Specifications

18 Issue 5, August 1994 Acorn Enhanced Expansion Card
Acorn Enhanced Expansion Card
Figure 17: Timings for synchronous cycle types
REF8M
PS
PWE
BD[0:15]
LA[2:15]
PR/W
CLK2
CLK8
BD[0:15]
write
read
PRE
t12
t13
t12
t15 t15
t16
t19
t21
t17 t18
t20
t22
t23
t14
MEMC expansion cards
MEMC expansion cards are not controlled by IOC,
although they share the same interface, so they have to
time their own cycles, with their own I/O control logic.
The interface has two control lines, IORQ (driven by
MEMC) and IOGT (driven by the expansion card or
IOC). IOGT is an open drain signal allowing multiple
devices to drive this signal. MEMC expansion cards are
decoded by LA[21] low, and IOC is decoded by LA[21]
high. But even when IOC is not selected, it continues to
control the external buffer enables, RBE and WBE. The
latching of the buffer must however be controlled by the
expansion card which is controlling the cycle. This is
done by pulling BL low. BL is an open drain signal.
I/O controller interface
I/O controllers use a handshaking system to synchronise
I/O peripherals with the system data bus. The interface
is timed with respect to the REF8M clock, and cycles
may be produced in multiples of 8 MHz clock ticks. When
the processor accesses the I/O controller address space
(while MEMC is in supervisor mode), MEMC starts an
I/O cycle by driving IORQ low and holding the processor
clocks (stretching the processor cycle when PH2 is
high). The I/O controller signals when it is ready to end
the I/O cycle by driving IOGT, low. The I/O cycle ends
when both IORQ and IOGT are seen low on the rising
edge of REF8M. Then MEMC drives IORQ high and
releases the processor clocks. The I/O controller
de-asserts IOGT which goes high on the next falling
edge of REF8M.
Sym Parameter Min Max
t12 CLK2 delay from CLK8 0 15
t13 PS delay from CLK8 5 50
t14 PS hold from CLK8 10
t15 PRE or PWE delay from CLK8 0 15
t16 write data setup to PS 100
t16a write data hold from PWE or CLK2 20
t17 read data setup to PRE or CLK2 50
t18 read data hold from PRE or CLK2 15
t19 address setup to PS 150
t20 add. hold from PRE or PWE or CLK210
t21 PR/W setup to PS 140
t22 PR/W hold from PRE or PWE or CLK2 10
t23 cycle time square wave 500