M16C80 Datasheet, PDF (223/358 Pages) Renesas Technology Corp – RENESAS 16-BIT SINGLE-CHIP MICROCOMPUTER M16C FAMILY / M16C/80 SERIES

M16C/80 Group
27. Usage Precaution
A/D Converter
(1) Write to each bit (except bit 6) of A/D control register 0, to each bit of A/D control register 1, and to bit
0 of A/D control register 2 when A/D conversion is stopped (before a trigger occurs).
In particular, when the Vref connection bit is changed from “0” to “1”, start A/D conversion after an
elapse of 1 µs or longer.
(2) When changing A/D operation mode, select analog input pin again.
(3) Using one-shot mode or single sweep mode
Read the correspondence A/D register after confirming A/D conversion is finished. (It is known by A/
D conversion interrupt request bit.)
Use the undivided main clock as the internal CPU clock.
(4) Using repeat mode, repeat sweep mode 0 or repeat sweep mode 1
(5) When f(XIN) is faster than 10 MHz, make the frequency 10 MHz or less by dividing.
(6) If A/D conversion is stopped by program while in progress of A/D conversion, the conversion result of
A/D converter becomes indeterminate. The contents of A/D registers irrelevant to A/D conversion
may become indeterminate. If A/D conversion is stopped by program while in progress of A/D conver-
sion, ignore the values of all A/D registers.
(7) Output impedance of sensor at A/D conversion (Reference value)
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 27.1 has to
be completed within a specified period of time T. Let output impedance of sensor equivalent circuit be
R0, microcomputer’s internal resistance be R, precision (error) of the A/D converter be X, and the A/
D converter’s resolution be Y (Y is 1024 in the 10-bit mode, and 256 in the 8-bit mode).
t
Vc is generally VC = VIN {1 – e ^– _{C (R0 + R)} }
And when t = T, VC=VIN – X VIN=VIN(1 – X )
Y
Y
Hence, R0 = –
T
e ^– _{C (R0 + R)} = X
Y
–
T _=ln X
C (R0 +R)
Y
T
–R
C • ln ^X
Y
Rev.1.00 Aug. 02, 2005 ^{Page 212} of 329
REJ09B0187-0100