Bus Operation
MOTOROLA
MC68307 USER’S MANUAL
3-11
3.1.5 CPU Space Cycle
A CPU space cycle, indicated when the internal function codes are all high, is a special pro-
cessor cycle. In the EC000 core, CPU space is used only for interrupt acknowledge cycles.
Figure 3-10 shows the encoding of an interrupt acknowledge cycle. No response is expected
or allowed from external devices. On the MC68307 this cycle is an indication of the internal
interrupt controller’s vector response.
As the MC68307 implementation does not provide function code pins to differentiate CPU
space cycles (the internal function code signals are all high in this type of cycle), care is
required when decoding addresses which may match the above address. No problems will
be encountered as long as the chip selects are used as a term in the decoding logic. This
will ensure that CPU space cycles are not decoded. The interrupt acknowledge cycle places
the level of the interrupt being acknowledged on address bits A3–A1 and drives all other
address lines high. The interrupt acknowledge cycle reads a vector number when the
MC68307 interrupt controller places a vector number on the data bus.
The timing diagram for an interrupt acknowledge cycle is shown in
Figure 3-11.3.1.6 8-Bit M68000 Dynamically-Sized Bus
M68000 8-bit bus cycles appear when the MC68307 dynamic bus sizing is enabled. This
bus sizing adds cycle-by-cycle control of data bus width to the EC000 core bus, as for the
MC68020, but with the difference that the bus width is controlled via the chip selects, rather
than external DSACK1 and DSACK0 inputs.
This provides the flexibility of differing bus widths for RAM, ROM and peripherals, without
the pin overhead of the MC68020 solution (DSACK1, DSACK0, SIZ1 and SIZ0).
Each of the four programmable chip selects has a default bus width of 8- or 16-bits associ-
ated with it. The initial bus width of CS0 is set upon reset by the state of the BUSW external
pin (0 for an 8-bit data bus, 1 for a 16-bit data bus).
The bus widths for CS2, CS3 and CS4 should be programmed during system initialization
using the BUSWx bits in the system configuration register (SCR), according to the system
design. The default after reset is 16-bits wide.
All bus accesses not matched by any of the chip selects or the internal peripheral address
ranges require an external DTACK input to terminate the cycle. The data bus width of such
cycles is 16-bits by default; the EBUSW bit in the system configuration register (SCR) can
be cleared to specify external DTACK cycles as 8-bit data bus.
Figure 3-10. Interrupt Acknowledge Cycle – Address Bus
1 1 1 111 111 1 1 111 111 111
INTERRUPT
ACKNOWLEDGE
LEVEL
1
31 0
23