Texas Instruments 99/4A - Page 64

INTERRUPT INTERFACE
A block diagram of the interrupt control section is shown in -Figure
24, p. 61. The interrupt inputs (6 dedicated (INTI — INT6), and 9
programmed 1 e ) are sampled on the -Falling edge of 0.3 and latched onto
the chip -For one 03 inverted (interrupts are active low) and ANDed
with its respective mask bit (mask=l, interrupt enabled). On the
rising edge o-F 03, the prioritizer and encoder senses the masked
interrupts and produces a -Four-bit prioritized code and INTREO are
latched off chip with a SYNCLATCH on the falling edge of the next 03,
which ensures proper synchronization to the processor. The interrupt
mask bits on the TNS 9901 PSI are individually set or reset under
software control . Any unused interrupt line should have its
associated mask disabled to avoid false interrupt: to do this, the
control bit (CRU bit zero), is first set to a zero for interrupt mode
operation. Writing to TNS 9901 CRU bits 1-15 indicates the status of
the respective interrupt inputs; thus, the designer can employ the
unused' (disabled) interrupt input lines as data inputs (true data
in).
INPUT/CUT INTERFACE
A block diagram of the TNS 9901 I/O interface is shown in figure 23,
p.60. Up to 16 individually controlled I/O ports are available (seven
dedicated, P0 — P6, and nine programmable) and as discussed above, the
unused dedicated interrupt lines also can be used as input lines (true
data in). Thus the TNS 9901 can be configured to have more than 16
inputs. RST 1 or RST2 (command bit ) is executed. An output port can
be read and indicates the present state of the pin. A pin programmed
to the output mode cannot be used as an input pin: applying an input
current to an output pin may cause damage to the TNS 9901. The TNS
9901 outputs are latched and buffered on chip, and inputs are buffered
onto the chip. The output buffers are NOS-TO TTL buffers and can
drive two standard TTL loads.
PROGRANNABLE PORTS
A total of nine pins on the TNS 9901 are user programmable as either
1/0 ports^.or interrupts. These pins will assume all char ac t er i st i cs
of the type pin they are programmed to be. Any pin which is not being
used for interrupts should have the appropriate interrupt mask
disabled (mask=0) to avoid erroneous interrupts to the CPU. To
program one of the pins as an interrupt, its interrupt mask: simply is
enabled and the line may be used as if it were one of the dedicated
interrupt lines. To program a pin as an 1/0 port, disable the
interrupt mask and use that pin as if it were one of the dedicated I/O
ports .