mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 20

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

by the state of the DATA0 line at the release of the RESET signal. The default bus width out of reset is 16
bits because the DATA0 line is pulled up to logic level one internally; however, the internal pull-up circuit is
weak, so it is best to follow the recommendations in 2.1 Using Data Bus Pins to Configure the MCU
For example, to design a system that uses 16-bit boot memory built from two 27C512 byte EPROMs, con-
nect the chip-select and output enable lines of the EPROMs to the CSBOOT line. Also connect MCU ad-
dress lines ADDR[16:1] to address lines [15:0] of the memories. Do not use ADDR0 of the MCU. This
system will be word accessible only.
In general, the MCU cannot make byte writes to word memory selected by CSBOOT. This lack of byte write
capability is not much of a practical limitation since the CSBOOT signal is generally used for read-only ac-
cess, and all CPU32 instructions must be word-aligned. However, if byte-write capability is required, the SIZ
and CSBOOT signals can be used to generate “high byte” and “low byte” chip-select signals. The only other
way to modify individual bytes is to use word moves, being careful to write the original data back to the un-
changed byte.
2.9.3 Using Chip-Select Signals to Enable External Memory
Chip-select signals can be configured for 8-bit or 16-bit ports. To use an 8-bit memory, connect the memory
element’s data lines to the upper half of the MCU data bus (DATA[15:8]). The MCU reads and writes an 8-
bit port on the upper half of the data bus. During write cycles, data is echoed on the lower half of the data
bus as well. Connect address line ADDR0 of the MCU to A0 of the memory. An example configuration is
shown in Figure 13. To use a 16-bit memory, connect the memory data lines to MCU data bus (DA-
TA[15:0]). Connect address line ADDR1 from the MCU to A0 of the memory.
2.9.3.1 How to Construct Word Memory from Two Byte Memories
For chip-select signals other than CSBOOT, forming word memory that is byte-accessible from two byte-
wide devices is simple. Use a separate chip-select pin for each device, and configure chip-select logic to
decode the upper and lower bytes, respectively. Each of the chip-select circuits must be configured as a 16-
bit port, even though only eight bits of memory are being accessed. This allows both byte and word writes
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ADDR[16:0]
MCU
DATA[15:0]
CSBOOT
R/W
CS0
Figure 13 Using Chip-Select Signals to Enable 8-Bit RAM
Freescale Semiconductor, Inc.
ROM ENABLE
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DATA[15:8]
ADDR[13:0]
DATA
R/W
32K X 8
RAM
ADDR
CE
DATA[15:0]
DATA
32K X 16
ROM
ADDR
CE
ADDR[16:1]
M68331/332TUT/D
MC68331/332
332TUT EXT MEM CONN 1