
CPU16
REFERENCE MANUAL
SYSTEM RESOURCES
MOTOROLA
3-9
The EBI transfers information between the MCU and external devices. It supports
byte, word, and long-word transfers. Data ports of 8 and 16 bits can be accessed
through the use of asynchronous cycles controlled by the data transfer (SIZ1 and
SIZ0) and data size acknowledge pins (DSACK1 andDSACK0). Multiple bus cycles
may be required for an operand transfer to an 8-bit port, due to misalignment or to port
width smaller than the operand size.
Port width is defined as the maximum number of bits accepted or provided during a
bus transfer. External devices must follow the handshake protocol described below.
3.5.1 Bus Control Signals
Control signals indicate the beginning of the cycle, the address space and size of the
transfer, and the type of cycle. The selected device controls the length of the cycle.
Strobe signals, one for the address bus and another for the data bus, indicate the va-
lidity of an address and provide timing information for data. The EBI operates asyn-
chronously for all port widths. A bus cycle is initiated by driving the address, size,
function code, and read/write outputs.
3.5.1.1 Function Codes
Function codes are automatically generated by the CPU16. Since the CPU16 always
operates in supervisor mode (FC2 = 1) FC1 and FC0 are encoded to select one of four
address spaces. One encoding (%00) is reserved. The remaining three spaces are
called program space, data space and CPU space. Program and data space are used
for instruction and operand accesses. CPU space is used for control information not
normally associated with read or write bus cycles, such as interrupt acknowledge cy-
cles, breakpoint acknowledge cycles, and low power stop broadcast cycles. Function
codes are valid while address strobe AS is asserted. The following table shows ad-
dress space encoding.
Table 3-2 Address Space Encoding
3.5.1.2 Size Signals
SIZ0 and SIZ1 indicate the number of bytes remaining to be transferred during an op-
erand cycle. They are valid while the AS is asserted. The following table shows SIZ0
and SIZ1 encoding.
Table 3-3 Size Signal Encoding
FC2
1
1
1
1
FC1
0
0
1
1
FC0
0
1
0
1
Address Space
Reserved
Data Space
Program Space
CPU Space
SIZ1
0
1
1
0
SIZ0
1
0
1
0
Transfer Size
Byte
Word
3 Byte
Long Word
F
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Go to: www.freescale.com
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