Pin Definitions and Functions
Port 4 is a 2-bit bidirectional I/O port. It is bit-wise
programmable for input or output via direction bits. For a pin
configured as input, the output driver is put into high-impedance
In case of an external bus configuration, Port 4 can be used to
output the segment address lines:
A16 Least Significant Segment Addr. Line
A17 Most Significant Segment Addr. Line
XTAL1: Input to the oscillator amplifier and input to the
internal clock generator
XTAL2: Output of the oscillator amplifier circuit.
To clock the device from an external source, drive XTAL1, while
leaving XTAL2 unconnected. Minimum and maximum high/low
and rise/fall times specified in the AC Characteristics must be
External Bus Configuration selection inputs. These pins are
sampled during reset and select either the single chip mode or
one of the four external bus configurations:
BUSACT EBC1 EBC0 Mode/Bus Configuration
0 0 0 8-bit demultiplexed bus
0 0 1 8-bit multiplexed bus
0 1 0 16-bit multiplexed bus
0 1 1 16-bit demultiplexed bus
1 0 0 Single chip mode
1 0 1 Reserved.
1 1 0 Reserved.
1 1 1 Reserved.
ROMless versions must have pin BUSACT tied to ‘0’.
Reset Input with Schmitt-Trigger characteristics. A low level at
this pin for a specified duration while the oscillator is running
resets the SAB 80C166. An internal pullup resistor permits
power-on reset using only a capacitor connected to VSS.
Internal Reset Indication Output. This pin is set to a low level
when the part is executing either a hardware-, a software- or a
watchdog timer reset. RSTOUT remains low until the EINIT
(end of initialization) instruction is executed.