XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
128
2.
Placing the Interrupt Level on the Address output
pins A[3:1].
When the 68000 μP has toggled all of its Function
Code output pins “high”, the “Function Code Decoder”
chip (U3) will read this value from the FC2–FC0 pins
as being the binary value for 7. As result, U3 will
assert its active-low Y7 output pin. At the same time,
the address lines A[3:1] are carrying the current
“Interrupt Level” of this IACK cycle (level = 6, or ‘1 1 0’
in this example) and applying this value to the A, B,
and C inputs of the “IACK Level Decoder” chip (U5).
Initially, all of the outputs of U5 are tri-stated. Due to
the fact that its active-low G2A and G2B inputs are
negated (e.g., at a logic “high”). However, when the
68000 μP begins the IACK cycle, it will assert its
Address Strobe (AS*) signal. This action will result in
asserting the G2A input pin of U5. Additionally, since
the Function Code Decoder chip has also asserted
its Y7 output pin this will, in turn assert the G2B input
pin of U5. At this point, the output of U5 will no longer
be tri-stated. U5 will read in the contents of its A, B,
and C inputs, and assert the appropriate output pin.
In this case, since U5 has the binary value of “6”
applied to its input, it will, in turn assert its active-low
Y6 output pin. The combination of the Int* output (of
the XRT7245) and Y6 (from U5) being asserted will
cause U6A to assert the active-low VPA* (Valid
Peripheral Address) input pin of the 68000 μP Any-
time the 68000 detects its VPA* pin being asserting
during an IACK cycle, it knows that this is an Auto-
vectored Interrupt Cycle. When the 68000 is operating
in an Auto-vectored Interrupt Cycle, it knows that it
will not receive an interrupt vector from the peripheral
device (e.g., the XRT7245 UNI in this case), and that
it must generate its own vector. In the very next bus
cycle, the 68000 μP is going to implement a “pseudo-
read” of the data bus. However, in reality no data will
be read from the XRT7245 device. The 68000 μP will
instead have determined that since this current IACK
is an Auto-vectored—Level 6 Interrupt cycle, which
corresponds to Vector Number 30, within the 68000
μP’s Exception Vector Table, Vector Number 30 cor-
responds to an Address Space of 78h, in the 68000
μP’s address space. In the case of this example, the
user is required to place an unconditional branch
statement (to the location of the XRT7245 Interrupt
Service Routine) at 78h in system level memory.
Table 10 presents the Auto-vector Table (e.g., the
relationship between the Interrupt Level and the
corresponding location in memory for this uncondi-
tional branch statement) for the MC68000 μP
T
ABLE
10: A
UTO
-
VECTOR
T
ABLE
FOR
THE
MC68000 M
ICROPROCESSOR
I
NTERRUPT
L
EVEL
V
ECTOR
N
UMBER
A
DDRESS
S
PACE
1
25
064h
2
26
068h
3
27
06Ch
4
28
070h
5
29
074h
6
30
078h
7
31
07Ch