MC68HC16Z1 Datasheet, PDF (174/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series

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5.9.3 Using Chip-Select Signals for Interrupt Acknowledge
Ordinary bus cycles use supervisor or user space access, but interrupt acknowledge
bus cycles use CPU space access. Refer to 5.6.4 CPU Space Cycles and 5.8 Inter-
rupts for more information. There are no differences in flow for chip selects in each
type of space, but base and option registers must be properly programmed for each
type of external bus cycle.
During a CPU space cycle, bits [15:3] of the appropriate base register must be config-
ured to match ADDR[23:11], as the address is compared to an address generated by
the CPU. ADDR[23:20] follow the state of ADDR19 in this MCU. The states of base
register bits [15:12] must match that of bit 11.
Figure 5-23 shows CPU space encoding for an interrupt acknowledge cycle. FC[2:0]
are set to %111, designating CPU space access. ADDR[3:1] indicate interrupt priority,
and the space type field (ADDR[19:16]) is set to %1111, the interrupt acknowledge
code. The rest of the address lines are set to one.
FUNCTION
CODE
ADDRESS BUS
INTERRUPT
ACKNOWLEDGE
20
111
23
19 16
0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 LEVEL 1
CPU SPACE
TYPE FIELD
CPU SPACE IACK TIM
Figure 5-23 CPU Space Encoding for Interrupt Acknowledge
Because address match logic functions only after the EBI transfers an interrupt ac-
knowledge cycle to the external address bus following IARB contention, chip-select
logic generates AVEC or DSACK signals only in response to interrupt requests from
external IRQ pins. If an internal module makes an interrupt request of a certain priority,
and the chip-select base address and option registers are programmed to generate
AVEC or DSACK signals in response to an interrupt acknowledge cycle for that priority
level, chip-select logic does not respond to the interrupt acknowledge cycle, and the
internal module supplies a vector number and generates an internal DSACK signal to
terminate the cycle.
Perform the following operations before using a chip select to generate an interrupt ac-
knowledge signal.
1. Program the base address field to all ones.
2. Program block size to no more than 64 Kbytes, so that the address comparator
checks ADDR[19:16] against the corresponding bits in the base address regis-
ter. (The CPU16 places the CPU space bus cycle type on ADDR[19:16].)
3. Set the R/W field to read only. An interrupt acknowledge cycle is performed as
a read cycle.
5-68
SYSTEM INTEGRATION MODULE
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M68HC16 Z SERIES
USER’S MANUAL