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MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-62
as normal address bus pins in expanded operating modes. Refer to 4.2.4 Noise information on the address bus disable bit (ABD).
The ADDR[23:19] pins have multiple functions as high-order address lines, chip
selects, or discrete outputs and are configured differently depending on operating
mode selection and data bus conditioning when RESET is released. The following
paragraphs contain a summary of pin configuration options for each external bus
configuration.
4.7.8.2 Data Bus Mode Selection
DATA[15:0] have weak internal pull-up devices. When pins are held high by the inter-
nal pull-ups, the MCU uses a default operating configuration. Specific lines can be held
low externally during reset to achieve alternate configurations.
NOTE
External bus loading can overcome the weak internal pull-up devices
on data bus lines and hold pins low during reset.
Use an active device to properly configure data bus lines while RESET is low. Data
bus configuration logic must release 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 released. If external mode selection logic causes a conflict of
this type, an isolation resistor on the driven lines may be required. Figure 4-19 shows
a recommended method for conditioning the data bus mode select signals.
The mode configuration drivers are conditioned with R/W and DS to prevent conflicts
between external devices and the MCU when RESET is asserted. If RESET is
asserted during an external write cycle, R/W conditioning (as shown in Figure 4-19)
prevents corruption of the data during the write. Similarly, DS conditions the mode con-
figuration drivers so that external reads are not corrupted when RESET is asserted
during an external read cycle.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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