System Integration Module
MOTOROLA
MC68307 USER’S MANUAL
5-33
When all the bits in this field are set to one, DTACK must be generated externally, and the
MC68307 waits for DTACK (input) to terminate its bus cycle.
After system reset, the bits of the DTACK field default to six wait states (M68000 cycles)
or 12 wait states (8051-cycles).
The DTACK generator uses the MC68307 internal processor clock to generate the pro-
grammable number of wait states. The number of wait states remains constant as pro-
grammed even if the processor clock speed is reduced using sleep modes.
NOTE
Do not assert DTACK externally when it is programmed to be
generated internally. The signal is bidirectional, and outputs as
an indication of the internally generated signal.
M23–M13—Base Address Mask
These bits are used to set the block size of a particular chip select line. The address com-
pare logic uses only the address bits that are not masked (i.e., mask bit set to one) to de-
tect an address match within its block size.
0 = The address bit in the corresponding base register (BR) is masked; the address
compare logic does not use this address bit. The corresponding external address
line value is a don't care in the comparison.
1 = The address bit in the corresponding BR is not masked; the address compare logic
uses this address bit.
For example, for a 64-Kbyte block, this field should be M13–M15 = 0 with the rest of the
base address mask bits (M23–M16) equal to one.
After system reset, the bits of the base address mask field default to ones, (selecting the
smallest block size of 8 Kbytes) to allow CS0 to select the ROM device containing the re-
set vector.
MRW—Mask Read/Write
This bit is used to disable or enable the comparison of read or write cycle during the chip
select matching process.
0 = The read/write bit (RW) in the base register (BR) is masked. The chip select is as-
serted for both read and write operations.
1 = The RW bit in the BR is not masked. The chip select is asserted for read-only or
write-only operations as programmed by the corresponding RW bit in BR3–BR0.
After system reset, this bit defaults to zero.