6-9
ST78C34
Rev. 3.00
written into the FIFO and the counter will not incre-
ment.
Two interrupt modes are available and are selected
with the INTSEL pin. If this pin is tied low, a latched
interrupt will result. In this mode, INT will transition low
when a “1” is written to Control Register bit-0. A reset
or reading the Status Register will clear the interrupt.
If INTSEL pin is tied high, INT will transition low when
a “1” is written to Control Register bit-0 and will
transition high when a write to the parallel port is
issued. This (non-latched) interrupt signal is always
available in Status Register bit-6 regardless of the
state of the INTSEL pin. Status Register bit-2 will
always contain the latched interrupt state. The polarity
of the INT pin may be inverted by setting Alternate
Function Register bit-6 high.
The ST78C34 provides additional programmable in-
terrupt output options by programming the Alternate
Function Register bit 4-5. INT output can be selected
as FIFO full or FIFO empty interrupt.
REGISTER DESCRIPTIONS
PORT REGISTER
Bi-directional printer port.
Writing to this register during output mode will transfer
the contents of the data bus to the PD7-PD0 ports .
Reading this register during input mode will transfer
the states of the PD7-PD0 to the data bus. This
register will be set to the output mode after reset.
PR BIT 7-0:
PD7-PD0 bi-directional I/O ports.
STATUS REGISTER
This register provides the state of the printer outputs
and the interrupt condition.
STR BIT 1-0:
This bits are set to “1” normally except when AFR bit
5-4 are both set to “1”.
STR BIT-2:
Interrupt condition.
0= an interrupt is pending
This bit will be set to “0” at the falling edge of the -ACK
input.
1= no interrupt is pending
Reading the STATUS REGISTER will set this bit to
“1”.
STR BIT-3:
ERROR input state.
0= ERROR input is in low state
1= ERROR input is in high state
STR BIT-4:
SLCT input state.
0= SLCT input is in low state
1= SLCT input is in high state
STR BIT-5:
PE input state.
0= PE input is in low state
1= PE input is in high state
STR BIT-6:
-ACK input state.
0= -ACK input is in low state
1= -ACK input is in high state
STR BIT-7:
BUSY or FIFO full signal.
0= BUSY input is in high state
1= BUSY input is in low state
FIFO is enabled.
0= FIFO is full
1= One or more empty locations in FIFO
COMMAND REGISTER
The state of the -STROBE, -AUTOFDXT, INIT, -
SLCTIN pins, and interrupt enable bit can be read by
this register regardless of the I/O direction.
COM BIT-0:
-STROBE input pin.
0= -STROBE pin is in high state
1= -STROBE pin is in low state
COM BIT-1:
-AUTOFDXT input pin.
0= -AUTOFDXT pin is in high state
1= -AUTOFDXT pin is in low state
