112
EPSON
S1C63666 TECHNICAL MANUAL
CHAPTER 4: PERIPHERAL CIRCUITS AND OPERATION (R/f Converter)
The capacitor for the CR oscillation circuit is commonly used for the reference resistance and sensors. The
CR oscillation circuit is configured with the reference resistance, the sensors and this capacitor and can
R/f-convert using the reference resistance and the sensor.
The R/f converter performs CR oscillation using each of the two resistances (sensor and reference
resistance) in the same period, and counts the CR oscillation clock. Difference in counted oscillation
frequency can be evaluated in terms of the difference between the respective resistance values. Measure-
ment results can be obtained from the changes in resistance values after correcting the difference accord-
ing to the program. Consequently, a resistance that has a resistance value equivalent to the middle of the
measurement range of the sensor to be used for measurement should be used as a reference resistance. An
element that does not change due to temperature or other environmental conditions must be used as the
reference resistance.
The following explains the operation of the CR oscillation circuit that is configured with the above
connection.
Figure 4.15.2.2 shows the CR oscillation circuit configured with the SEN0 terminal.
V
SS
V
SS
Tr1
SEN0
CR oscillation
control circuit
➁
➀
➀
V
DD
RFIN
Tr2
To measurement
counter
➁
C
RF
R
SEN0
Fig. 4.15.2.2 CR oscillation circuit
The Tr1 turns on first, and the capacitor (C
RF
) connected between the V
SS
and RFIN terminals is charged
through the sensor (R
SEN0
). If the voltage level of the RFIN terminal increases, the Tr1 turns off and the
Tr2 turns on. As a result, the capacitor becomes discharged, and oscillation is performed according to CR
time constant. The time constant changes as the sensor resistance value fluctuates, producing a difference
from the oscillation frequency of the reference resistance.
The above example is in the case of a SEN0 terminal. Controlling whether the reference resistance or the
sensor to be CR-oscillated (controlling a transistor of terminal) is done by the CR oscillation control
circuit.
Oscillation waveforms are shaped by the schmitt trigger and transmitted to the measurement counter.
The clock transmitted to the measurement counter is also output from the RFOUT terminal while the
sensor is oscillating. As a result, oscillation frequency can be identified by the oscilloscope. Since this
monitor has no effect on oscillation frequency, it can be used to adjust R/f conversion accuracy.
Oscillation waveforms and waveforms output from the RFOUT terminal are shown in Figure 4.15.2.3.
The "L" pulse width of the RFOUT output must be 10 µsec or more (when V
DD
= 3.0 V, R
SEN0/1
= 50 k
Ω
,
C
RF
= 1000 pF).
RFIN terminal
RFOUT output
V
DD
V
SS
V
DD
V
SS
10
µ
sec or more
Fig. 4.15.2.3 Oscillation waveform