Program Interface Library AR_SET_TIMERRATE
CEI-100/CEI-200/CEI-x20 User’s Manual 144
AR_SET_TIMERRATE
void ar_set_timerrate (CEI_INT16 board, CEI_INT16 rate);
The ARINC board contains an onboard timer that can be programmed to
interrupt at various frequencies (ticks). A 32-bit integer is incremented for
each of the ticks. This tick count assists the host computer in accurately
timing message retransmission. It is also used as a timetag when time
tagging is enabled. The integer's value is read by using
AR_GET_TIMERCNTL or cleared using AR_RESET_TIMERCNT.
For the CEI-100/200, this routine must be called after AR_LOADSLV but
before AR_GO. The CEI-100 default timer rate is 0 (off). The CEI-
200/x20 default timer rate is 20000 (5 milliseconds).
On the CEI-200/x20, the timer count is initialized with a 16-bit value. The
value is given as a number of clocks where 4 clock cycles equals 1
microsecond. The default value timer rate is 20000 (decimal), which is
equivalent to 5000 microseconds. Any valid 16-bit value can be
programmed here. For example, if the timer rate were reprogrammed to a
value of 4000, the value of a tick would be 1000 microseconds. The
minimum acceptable timer rate parameter that should be programmed for a
CEI-x20 board is 1440 (360 microseconds).
On the CEI-100, the timer count is selected from a table of possible values
as defined below.
Increasing the timer rate also increases the overhead on the interface. For
a lightly loaded interface, this is not important, but for a busy one it can
impact performance, possibly causing ARINC data loss or timer tick loss.
The selected timer rate for the CEI-200 should not exceed 360
microseconds during high-speed operation of all twelve channels. As
fewer channels are used, the resolution may increase.
The following limitations apply to the CEI-100:
If time tagging isn’t enabled and all three channels are running at the
high bit rate, a maximum rate of 500 µsec may be used.
If only two receivers are running, a maximum rate of 200 µsec may be
used. A maximum of 100 µsec may be used if only one receiver is
running.
If time tagging is enabled, don’t run all three channels simultaneously
at the high bit rate or data may be lost.
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