Motorola USB08 - Manual (Page 50)

Designer Reference Manual USB08 Evaluation Board
50 Software Module Descriptions MOTOROLA
Software Module Descriptions
practically without function; however, it can be used in the debugging
phase for seeking out unexpected (spurious) interrupts. In addition, the
allocation of all interrupt vectors is important for the implementation of
the FLASH memory security feature (read-out protection). For additional
information, refer to the MC68HC908JB8 Technical Data, Motorola
document order number MC68HC908JB8/D.
The reset vector refers to the start address of the application. This point
of entrance is located in the C startup module.
3.7 C Startup Module CRTSJB8.S
The C startup module used essentially corresponds to the standard
startup module from the Cosmic C Compiler package with one
exception. Immediately after a reset (and after the initialization of the
stack pointer), a subroutine reference was inserted to _HC08Setup().
This subfunction is defined in
U08MAIN.C and performs urgent
accesses to M68HC08 control registers which should be completed
immediately after the reset.
3.8 Push Button Module U08KEY.C
Port A (PTA) of the MC68HC908JB8 has eight port bits for keyboard
connection. Each of these lines can cause an interrupt. The port bits
individually can be configured for use within the MC68HC908JB8
keyboard interrupt module (KBI).
The USB08 evaluation board uses three single keys which are
connected to port lines PTA[4:6]. The switch noise reduction is
performed via a resistor-capacitor (RC) combination at each key. This
combination is made by:
• M68HC08 internal pullup resistors and a capacitor (parallel to the
key)
• Hysteresis of the port A input Schmitt triggers