
CPU16
REFERENCE MANUAL
SYSTEM RESOURCES
MOTOROLA
3-11
tor number to locate an interrupt handler routine. If AVEC is continuously asserted, au-
tovectors will be generated for all external interrupt requests. AVEC is ignored during
all other bus cycles.
3.5.5 Data Transfer Mechanism
EBI architecture supports byte, word, and long-word operands, allowing access to 8-
and 16-bit data ports through the use of asynchronous cycles controlled by the data
transfer and size acknowledge inputs (DSACK1and DSACK0).
3.5.5.1 Dynamic Bus Sizing
The EBI dynamically interprets the port size of the addressed device during each bus
cycle, allowing operand transfers to or from 8- and 16-bit ports. During an operand
transfer cycle, the slave device signals its port size and indicates completion of the bus
cycle to the EBI through the use of the DSACKx inputs, as shown in the following table.
For example, if the CPU16 is executing an instruction that reads a long-word operand
from a 16-bit port, the EBI latches the 16 bits of valid data and 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 EBI transfers valid data.
The EBI always attempts to transfer a maximum amount of data during each bus cycle.
For a word operation, it is assumed that the port is 16 bits wide when the bus cycle
begins. Operand bytes are designated as shown in
Figure 3-2
. OP0 is the most sig-
nificant byte of a long-word operand, and OP3 is the least significant byte. 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 3-4 Effect of DSACK Signals
DSACK1
1
1
0
0
DSACK0
1
0
1
0
Result
Insert Wait States in Current Bus Cycle
Complete Cycle — Data Bus Port Size is 8 Bits
Complete Cycle — Data Bus Port Size is 16 Bits
Reserved
F
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