National Instruments PCI-6601, User Manual

Page: 15 / 52

Chapter 2 Device Overview
NI 660x User Manual 2-2 ni.com
Fig
u
re 2-1 shows an example of prescaling.
Figure 2-1. Prescaling Example
Prescaling is intended for
u
se with two co
u
nter period and freq
u
ency
meas
u
rements where the meas
u
rement is made on a contin
u
o
u
s, repetitive
signal. The prescaling co
u
nter cannot be read, so yo
u
cannot determine how
many edges have occ
u
rred since the previo
u
s roll-over. Yo
u
can also
u
se
prescaling for co
u
nting edges if it is acceptable to have an error of
u
p to
seven when
u
sing 8X prescaling or one when
u
sing 2X prescaling.
Pad Synchronization
The NI 660 x devices allow synchronization of their PFI lines and RTSI
lines at the I/O pads. This is called pad synchronization in this doc
u
ment,
and digital synchronization in the NI-DAQmx API. Yo
u
cannot
u
se digital
filtering while enabling this feat
u
re.
Pad synchronization is
u
sef
u
l when several co
u
nters are meas
u
ring or
operating off the same external signal. For example, s
u
ppose
co
u
nters 0 and 1 are config
u
red for triggered p
u
lse generation and each
co
u
nter
u
ses the same external trigger (this external signal is connected to
PFI 38 on the I/O connector and both co
u
nters have PFI 38 selected as their
GATE). After the trigger signal propagates thro
u
gh the I/O pad of the
ASIC, the time for the signal to reach the GATE of each co
u
nter within the
ASIC may differ by a few nanoseconds.
This signal is sampled at the co
u
nters’ GATEs
u
sing the selected SOURCE.
Beca
u
se of different propagation times for the paths to the two GATEs,
it is possible for the co
u
nters to detect the trigger on different edges on
SOURCE. Th
u
s, one co
u
nter co
u
ld see the trigger one SOURCE period
after the other. If yo
u
want to allow the co
u
nters to see the changes in the
signal at the same instance, yo
u
sho
u
ld
u
se pad synchronization. D
u
ring
pad synchronization, the signal is offset by one clock cycle.
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1