System Integration Module
5-36
MC68307 USER’S MANUAL
MOTOROLA
which are not currently configured as outputs. If a port line is reconfigured from being an
input to being an output, then the most recently latched value appears on that pin.
If the corresponding port A bit is configured as a general-purpose input in the PADDR and
PACNT, then the current status of that pin can be read by reading PADAT and examining
the value in that bit position. Reading PADAT bits for pins configured as general-purpose
outputs simply returns the output value as stored in the latch. Reading PADAT bits for pins
configured as dedicated inputs/outputs for on-chip peripherals returns the current state of
the pin, whether it is an input or an output.
1 = The corresponding port A bit (input or output) currently holds a logic 1 (high) value
(read cycle) OR a logic 1 is to be stored to that bit (write cycle).
0 = The corresponding port A bit (input or output) currently holds a logic 0 (low) value
(read cycle) OR a logic 0 is to be stored to that bit (write cycle).
5.2.3.4 PORT B CONTROL REGISTER (PBCNT). This 16-bit read/write register is used by
the user to specify the function of the I/O lines in port B, the 16-bit I/O port, as general-
purpose I/O, or dedicated I/O for the on-chip peripherals.
From a cold reset, these register bits are all cleared, thus configuring all port B I/O lines as
general-purpose inputs. Bootstrap software must write an appropriate value into PBCNT to
reflect the system hardware setup, before any of the peripherals which need port B pins can
be used properly. Pullup resistors may be required if the dedicated peripheral outputs (which
share port B pins) are used, as these signals would otherwise momentarily float at cold reset
until configured in the PBCNT register.
CB15–CB0—Port B Pin Assignment Control
These bits are used to determine whether the corresponding port B input/output line are
configured as general-purpose functions or dedicated functions.
0 = The corresponding port B I/O bit is to be a general-purpose I/O bit.
1 = The corresponding port B I/O bit is to be a
dedicated I/O bit.
5.2.3.5 PORT B DATA DIRECTION REGISTER (PBDDR). This 16-bit read/write register
is used by the user to program the general-purpose I/O lines in port B, the 16-bit I/O port,
as inputs or outputs.
From a cold reset, these register bits are all cleared, configuring all port B I/O lines as
general-purpose inputs. Bootstrap software must write an appropriate value into PBDDR if
PBCNT
MBASE+$016
15
0
CB15
CB14
CB13
CB12
CB11
CB10
CB9
CB8
CB7
CB6
CB5
CB4
CB3
CB2
CB1
CB0
RESET:
0
Read/Write
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