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THEORY OF OPERATATION

4.7

[3] STCG (GENERATOR VIAS)

U13, VUSR, and U29, VCOM, are Versatile Interface Adapters (VIA). Each of them contains 2-
8 bit I/O ports with handshaking, 2-16 bit counters, an 8 bit shift IN-or-OUT register, and
extensive interrupt capabilities.

All of the functions of the VIAs are under program control, and many of them are changed
dynamically. As a convenience, the VIAs have been named according to their primary function.
VUSR provide user interface via the Keyboard. VCOM provides communication with the
Translator Assembly. One of the counters of VUSR counts

∅

2 to generate 100KHz. This

100KHz becomes the reference frequency for the Generator. It is further divided by U23 to
provide 10KHz, which interrupts VUSR (at CA1) to “clock” the Firmware Minor Time. 10KHz
is further divided by hardware (U23) to 1KHz which becomes the IRIG B carrier after filtering.
VUSR, CB2, provides the signal GEN 1PPS, which is used to Start/Stop the counter chain, and
also to synchronize it to the Firmware Minor Time. This synchronizing signal does not appear
once per second and may occur only rarely. VUSR PA0-PA7 are used to service the Keyboard.
The Keyboard is a 4X4 matrix. It is periodically scanned for a connection. If a connection
(closure) exists, it is debounced in firmware, then acted upon. Due to the scanning technique,
simultaneous closures are not permitted. VCOM, CA2, is used as an inhibit to prevent I/O
interrupts during time sensitive parts of the program.

The second counter in VUSR generates periodic interrupts which are used to debounce the
Keyboard. VUSR PB0 outputs W/, which is used to write to the LED Display. VUSR, PB1,
outputs ENL, which enables the LCD Display.

VUSR, CA2, is used as a Power Fail (FP) output during the Power-Down sequences to prevent
multiple sequencing. VUSR, PB4 is used as a Clock Inhibit (C1H) for the Backup Clock. VUSR,
PB5, is used as an input to sample the Power-On (PON) status during Power-Down and Power-
Up sequence. This permits optional circuitry to store the Generator Time for subsequent Serial or
Parallel output. VUSR, CB1, is an optional input with interrupt capability. Depending on the
configuration, it may be used to either Start or Synchronize the Generator.

VCOM, PA0-PA7, and CA1 and CA2 are used for the GEN COMM half of the I/O with the
Translator assembly. All lines tie to the corresponding lines of the TRANS COMM except that
CA1 and CA2 are crossed. This is a bi-directional data path over which time and other
information pass between the Generator and Translator. Data transfer is asynchronous. Data on
the PA0-PA7 bus is latched into the receiving bus by a transition on the sending bus CA2
(receiver’s CA1). This transition also sets a flag in the receiver’s status register. When the flag
is discovered, it is acknowledged by a transition on the receiver’s CA2 (sender’s CA1).
Generally transfer to the Translator causes an interrupt (in the Translator) while that to the
Generator merely sets a flag which must be polled to be recognized. In some cases however, the
Generator is enabled to be interrupted.

VCOM, PB0-PB3, are used as data outputs to the Backup Clock U1, the LCD Display, and the
LED Displays. VCOM, PB4-PB6, are used, in conjunction with C1H, to write an address, write