MC68336/376
SYSTEM INTEGRATION MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
5-23
Selects for more information. The autovector function is disabled when there is an
5.5.2 Dynamic Bus Sizing
The MCU dynamically interprets the port size of an addressed device during each bus
cycle, allowing operand transfers to or from 8-bit and 16-bit ports.
During an operand transfer cycle, an external device signals its port size and indicates
completion of the bus cycle to the MCU through the use of the DSACK inputs, as
shown in Table 5-11. Chip-select logic can generate data and size acknowledge sig-
If the CPU is executing an instruction that reads a long-word operand from a 16-bit
port, the MCU latches the 16 bits of valid data and then runs another bus cycle to
obtain the other 16 bits. The operation for an 8-bit port is similar, but requires four read
cycles. The addressed device uses the DSACK signals to indicate the port width. For
instance, a 16-bit device always returns DSACK for a 16-bit port (regardless of
whether the bus cycle is a byte or word operation).
Dynamic bus sizing requires that the portion of the data bus used for a transfer to or
from a particular port size be fixed. A 16-bit port must reside on data bus bits [15:0],
and an 8-bit port must reside on data bus bits [15:8]. This minimizes the number of bus
cycles needed to transfer data and ensures that the MCU transfers valid data.
The MCU always attempts to transfer the maximum amount of data on all bus cycles.
For any bus access, it is assumed that the port is 16 bits wide when the bus cycle
begins.
Operand bytes are designated as shown in Figure 5-9. OP[0:3] represent the order of
access. For instance, OP0 is the most significant byte of a long-word operand, and is
accessed first, while OP3, the least significant byte, is accessed last. The two bytes of
a word-length operand are OP0 (most significant) and OP1. The single byte of a byte-
length operand is OP0.
Table 5-11 Effect of DSACK Signals
DSACK1
DSACK0
Result
1
Insert Wait States in Current Bus Cycle
1
0
Complete Cycle — Data Bus Port Size is 8 Bits
0
1
Complete Cycle — Data Bus Port Size is 16 Bits
00
Reserved