Chapter 2 Analog Input Timing/Control©National Instruments Corporation2-127DAQ-STC Technical Reference ManualFigure 2-50. SI Control Circuit State Transitions2.8.3.5 SI2 CounterThe SI2 counter is a 16-bit down counter with dual-load registers. The SI2 counter counts theinterval between samples, as well as the delay from the START signal to the first samplepulse. The SI2 counter uses the same clock that is selected for the SI counter (SI_SRC) orAI_IN_TIMEBASE1. The counter load registers are directly accessible from the registermap. If the counter is disarmed, AI_SI2_Load will load the counter with the value from theselected load register.During normal operation, the SI2 counter will synchronously reload from the selected loadregister following SI2_TC. The AI_SI2_Reload_Mode option allows the SI2 counter toalternate load registers once after every STOP. The SI2 control circuit generates the countenable signals.2.8.3.6 SI2 ControlThe 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, thecounter begins in state WAIT1 and remains there until the counter is armed and a START1pulse is received. When these two events occur, the counter transitions to state WAIT2 to waitfor START to be asserted. Once START is received, the counter transitions to state CNT andbegins counting. When STOP is received, the counter returns to the WAIT2 state to wait foranother START.ABEGHIJMNPSI_START1AI_SI_ArmSI_TCSC_TCAI_End_On_End_Of_ScanAI_End_On_SC_TCAI_ContinuousSTOPAI_Trigger_OnceAI_SI_TonceT = (G'H' + H'I'J)P'SI_LOAD = E + AI_SI_LoadSI_CE = AI_SI_Arm (CNT1(n+1))SI_DISARM = CNT1(n) WAIT1(n+1) (H + I + N)(AB)'WAIT1CNT1ABMET'(MET')'
