Rohm LAPIS SEMICONDUCTOR ML620Q503, User Manual

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ML620Q503/Q504 User's Manual
Chapter 24 RC Oscillation Type A/D Converter
FEUL620Q504 24-10
24.3.1 RC Oscillator Circuits
RC-ADC performs A/D conversion by converting the oscillation frequency ratio between a reference resistor (or
capacitor) and a resistive sensor (or capacitive sensor) such as a thermistor to digital data.
By making RC oscillation occur both on the reference side and on the sensor side with the reference capacitor the
error factor that the RS oscillator circuit itself is eliminated, thereby making it possible to perform the A/D
conversion of the characteristics of the sensor itself.
Also, by calculating the ratio between the oscillation frequency on the reference side and that on the sensor side
and then calculating the correlation between the calculated ratio and temperatures that the sensor characteristics
have in advance, a temperature can be obtained based on that calculated ratio.
Table 24-1 lists the eight types of oscillation modes, one of which is selected by the RC-ADC mode register
(RADMOD) OM3–0 bits.
Table 24-1 Oscillation Modes from Which Selection Is Made by OM3–0 Bits
mode
No.
RADMOD RCOSC0 output pin RCOSC1 output pin
mode
OM3 OM2 OM1 OM0 RS0 RT0 RCT0 CS0 RS1 RT1 CS1
0 0 0 0 0 Z Z Z Z Z Z Z IN0 External clock input mode
1 0 0 0 1 1/0 Z Z 0/1 Z Z Z
RS0–CS0
oscillation
RCOSC0
oscillation
mode
2 0 0 1 0 Z 1/0 Z 0/1 Z Z Z
RT0–CS0
oscillation
3 0 0 1 1 Z Z 1/0 0/1 Z Z Z
RT
0-1
–CS0
oscillation
4 0 1 0 0 1/0 Z 0/1 Z Z Z Z
RS0–CT0
oscillation
5 0 1 0 1 Z Z Z Z 1/0 Z 0/1
RS1–CS1
oscillation
RCOSC1
oscillation
mode
6 0 1 1 0 Z Z Z Z Z 1/0 0/1
RT1–CS1
oscillation
7 0 1 1 1 Z Z Z Z Z Z Z IN1 External clock input mode
8 1 * * * Z Z Z Z Z Z Z (Prohibited)
Note) * : Indicates arbitrary.
Z : Indicates high-impedance output.
1/0, 0/1 : Indicates active output.
(Prohibited)
: The oscillator clock is not supplied even by setting the RARUN bit to “1” or by starting A/D
conversion.
In Table 24-1, mode No.0 and mode No.7 are modes where external clocks to be input to the IN0 or IN1 pin are
used for measurement with the RC oscillator circuit stopped.
As shown in Table 24-1, the two oscillator circuits, RCOSC0 and RCOSC1, are so specified that they cannot
operate concurrently in order to prevent interference in oscillation from occurring when they oscillate
concurrently.
The relationship between an oscillation frequency f
RCCLK
and an RC constant is expressed by the following
equation:
1
= t
RCCLK
= k
RCCLK
•R•C
f
RCCLK
The t
RCCLK
is the period of the oscillator clock, k
RCCLK
the proportional constant, and R x C the product of
capacitances CS, CT, (CS+CVR) and (CT+CVR) and resistances RS and RT. The value of k
RCCLK
slightly
changes depending on the value of the supply voltage VDD, RI, R, or C.
Table 24-2 lists the typical k
RCCLK
values.