Chapter 2
Analog Input Timing/Control
©
National Instruments Corporation
2-127
DAQ-STC Technical Reference Manual
Figure 2-50.
SI Control Circuit State Transitions
2.8.3.5 SI2 Counter
The SI2 counter is a 16-bit down counter with dual-load registers. The SI2 counter counts the
interval between samples, as well as the delay from the START signal to the first sample
pulse. The SI2 counter uses the same clock that is selected for the SI counter (SI_SRC) or
AI_IN_TIMEBASE1. The counter load registers are directly accessible from the register
map. If the counter is disarmed, AI_SI2_Load will load the counter with the value from the
selected load register.
During normal operation, the SI2 counter will synchronously reload from the selected load
register following SI2_TC. The AI_SI2_Reload_Mode option allows the SI2 counter to
alternate load registers once after every STOP. The SI2 control circuit generates the count
enable signals.
2.8.3.6 SI2 Control
The SI2 counter is controlled by a circuit whose state transitions are shown in Figure 2-51.
The SI2 counter control circuit has three states: WAIT1, WAIT2 and CNT. On power up, the
counter begins in state WAIT1 and remains there until the counter is armed and a START1
pulse is received. When these two events occur, the counter transitions to state WAIT2 to wait
for START to be asserted. Once START is received, the counter transitions to state CNT and
begins counting. When STOP is received, the counter returns to the WAIT2 state to wait for
another START.
A
B
E
G
H
I
J
M
N
P
SI_START1
AI_SI_Arm
SI_TC
SC_TC
AI_End_On_End_Of_Scan
AI_End_On_SC_TC
AI_Continuous
STOP
AI_Trigger_Once
AI_SI_Tonce
T = (G'H' + H'I'J)P'
SI_LOAD = E + AI_SI_Load
SI_CE = AI_SI_Arm (CNT1(n+1))
SI_DISARM = CNT1(n) WAIT1(n+1) (H + I + N)
(AB)'
WAIT1
CNT1
AB
MET'
(MET')'
