M68HC11
REFERENCE MANUAL
PARALLEL INPUT/OUTPUT
MOTOROLA
7-47
ers. The CWOM bit simultaneously affects all eight port C bits. The P-channel device
forms a P-N junction between the V
DD
supply and the output pin so that the pin cannot
be pulled more than a diode drop above the V
DD
supply. For this reason, the wired-
OR mode cannot be used for level conversion the way open-collector TTL devices are
sometimes used. In a TTL system, a brief contention between two push-pull drivers,
though not good practice, generally has no serious consequences. In a CMOS sys-
tem, a brief contention between push-pull drivers can induce destructive latchup. In
cases where two CMOS output drivers could be in contention, they should be config-
ured for wired-OR operation. If there is a brief contention between the time one driver
is turned on and the other is turned off, there will be no danger of latchup damage.
HNDS — Handshake/Simple Strobe Mode Select
When HNDS is zero, the simple strobe mode is selected. In the simple strobe mode,
the STRB pin is pulsed for two E-clock cycles after each write to port B. Also, port C
data is asynchronously latched into the PORTCL register each time the selected edge
is detected at the STRA pin. When HNDS is set to one, either full-input or full-output
handshake mode is selected. All full-handshake modes use port C, the STRA strobe
input pin, and the STRB handshake output pin. Since the handshake I/O subsystem
does not use port B when a full-handshake mode is selected, port B defaults to being
a general-purpose output port.
OIN — Output/Input Handshake Select
This bit has no effect unless HNDS is one. When HNDS is one, OIN further qualifies
the handshake mode. When OIN is one, full-output handshake is selected. When OIN
is zero, full-input handshake is selected.
PLS — Strobe B Pulse Mode Select
This control bit determines whether the STRB pin is configured for pulsed or inter-
locked operation. In interlocked mode, once STRB is asserted, it will remain active un-
til an acknowledge edge is detected at the STRA pin. The interlocked mode is selected
when PLS is zero. Interlocked mode cannot be specified unless HNDS is logic one. In
pulsed mode, STRB is negated exactly two E-clock cycles after it is asserted. When
the simple strobe mode is selected (HNDS = 0), the pulsed mode is assumed, even if
PLS is set to one. Additional information about strobe B can be found in
7.3.3 R/W
(STRB) Pin
.
EGA — Edge Select for Strobe A
This control bit selects which polarity edge will be recognized at the STRA input pin.
When EGA is zero, falling edges are detected and rising edges are ignored. When
EGA is one, only rising edges are recognized at the STRA pin. When the three-state
variation of the full-output handshake mode is being used, EGA also specifies the level
on STRA that will cause port C output buffers to be enabled. The output enable for port
C pins is an active-high internal signal, which is the exclusive OR of EGA with the level
at the STRA pin. Thus, the trailing edge of the enable signal on the STRA pin will be
the selected active edge used by the handshake sequence.
INVB — Invert Strobe B
The STRB signal is developed in an S/R flip-flop in the STRB pin logic. The INVB con-
trol bit selects either the Q or Q output of this flip-flop to be coupled out of the STRB