
M68HC11
REFERENCE MANUAL
PARALLEL INPUT/OUTPUT
MOTOROLA
7-23
DDRC data, data in the PORTCL register can change at any time since the strobe A
latching edge is asynchronous. Since it is undesirable for data to be changing at the
instant the CPU is latching in this data, the user should avoid this synchronization haz-
ard. Usually, the system design automatically solves the problem because an edge on
STRA cannot normally occur during a read of PORTCL. For example, in a full-input
handshake, PORTCL is only read in response to recognizing that the STAF has been
set. In such a case, the edge that caused STAF to be set and data to be latched into
PORTCL will have occurred several cycles before the PORTCL read could possibly
occur. Also, in the full-handshake protocol, the external device is inhibited from latch-
ing new data into PORTCL until the previous data is read from PORTCL. This inhibit
is accomplished by the STRB handshake output.
Writes to port C at PORTC or PORTCL and writes to DDRC are controlled by
WPORTC, WPORTCL, and WDDRC, respectively. All three signals are synchronized
to the falling edge of the internal PH2 clock, which corresponds to the center of the E
high time.
7.3.4.5 Special Considerations for Port C on MC68HC24 PRU
Since the external PRU does not have access to the internal PH2 clock of the
MC68HC11A8, there are slight differences in the timing of port B, port C, STRA, and
STRB activities.
Figure 7-14
shows the differences between internal MC68HC11A8
writes to port C and MC68HC24 writes to port C.
7.3.5 AS (STRA) Pin
In expanded modes, this pin acts as the AS control signal, which is used to demultiplex
low-order addresses from data at port C. In single-chip modes, this pin acts as the
STRA input, which serves the handshake I/O subsystem on the MC68HC11A8. The
MC68HC24 can be used to regain the STRA functions when the MCU is operating in
an expanded mode.
7.3.5.1 AS (STRA) Pin Logic
Refer to
Figure 7-15
for the following discussion. When the MC68HC11A8 is operat-
ing in a single-chip mode, the MDA control bit is zero; thus, both the P- and N-channel
output drivers are disabled. While the MCU is operating in an expanded mode, the
MDA control bit enables the output driver logic. As long as the MCU is not in stop
mode, the AS signal is buffered and driven out the AS pin.