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98640A Analog Input Interface
Control Lines
There are two control lines of interest for external pacing:
IP ACDA determines whether the readings are paced by the internal pacing timer on the A -to - D
card. If IPACDA is low, the internal pacing timer of the card is used; if IPACDA is high, the inter-
nal pacing timer is bypassed and readings are taken at the free run speed of the card (one reading
every 18 microseconds). Note that IPACDA must be high when readings start in order for the
timing of the first reading of a series to be accurately known. (IP ACDA can be set low after the
start of readings if you want the readings to be paced by the internal pacing timer.)
EPCON controls whether or not any readings are taken. If EPCON is low, readings are taken when-
ever they are requested. If EPCON is high, requested readings are held off; a read request will not
complete until EPCON goes low again.
In summary, when EPCON is low (open), readings are taken at the free run speed of the card (if
IPACDA is high) or at the time programmed into the internal pacing timer (if IP ACDA is low [open]).
When EPCON is high, readings stop.
Applica tions
External pacing applications divide into two general types: single externally-paced readings and bursts
of externally-triggered internally-paced readings.
Sil!&J£..read~.llg~
The idea behind taking single externally-paced readings is that you keep EPCON
high until you want to take a reading, set it low only long enough to take the reading, and then set it
high again. The steps in taking a single reading are:
1)
Set IPACDA high. IPACDA will remain high for the duration of externally paced readings.
2) Set EPCON high. This holds off all readings.
3) Issue an analog read request to the A -to-D card via a procedure similar to that use the program
sample above.
4) When it is time to take a reading, set EPCON low. Keep it low for 1 to 15 microseconds, then set
it high again. This will allow one (and only one) reading to be taken.
5)
If
your routine takes mUltiple readings, repeat step 4 until you have taken all of the readings.
(Remember to take 2 extra readings to push your results through the pipeline.)
Burs!!.
of-B£~di~
The idea behind taking readings in bursts is that you request multiple readings
with an assembly language routine, and then take those readings in one burst by setting EPCON low
until all of the readings have been taken. These readings can be taken at the free run speed of the
card, or they can be paced by the card's internal pacing timer. The following steps are for triggering
burst readings that are paced by the internal pacing timer.
1) Set IP ACDA and EPCON high.
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