Typewriter User Manual

4- 36 MC68340 USER’S MANUAL MOTOROLA
4.4 MC68340 INITIALIZATION SEQUENCE
The following paragraphs discuss a suggested method for initializing the MC68340 after
power-up.
4.4.1 Startup
RESET is asserted by the MC68340 during the time in which V
CC
is ramping up, the VCO
is locking onto the frequency, and the MC68340 is going through the reset operation. After
RESET is negated, four bus cycles are run, with global CS0 being asserted to fetch the
32-bit supervisor stack pointer (SSP) and the 32-bit program counter (PC) from the boot
ROM. Until programmed differently,
CS0 is a global, 16-bit-wide, three-wait-state chip
select.
CS0 can be programmed to continue decode for a range of addresses after the
V-bit is set, provided the desired address range is first loaded into the
CS0 base address
register. After the V-bit is set for
CS0, global chip select can only be restarted with a
system reset.
After the SSP and the PC are fetched, the module base address register (MBAR) should
be initialized, and the MBAR V-bit should be set (CPU space address $0003FF00) with
the desired base address for the internal modules.
4.4.2 SIM40 Module Configuration
The order of the following SIM40 register initializations is not important; however, time can
be saved by initializing the SYNCR first to quickly increase to the desired processor
operating frequency. The module base address register must be initialized prior to any of
following steps.
Clock Synthesizer Control Register (SYNCR):
• Set frequency control bits (W, X, Y) to specify frequency.
• Select action taken during loss of crystal (RSTEN bit): activate a system reset or
operate in limp mode.
• Select system clock and CLKOUT during LPSTOP (STSIM and STEXT bits).
Module Configuration Register (MCR)
• If using the software watchdog, periodic interrupt timer, and/or the bus monitor, select
action taken when FREEZE is asserted (FRZx bits).
• Select port B configuration (FIRQ bit). Note that this bit is used in combination with
the bits in the PPARB to program the function of the port B pins.
• Select the access privilege for the supervisor/user registers (SUPV bit).
• Select the interrupt arbitration level for the SIM40 (IARBx bits).
Autovector Register (AVR)
• Select the desired external interrupt levels for internal autovectoring.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
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