Output On Terminal Count (Mode 0); Programmable One-Shot (Mode 1); Rate Generator (Mode 2); Square Wave Generator (Mode 3) - NCR PC4I Technical Reference

PROGRAMMABLE INTELLIGENCE
COUNTER is a two-bit binary value 0-2, denoting the
number of the Counter to be accessed. Therefore, to
access Counter 2, D7 should be set and D6 zero.
R/W SELECT determines the way in which the specified
Counter is to be loaded or read. The type of
operation to be carried out (read or load) depends on
whether an IN or OUT instruction is being used (the
Timer has pin connections for / RD and /WR signals).
The significance of the binary value contained in
these two bits is as follows:
0 Counter Latching (see below)
1 Read/load more significant byte of Counter
2 Read/load less significant byte of Counter
3 Read/load both Counter bytes (less significant
first)
BCD: if this bit is set, the 16 bits of the selected
Counter are used as a 4-digit BCD counter. If this
bit is zero, the Counter represents a 16-bi t binary
value.
MODE may be a binary value 0-5 in three bits. The
following modes can be implemented in the NCR
PERSONAL COMPUTER hardware (applications requiring a
rising edge at the GATE can be implemented only with
Timer 2):
0 - Output on terminal count
1 - Programmable one-shot
2 - Rate generator
3 - Square wave generator
4 - Software triggered strobe
5 - Hardware triggered strobe
These Timer modes are described in detail in the
following sections. Timing characteristics for the
Timer modes are illustrated in Figure 2.3.
Output on Terminal Count (Mode 0)
Following the loading of the Counter, the signal
OUTput pin for that Counter goes low, and remains low
until the Counter has decremented to zero. The OUT
signal then goes high, and remains high until the
Counter is next programmed. If you write a new value
2-6