Operation Of R/F Conversion - Epson S1C63616 Technical Manual

(2) R/f conversion using an AC bias resistive sensor such as a humidity sensor
This conversion is possible only in channel 1, and this method is selected by setting ERFx to "10B". This
is basically the same as the R/f conversion described above (1), but the AC bias circuit works for a sen-
sor (e.g. humidity sensor) to which DC bias cannot be applied for a long time. The oscillating operation
by reference resistance is the same as the R/f conversion described above (1).
Figure 4.13.3.3 shows the connection diagram of external devices.
The oscillation waveform is the same as Figure 4.13.3.2.

4.13.4 Operation of R/f conversion

Counter
The R/f converter incorporates two types of counters: measurement counter MCxx and time base
counter TCxx. The measurement counter is a 20-bit up counter that counts the CR oscillation clock with
the reference resistance or sensor selected by software. The R/f conversion results can be obtained by
reading this counter. The time base counter is a 20-bit up/down counter to equal both oscillation times
for the reference resistance and the sensor. The time base counter uses the R/f converter clock selected
by the RFCKSx register (OSC1 or OSC3). Each counter permits reading and writing on a 4-bit basis.
First start an R/f conversion for the reference resistance. The measurement counter starts counting up
and the time base counter starts counting down. The counters stop counting when the measurement
counter overflows (counter = "00000H"). By resetting the time base counter to "00000H" before starting
an R/f conversion for the reference resistance, the reference oscillation time will be obtained from the
time base counter.
Then start an R/f conversion for the sensor, the measurement counter starts counting up from "00000H"
and the time base counter starts counting up from the counted value. The counters stop counting when
the time base counter overflows (counter = "00000H"). The oscillation time in this phase is the same as
that of the reference resistance.
Therefore, by converting a appropriate initial value for counting of the oscillation of the reference
resistance into a complement (value subtracted from "00000H") and setting it into the measurement
counter before starting to count, the number of counts for the sensor oscillation is obtained by reading
the measurement counter after the R/f conversion. In other words, the difference between the reference
resistance and sensor oscillation frequencies can be found easily. For instance, if resistance values of the
reference resistance and the sensor are equivalent, the same value as the initial value before converting
into a complement will be obtained as the result.
The time base counter allows reading of the counter value and presetting of data. By saving the counter
value after the reference oscillation has completed into the RAM, the subsequent reference oscillation
phase can be omitted. The sensor oscillation can be started after setting the saved value to the time base
counter and "00000H" to the measurement counter.
Note: When setting the measurement counter, always write 5 words of data continuously in order from the
lower address (FF62H → FF63H → FF64H → FF65H → FF66H). Furthermore, an LD instruction
should be used for writing data to the measurement counter and a read-modify-write instruction (AND,
OR, ADD, SUB, etc.) cannot be used.
HUD
R
SEN
SEN1
REF1
R
REF
RFIN1
C
RFC
Channel 1
V
SS
Fig. 4.13.3.3 Connection diagram of resistive humidity sensor
R : Resistive sensor (e.g. humidity sensor)
SEN
R : Reference resistor
REF
C : Oscillating capacitor
RFC
SIC63616-(Rev. 1.0) NO. P152
3240-0412