201
CHAPTER 14 FREQUENCY COUNTER
User’s Manual U15104EJ2V0UD
14.5 Notes on Frequency Counter
(1) Notes on using frequency counter
Because signals are input to the frequency counter from an input pin (FMIFC or AMIFC pin) with an AC amplifier
as shown in Figure 14-7, cut the DC component of the input signals by using capacitor C.
If the FMIFC or AMIFC pin is selected by the IF counter mode select register, switch SW1 turns ON, and switch
SW2 turns OFF. As a result, the voltage on the pin is about 1/2V
DD
.
Unless the voltage has risen to a sufficient intermediate level at this time, counting may not be performed
normally because the AC amplifier is not in the normal operating range.
Therefore, make sure that sufficient wait time elapses after a pin has been selected and before counting is
started (IFCST = 1).
Figure 14-7. Frequency Counter Input Pin Circuit
(2) Notes in HALT mode
The FMIFC and AMIFC pins hold the status immediately before the HALT status was set.
To release the HALT mode by using the interrupt of the frequency counter at this time, the following point must
be noted.
The gate will not be opened if the HALT instruction is executed after counting has been started by IFCST before
the gate is actually opened.
Therefore, wait for at least 1 ms before executing the HALT instruction.
Figure 14-8. Gate Status When HALT Instruction Is Executed
OPEN
CLOSE
1 ms MAX.
Timing to open gate
Sets IFCST
Interrupt request is not issued if HALT instruction is executed
during this period because gate is not opened.
Gate
SW1
SW2
To internal counter
V
DD
PLL
R
C
External frequency
FMIFC
or AMIFC pin