight memory banks, each with their own device
and timing attributes. Memory control services are provided to both internal and external
masters. The MPC823 supports circuitboard test strategies through a user-accessible test
The PCMCIA host adapter module provides all control logic for a PCMCIA interface. A host
adapter interface fully compliant with the PCMCIA Standard, Release 2.1+ (PC Card -16)).
It can support one PCMCIA socket with a maximum of eight memory or I/O windows.
Note: The MPC823’s external address bus is 26 bits wide, while the internal address
bus is 32 bits wide. Therefore, external accesses are considered internally as
26-bit accesses (A[6:31]) with A[0:5] equal to 0, while internal accesses are full
32-bit accesses.
Note: Both the MPC823 and MPC821 have the same PCMCIA module except that the
MPC823 has only one valid slot (Slot B). Programming a window to be assigned
to Slot A may cause an erroneous operation.