
MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-60
When a match occurs, the chip-select signal is asserted. Assertion occurs at the same
time as AS or DS assertion in asynchronous mode. Assertion is synchronized with
ECLK in synchronous mode. In asynchronous mode, the value of the DSACK field
determines whether DSACK is generated internally. DSACK[3:0] also determines the
number of wait states inserted before internal DSACK assertion.
The speed of an external device determines whether internal wait states are needed.
Normally, wait states are inserted into the bus cycle during S3 until a peripheral
asserts DSACK. If a peripheral does not generate DSACK, internal DSACK generation
must be selected and a predetermined number of wait states can be programmed into
the chip-select option register. Refer to the SIM Reference Manual (SIMRM/AD) for
further information.
5.9.3 Using Chip-Select Signals for Interrupt Acknowledge
Ordinary bus cycles use supervisor or user space access, but interrupt acknowledge
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.
Figure 5-21 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.
Figure 5-21 CPU Space Encoding for Interrupt Acknowledge
Because address match logic functions only after the EBI transfers an interrupt
acknowledge 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,
11111111111111111111
1
111
LEVEL
19
16
23
FUNCTION
CODE
20
0
CPU SPACE
TYPE FIELD
ADDRESS BUS
INTERRUPT
ACKNOWLEDGE
CPU SPACE IACK TIM