DSP 104-plus, Reference Manual

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The printer port can optionally be configured as an output-only port, an Enhanced
Parallel Port (EPP) and as an Extended Capabilities Printer Port (ECP). In EPP mode
greater throughput is provided by automatically generating strobe signals. In ECP
mode a 16-byte FIFO is provided. Users must provide their own software for these
modes. The parallel port mode can be set with the BIOS Setup program (use the
Advanced / I/O Device Configuration). The port can also be disabled using this Setup
program. See section 6.2 for details of the Setup program.
The I/O signals on the printer port can be treated as general purpose digital input and
output signals, and as such can be used for other applications, such as driving an
alphanumeric LCD display, or inputting from switches, for example.
The printer port control output signals (/STROBE, /AUTOFD, /INIT and /SLCTIN) are
open drain with 4k7 pull-up resistors fitted. The printer port status inputs (BUSY, PE,
SLCT, /ACK and /ERROR) have pull-up and pull-down resistors present, inside the
super I/O chip. There are pull-up resistors on /ACK and /ERROR, and pull-down
resistors on the other three signals. The printer port data lines (PD0-7) are driven to
TTL levels when outputs. When inputs they do not have pull-up resistors fitted.
3.6 REAL TIME CLOCK
Calendar/Clock facilities are provided in PC computers. The calendar/clock module is
often known as the Real Time Clock, or RTC. These RTC functions emulate those
found in the Motorola MC146818 chip, and include time of day functions, calendar
functions and CMOS RAM for storing setup parameters. An alarm facility is also
provided; this allows an interrupt to be generated when a particular time is reached.
Calendar/clock functions are implemented within the PC97317 Super I/O chip.
The calendar/clock hardware is read by the BIOS, which maintains time and date on
behalf of the operating system. The BIOS does this in a millennium-compliant
fashion.
The calendar/clock logic may be accessed through the MS-DOS calls (interrupt 1AH)
or with MS-DOS TIME and DATE commands. As well as the calendar clock functions
there are 242 bytes of static RAM (usually called “CMOS SRAM”) that are backed up
by the battery. Some of this is used to store configuration parameters used by the
BIOS. The serial EEPROM can be used to store these parameters in systems that
have no battery - see section 6.7 for details.
The 242 bytes of SRAM are divided between 114 bytes that are normally accessible
through the RTC index and data registers at I/O addresses 70h and 71h, and 128
further bytes that are accessible in a less direct manner. Users who want to make
use of the further 128 bytes should understand how to do this by reading the
PC97317 data sheet. These 128 bytes are not used by the BIOS.
A battery can be used to provide power to maintain the clock and CMOS RAM when
the main +5V power supply is not present. This external battery should be connected
between the BATT input and GND of J4. The battery voltage should be between 3.0V
and 3.6V and can be either be a rechargeable battery (e.g. NiMH) or a non-
rechargeable battery (e.g. Lithium).
Do not connect a 5V supply to the battery pin. This will prevent the TP400 from
booting. Leaving the battery pin floating may also prevent booting. If no battery