Technical data

MOTOROLA MC68HC912B32
52 MC68HC912B32TS/D
9 Resets and Interrupts
CPU12 exceptions include resets and interrupts. Each exception has an associated 16-bit vector, which
points to the memory location where the routine that handles the exception is located. Vectors are
stored in the upper 128 bytes of the standard 64-Kbyte address map.
The six highest vector addresses are used for resets and non-maskable interrupt sources. The remain-
der of the vectors are used for maskable interrupts, and all must be initialized to point to the address of
the appropriate service routine.
9.1 Exception Priority
A hardware priority hierarchy determines which reset or interrupt is serviced first when simultaneous
requests are made. Six sources are not maskable. The remaining sources are maskable, and any one
of them can be given priority over other maskable interrupts.
The priorities of the non-maskable sources are:
1. POR or RESET
pin
2. Clock monitor reset
3. COP watchdog reset
4. Unimplemented instruction trap
5. Software interrupt instruction (SWI)
6. XIRQ signal (if X bit in CCR = 0)
9.2 Maskable Interrupts
Maskable interrupt sources include on-chip peripheral systems and external interrupt service requests.
Interrupts from these sources are recognized when the global interrupt mask bit (I) in the CCR is
cleared. The default state of the I bit out of reset is one, but it can be written at any time.
Interrupt sources are prioritized by default but any one maskable interrupt source may be assigned the
highest priority by means of the HPRIO register. The relative priorities of the other sources remain the
same.
An interrupt that is assigned highest priority is still subject to global masking by the I bit in the CCR, or
by any associated local bits. Interrupt vectors are not affected by priority assignment. HPRIO can only
be written while the I bit is set (interrupts inhibited). Table 17 lists interrupt sources and vectors in default
order of priority.
Table 17 Interrupt Vector Map
Vector Address Interrupt Source
CCR
Mask
Local Enable
Register (Bit)
HPRIO Value to
Elevate
$FFFE, $FFFF Reset None None –
$FFFC, $FFFD COP clock monitor fail reset None COPCTL (CME, FCME) –
$FFFA, $FFFB COP failure reset None COP rate selected –
$FFF8, $FFF9 Unimplemented instruction trap None None –
$FFF6, $FFF7 SWI None None –
$FFF4, $FFF5 XIRQ X bit None –
$FFF2, $FFF3 IRQ I bit INTCR (IRQEN) $F2
$FFF0, $FFF1 Real time interrupt I bit RTICTL (RTIE) $F0
$FFEE, $FFEF Timer channel 0 I bit TMSK1 (C0I) $EE
$FFEC, $FFED Timer channel 1 I bit TMSK1 (C1I) $EC
$FFEA, $FFEB Timer channel 2 I bit TMSK1 (C2I) $EA