
M68HC11
REFERENCE MANUAL
PARALLEL INPUT/OUTPUT
MOTOROLA
7-15
Figure 7-10 R/W (STRB) Pin Logic
The start strobe B (STARTSTRB) is an active-low signal from the handshake I/O sub-
system. The conditions that enable strobe B depend on the strobe or handshake mode
in effect. The strobe and handshake modes are controlled by the state of the HNDS
and output/input (OIN) control bits in the PIOC register. When HNDS is zero, the sim-
ple strobe mode is selected, and the state of the OIN control bit is ignored. In simple
strobe mode, STARTSTRB is asserted at the rising edge of the internal PH2 clock fol-
lowing a write to the PORTB register. This time corresponds to the center of the E low
time following the write to port B. When HNDS is one and OIN is zero, full-input hand-
shake is selected. In full-input handshake mode, STARTSTRB is asserted at the rising
edge of the internal PH2 clock following a read of the PORTCL register. This time cor-
responds to the center of the E low time following the read of port C latched data.
When HNDS and OIN are one, full-output handshake is selected. In full-output hand-
shake mode, STARTSTRB is asserted at the rising edge of the internal PH2 clock fol-
lowing a write to the PORTCL register. This time corresponds to the center of the E
low time following the write to port C at PORTCL address.
The end strobe B (ENDSTRB) is an active-low signal from the handshake I/O sub-
system. Three possible conditions can cause the ENDSTRB to be asserted. If the
HNDS bit is zero or if the pulse (PLS) control bit is one, ENDSTRB is asserted exactly
two full E-clock periods after STARTSTRB was asserted. This configuration corre-
sponds to the simple strobe mode or a full-handshake mode where strobe B is config-
ured for pulsed-mode operation. The second condition causing ENDSTRB to be
asserted corresponds to the full-handshake modes where strobe B has been config-
[2]
R/W
(STRB)
VDD
P
PIN
N
P
MDA
Q
D
C
Q
HFF
R/W (INTERNAL)
AS
STARTSTRB
ENDSTRB
[3]
[1]
INVB
INVB
Q
Q
[4]
[5]
[6]