LAPIS Semiconductor ML610472, Руководство пользователя

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ML610471/472/473/Q471/Q472/Q473 User's Manual
Chapter 17 RC Oscillation Type A/D Converter
17-16
request is generated. (Section (a)). Also, the generation of interrupt request releases HALT mode (section (b)) and at the
same time, A/D conversion operation stops. (Section (c), RARUN bit = "0"). At this time, Counter A is set to “0000H”.
The content of Counter B at this time is expressed by the following expression:
t
BSCLK
nB0 = nA0
•
t
RCCLK
(RS1)
... Expression B
That completes the operations in First Step.
<Second step>
c
Calculate “10000H – nB0” from the content of Counter B “nB0” and set the obtained value in Counter B.
At this point, Counter A needs to be cleared; however, no processing is required since the counter is already set to
“0000H”.
d
Write “12H” in RADMOD to select Counter B reference mode and the oscillation mode that uses thermistor RT1.
e
Write “01H” in RADCON to start A/D conversion operation.
f
Write “1” in the HLT bit of SBYCON to set the device to HALT mode.
The RC oscillator circuit (RCOSC1) oscillates with thermistor RT1 from this point until Counter B overflows. This period
is equal to the product of “nB0” obtained in the First Step and the oscillation period t
RCCLK
(RT1) using RT1.
When Counter B overflows, the RADINT signal is set to “1” and an RC-ADC interrupt request is generated. (Section (d)).
Also, the generation of interrupt request releases HALT mode (section (e)) and at the same time, A/D conversion operation
stops. (Section (f), RARUN bit = "0").
This completes the operations in Second Step.
The content of Counter A at this time becomes the A/D conversion value nA1, which is expressed by the following
expression:
t
RCCLK
(RT1)
nA1 = nB0
•
t
BSCLK
... Expression C
From expressions B and C, nA1 is expressed by the following expression:
t
RCCLK
(RT1)
nA1 = nA0
•
t
RCCLK
(RS1)
... Expression D
where t
RCCLK
(RS1) is the oscillator clock period by reference resistor RS1 and t
RCCLK
(RT1) the oscillator clock period
by thermistor RT1.
Since the oscillation period is expressed by "t
RCCLK
= k
RCCLK
x R x C", t
RCCLK
(RS1) and t
RCCLK
(RT1) are expressed by
the following expressions:
t
RCCLK
(RS1) = k
RCCLK
•
(CS1+CVR1)
•
RS1
... Expression E
t
RCCLK
(RT1) = k
RCCLK
•
(CS1+CVR1)
•
RT1
When expression E is substituted for expression D, nA1 will be:
RT1
nA1 = nA0
•
RS1
Since “nA0” (“4B0H” in this example) and RS1 are constants whose values are fixed, “nA1” is a digital value
proportional to RT1. This very “nA1” corresponds to “nT0” in expression A.
That concludes the description of the A/D conversion method using a thermistor. “nA1” that has been obtained must
further be converted to a value such as a temperature indication value for thermometer by program according to the
temperature-to-resistance characteristics of the thermistor.