MC68711E20CFNE3 Datasheet, PDF (101/242 Pages) –

Port E
6.6 Port E
Port E is used for general-purpose static inputs or pins that share functions with the analog-to-digital (A/D)
converter system. When some port E pins are being used for general-purpose input and others are being
used as A/D inputs, PORTE should not be read during the sample portion of an A/D conversion.
Address: $100A
Bit 7
6
5
4
3
2
1
Bit 0
Read:
PE7
PE6
PE5
PE4
PE3
PE2
PE1
PE0
Write:
Reset:
Indeterminate after reset
Alternate Function: AN7
AN6
AN5
AN4
AN3
AN2
AN1
AN0
Figure 6-9. Port E Data Register (PORTE)
6.7 Handshake Protocol
Simple and full handshake input and output functions are available on ports B and C pins in single-chip
mode. In simple strobed mode, port B is a strobed output port and port C is a latching input port. The two
activities are available simultaneously.
The STRB output is pulsed for two E-clock periods each time there is a write to the PORTB register. The
INVB bit in the PIOC register controls the polarity of STRB pulses. Port C levels are latched into the
alternate port C latch (PORTCL) register on each assertion of the STRA input. STRA edge select, flag,
and interrupt enable bits are located in the PIOC register. Any or all of the port C lines can still be used
as general-purpose I/O while in strobed input mode.
Full handshake modes use port C pins and the STRA and STRB lines. Input and output handshake
modes are supported, and output handshake mode has a 3-stated variation. STRA is an edge-detecting
input and STRB is a handshake output. Control and enable bits are located in the PIOC register.
In full input handshake mode, the MCU asserts STRB to signal an external system that it is ready to latch
data. Port C logic levels are latched into PORTCL when the STRA line is asserted by the external system.
The MCU then negates STRB. The MCU reasserts STRB after the PORTCL register is read. In this mode,
a mix of latched inputs, static inputs, and static outputs is allowed on port C, differentiated by the data
direction bits and use of the PORTC and PORTCL registers.
In full output handshake mode, the MCU writes data to PORTCL which, in turn, asserts the STRB output
to indicate that data is ready. The external system reads port C data and asserts the STRA input to
acknowledge that data has been received.
In the 3-state variation of output handshake mode, lines intended as 3-state handshake outputs are
configured as inputs by clearing the corresponding DDRC bits. The MCU writes data to PORTCL and
asserts STRB. The external system responds by activating the STRA input, which forces the MCU to drive
the data in PORTC out on all of the port C lines. After the trailing edge of the active signal on STRA, the
MCU negates the STRB signal. The 3-state mode variation does not allow part of port C to be used for
static inputs while other port C pins are being used for handshake outputs. Refer to the 6.8 Parallel I/O
Control Register for further information.
M68HC11E Family Data Sheet, Rev. 5.1
Freescale Semiconductor
101