SYSTEM INTEGRATION MODULE
M68HC16 Z SERIES
5-50
USER’S MANUAL
5.7.3.1 Data Bus Mode Selection
All data lines have weak internal pull-up devices. When pins are held high by the in-
ternal pull-ups, the MCU uses a default operating configuration. However, specific
lines can be held low externally during reset to achieve an alternate configuration.
NOTE
External bus loading can overcome the weak internal pull-up drivers
on data bus lines and hold pins low during reset.
Use an active device to hold data bus lines low. Data bus configuration logic must re-
lease the bus before the first bus cycle after reset to prevent conflict with external
memory devices. The first bus cycle occurs ten CLKOUT cycles after RESET is re-
leased. If external mode selection logic causes a conflict of this type, an isolation re-
sistor on the driven lines may be required.
shows a recommended
method for conditioning the mode select signals.
Figure 5-18 Preferred Circuit for Data Bus Mode Select Conditioning
RESET
DS
R/W
V
DD
V
DD
V
DD
IN8
OUT8
TIE INPUTS
HIGH OR LOW
AS NEEDED
OE
TIE INPUTS
HIGH OR LOW
AS NEEDED
DATA0
DATA7
DATA8
DATA15
10 k
Ω
10 k
Ω
820
Ω
OUT1
IN1
74HC244
IN8
OUT8
OE
OUT1
IN1
74HC244
DATA BUS SELECT CONDITIONING
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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