MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-61
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 CPU32 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.
4. Set the BYTE field to lower byte when using a 16-bit port, as the external vector
for a 16-bit port is fetched from the lower byte. Set the BYTE field to upper byte
when using an 8-bit port.
If an interrupting device does not provide a vector number, an autovector acknowl-
edge must be generated, either by asserting the AVEC pin or by generating AVEC
internally using the chip-select option register. This terminates the bus cycle.
5.9.4 Chip-Select Reset Operation
The least significant bit of each of the 2-bit chip-select pin assignment fields in
CSPAR0 and CSPAR1 each have a reset value of one. The reset values of the most
significant bits of each field are determined by the states of DATA[7:1] during reset.
There are weak internal pull-up drivers for each of the data lines so that chip-select
operation is selected by default out of reset. However, the internal pull-up drivers can
be overcome by bus loading effects.
To ensure a particular configuration out of reset, use an active device to put the data
lines in a known state during reset. The base address fields in chip-select base
address registers CSBAR[0:10] and chip-select option registers CSOR[0:10] have the
reset values shown in Table 5-21. The BYTE fields of CSOR[0:10] have a reset value
of “disable”, so that a chip-select signal cannot be asserted until the base and option
registers are initialized.