MC68HC11E0CFNE3 Datasheet, PDF (25/242 Pages) Freescale Semiconductor, Inc – M68HC11E Family

Pin Descriptions
1.4.9 STRA/AS
The strobe A (STRA) and address strobe (AS) pin performs either of two separate functions, depending
on the operating mode:
• In single-chip mode, STRA performs an input handshake (strobe input) function.
• In the expanded multiplexed mode, AS provides an address strobe function.
AS can be used to demultiplex the address and data signals at port C. Refer to Chapter 2 Operating
Modes and On-Chip Memory.
1.4.10 STRB/R/W
The strobe B (STRB) and read/write (R/W) pin act as either an output strobe or as a data bus direction
indicator, depending on the operating mode.
In single-chip operating mode, STRB acts as a programmable strobe for handshake with other parallel
devices. Refer to Chapter 6 Parallel Input/Output (I/O) Ports for further information.
In expanded multiplexed operating mode, R/W is used to indicate the direction of transfers on the external
data bus. A low on the R/W pin indicates data is being written to the external data bus. A high on this pin
indicates that a read cycle is in progress. R/W stays low during consecutive data bus write cycles, such
as a double-byte store. It is possible for data to be driven out of port C, if internal read visibility (IRV) is
enabled and an internal address is read, even though R/W is in a high-impedance state. Refer to
Chapter 2 Operating Modes and On-Chip Memory for more information about IRVNE (internal read
visibility not E).
1.4.11 Port Signals
Port pins have different functions in different operating modes. Pin functions for port A, port D, and port
E are independent of operating modes. Port B and port C, however, are affected by operating mode. Port
B provides eight general-purpose output signals in single-chip operating modes. When the microcontroller
is in expanded multiplexed operating mode, port B pins are the eight high-order address lines.
Port C provides eight general-purpose input/output signals when the MCU is in the single-chip operating
mode. When the microcontroller is in the expanded multiplexed operating mode, port C pins are a
multiplexed address/data bus.
Refer to Table 1-1 for a functional description of the 40 port signals within different operating modes.
Terminate unused inputs and input/output (I/O) pins configured as inputs high or low.
1.4.12 Port A
In all operating modes, port A can be configured for three timer input capture (IC) functions and four timer
output compare (OC) functions. An additional pin can be configured as either the fourth IC or the fifth OC.
Any port A pin that is not currently being used for a timer function can be used as either a general-purpose
input or output line. Only port A pins PA7 and PA3 have an associated data direction control bit that allows
the pin to be selectively configured as input or output. Bits DDRA7 and DDRA3 located in PACTL register
control data direction for PA7 and PA3, respectively. All other port A pins are fixed as either input or
output.
PA7 can function as general-purpose I/O or as timer output compare for OC1. PA7 is also the input to the
pulse accumulator, even while functioning as a general-purpose I/O or an OC1 output.
M68HC11E Family Data Sheet, Rev. 5.1
Freescale Semiconductor
25