M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-21
pins are used as a bidirectional data bus. The AS signal is used as an active-high latch
enable to an external address latch. Address information is allowed through this exter-
nal transparent latch while AS is high, and the stable address information is latched
when AS is low. The E clock is used to enable external devices to drive data into the
CPU during the second half of a read bus cycle (E clock high). The R/W signal indi-
cates the direction of data — high for read cycles, low for write cycles.
NOTE
The AS/STRA pin is an output in expanded modes and an input in
single-chip modes. Do not forget to terminate this pin as an unused
input in single-chip modes.
2.3 Termination of Unused Pins
Because the MC68HC11A8 is a CMOS device, unused input pins must be terminated
to assure proper operation and reliability.
Figure 2-14
shows a CMOS inverter, which
is representative of circuitry found on CMOS input pins. When the input is logic zero,
the P-channel transistor is on (conducting), and the N-channel transistor is off. When
the input is logic one, the P-channel transistor is off, and the N-channel transistor is
on. These transistors are actually linear devices with relatively broad switch points. As
the input passes through midsupply, there is a region where both transistors conduct
to some degree. Under normal circumstances, the input does not remain in this linear
region for very long. Once the inverter has completely switched so that only one of the
two transistors is conducting, there is virtually no current flow. This principle is why the
overall current drain of a CMOS device is directly proportional to the rate of switching.
Table 2-2 Ports B and C, STRA, and STRB Pins
Port
B
B
B
B
B
B
B
B
C
C
C
C
C
C
C
C
Bit
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
Single-Chip and Bootstrap Mode
PB0
PB1
PB2
PB3
PB4
PB5
PB6
PB7
PC0
PC1
PC2
PC3
PC4
PC5
PC6
PC7
STRA
Input Strobe (Edge In)
STRB
Expanded-Multiplexed and Special Test Mode
ADDR8
ADDR9
ADDR10
ADDR11
ADDR12
ADDR13
ADDR14
ADDR15
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
AS
R/W
Output
Output
Output
Output
Output
Output
Output
Output
Input/Output
Input/Output
Input/Output
Input/Output
Input/Output
Input/Output
Input/Output
Input/Output
Address Output
Address Output
Address Output
Address Output
Address Output
Address Output
Address Output
Address Output
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address/DataMultiplexed
Address Strobe (Out)
Read/Write Select
Output Strobe