Datum bc630AT, User Manual

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OPERATION AND SOFTWARE INTERFACE
Datum Inc bc630AT Real Time Clock Module 3-13
Heartbeat rates above 2000 pulses per second are not recommended. The Heartbeats are
synchronized to on-time, therefore a heartbeat rate of 1000 would generate a positive pulse each
millisecond. (i.e. at 1.000 000, 1.000 001, ...1.999 999 , 2.000 000 seconds). The rising edge of the
heartbeat pulse occurs on-time. This edge is generated by hardware and should be used as the
reference. The falling edge is controlled by software resulting in a small variance of the pulse width.
The heartbeat pulses are available on J1 pin 15 and the BNC connector (if jumpered for heartbeats).
The heartbeats can generate XT/AT bus interrupts if selected as the interrupt source in the Masks
Register (See Section 3.1.4). The bc630AT generates heartbeat interrupts to the PC by writing 60H to
the Interrupt Register, offset 0FH. The interrupt is cleared when the host PC reads the Interrupt
Register. The host can optionally use the Heartbeat Interrupt in a polled mode.
The bc630AT achieves synchronization of the heartbeats to the time source by computing the heartbeat
pulse/interrupt period by dividing the Time Code Period (see section 3.5) by the programmed heartbeat
rate (HBRATE1, HBRATE0). The bc630AT does the heartbeat period calculations in response to the
'Initialize' and 'Synchronize Heartbeats' commands and at power on. The bc630AT must be decoding
time code or 'flywheeling' with a valid Time Code Period to insure accurate heartbeat synchronization.
The 'Initialize' command is valid for heartbeat synchronization only when the 'Initialization mode' is set to
normal in the Time Code Select register (see section 3.1.3). When other 'Initialization modes' are
selected the initial heartbeat period will be calculated based on the time code period specified by the
mode.
The 'Synchronize Heartbeats' command allows on the fly heartbeat programming without disruption of
the bc630AT's clock. It is a valuable command for changing the heartbeat rate or re-synchronizing an
existing heartbeat rate to the time code. The heartbeats become valid at the on-time mark after the
command completes.
Varying temperature changes to the bc630AT may result in a small drift of the Time Code Period
measurement. When this occurs the heartbeats will become slightly out of sync near the end of the one
second time period (possibly a few microseconds). To assure the highest synchronization accuracy’s of
the heartbeat pulses/interrupts, the host XT/AT computer may issue the 'Synchronize Heartbeats'
command when a drift in the time code period is detected or at a periodic rate.
Note : The heartbeat feature is not available for the IRIG A time code format.
3.4.5 PROPAGATION DELAY COMPENSATION
When the time code source is located an appreciable distance from the bc630AT, a significant
propagation delay will be introduced. The bc630AT incorporates a propagation delay compensation
feature which removes the effects of this delay. To utilize this feature simply load the desired value into
the PROPDEL0 and PROPDEL1 registers and then 'Initialize' the bc630AT.
The PROPDEL0 (MSB) and PROPDEL1 (LSB) registers are loaded with a 16-bit signed value
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