Maxim Integrated Secure Microcontroller - Manual (Page 99)

Secure Microcontroller User’s Guide
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12. PARALLEL I/O
The secure microcontroller provides four 8-bit bidirectional ports for general-purpose I/O functions. Each
port pin is bit and byte addressable using four SFRs that control the respective port latch. Each bit has an
associated latch (accessed via SFR), input buffer circuit, and output driver circuit. Ports 0, 2, and 3 also
have alternate functions that can be used in place of general I/O. All of the SFR latches for the parallel
port pins are written with 1’s during a hardware reset.
Figure 12-1 through Figure 12-4 illustrates
functional circuit diagrams for bits within each of the four I/O ports. Port 1 has no alternate function; it is
always available for parallel I/O functions.
Ports 0 and 2 can serve as a multiplexed Expanded Memory bus for applications needing memory
mapped I/O. In the DS5001/2FP the Ports 0 and 2 can also serve as a slave RPC interface to a host
microprocessor. Port 3 pins each have individual, optional functions described below. Enabling the
optional function by writing a 1 to the associated latch bit in the Port 3 SFR automatically converts the
I/O pin into its alternate function. For example, enabling the serial port automatically converts P3.0 and
P3.1 into the RXD and TXD function. Alternate functions pins and general I/O pins can be enabled
independent of each other. Enabling selected pins to perform their alternate function leaves the other as
bit addressable I/O pins.
PIN NAME FUNCTION
P3.7
RD
Expanded Data Memory Read Strobe
P3.6 WR Expanded Data Memory Write Strobe
P3.5 T1 Timer/Counter 1 Input
P3.4 T0 Timer/Counter 0 Input
P3.3 INT1 External Interrupt 1 Input
P3.2 INT0 External Interrupt 0 Input
P3.1 TXD Serial Port Transmit Data
P3.0 RXD Serial Port Receive Data
In many cases it may be desirable to use a combination of pure I/O and alternate function pins on port 3.
For example, a user may decide to use the serial port and INTO pins, leaving 5 pins available for use as
general purpose I/O (assuming P3.6 and P3.7 are not being used to access external memory). SETB and
CLR commands can be used to access the general I/O pins without any effect on the pins being used in
their alternate function. If the MOV command is used to write to port 3, however, software must always
write a logic 1 to the pins that are being used in their alternate function. Failure to do so will disturb their
function, resulting in serial port data corruption or disabling of the alternate function in the case of other
pins.