CHAPTER THREE
3-12
bc620/627AT Time and Frequency Processor
Symmetricom, Inc.
3.2.3.1 EXTERNAL EVENT INPUT
The external event input provides a means of capturing time based on an event that occurs
externally to the TFP. Use the EVENTEN bit (CR0 bit 3) to enable the external event time
capture. Use the EVSENSE bit (CR0 bit 2) to select the active edge. When the active edge of
the external event input occurs, INTSTAT bit 0 will be set, and if MASK bit 0 is set a PC AT
bus interrupt will be generated.
3.2.3.2 PROGRAMMABLE PERIODIC OUTPUT
The programmable periodic output will capture time in the EVENT0 - EVENT8 registers if the
HBEN bit (CR0 bit 0) is set to 1. When the rising edge of the periodic output occurs, INTSTAT
bit 1 will be set, and if MASK bit 1 is set a PC AT bus interrupt will be generated. The periodic
output rate is programmable over the range from 2.5 MHz to 2.3 MHz.
3.2.3.3 TIME CAPTURE LOCKOUT
Time capture lockout is used with the event capture registers to prevent them from being
overwritten before they can be read. The lockout (if enabled) will allow a capture signal
(external event or periodic) to capture time once, then subsequent capture signals will be blocked
until the lockout is released. The lockout is released by reading the UNLOCK register. The
time capture lockout feature is enabled with LOCKEN (CR0 bit 0).
A write instruction to the UNLOCK register will latch time in the EVENT0 - EVENT8 registers.
3.2.4 TIME COINCIDENCE STROBE OUTPUT
The TFP provides a time coincidence strobe output which generates a one millisecond wide
pulse (rising edge on time) at the time set in the STROBE2 - STROBE6 registers. Use STREN
(CR0 bit 4) to enable/disable the strobe output pulse. When changing the STROBE2 -
STROBE6 registers, the strobe output should be disabled to prevent false strobe outputs. The
strobe can be operated in one of two modes: “major/minor” mode or “minor only” mode. In
major/minor mode, the strobe output pulse occurs when the time matches both the programmed
major time (hours - seconds) and the programmed minor time (milliseconds). In minor only
mode, the strobe output pulse occurs once per second when the time matches the programmed
minor time. The rising edge of the strobe will set INTSTAT bit 2, and if MASK bit 2 is set a PC,
AT bus interrupt will be generated.
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