MOTOROLA
7-48
PARALLEL INPUT/OUTPUT
M68HC11
REFERENCE MANUAL
pin. If INVB is zero, the Q of this latch is coupled out of the STRB pin, and STRB sig-
nals are active low. If INVB is one, the Q of this latch is used, and STRB signals are
active high. Changes to INVB do not affect the state of the internal S/R flip-flop.
7.4.5 Non-Handshake Uses of STRA and STRB Pins
When not being used for handshake functions, the STRA pin can be used as a gener-
al-purpose edge-detection interrupt source, which is fairly common use for the STRA
pin. The STAF is set each time a selected edge is recognized. The STAI control bit
allows strobe A edges to force a maskable interrupt to the IRQ vector. The EGA control
bit allows the user to select either rising edges or falling edges as the triggering edge
for the strobe A input.
Though not a very common practice, the STRB pin can be used as an extra static out-
put. When full-input handshake mode is selected, STRB remains at its inactive level
until the PORTCL register is read. If PORTCL is never read, STRB stays at its inactive
level indefinitely. The INVB control bit allows the user to switch the inactive level from
one to zero by writing to the PIOC register. In this scheme, it is important never to read
the PORTCL register because this would cause STRB to automatically go to its active
level. Other similar schemes may be developed to meet specific application needs.
Usually when the STRA and STRB pins are being used for non-handshake functions,
the handshake I/O subsystem would be configured for full-input handshake mode be-
cause the other two modes result in interactions between the strobe pins and the port
B and C pins. If simple strobe mode is selected, any write to port B will generate a
pulse on the STRB pin. If full-output handshake is selected, each time the STRA pin
goes to its selected active level, all port C pins are forced to be outputs (even if the
DDRC bits indicate they should be inputs). These interactions are a normal conse-
quence of the handshake I/O functions but could interfere with non-handshake use of
the STRA and STRB pins. For this reason, users are encouraged to study the opera-
tion of the handshake I/O subsystem carefully if they plan to use STRA and STRB for
non-handshake functions.