M32C8A Datasheet, PDF (287/372 Pages) Renesas Technology Corp – RENESAS MCU M16C FAMILY / M32C/80 Series

M32C/8A Group
18. A/D Converter
18.3 Read from the AD0i Register (i = 0 to 7)
Use the following procedure to read the AD0i register by program.
• In one-shot mode and single sweep mode:
Ensure that the A/D conversion is completed before reading the corresponding AD0i register. The IR bit in the
AD0IC register becomes 1 when the A/D conversion is completed.
• In repeat mode, repeat sweep mode 0, and repeat sweep mode 1:
Read the AD0i register after setting the CPU clock as follows.
(1) Set the CM07 bit in the CM0 register to 0 (clock selected by the CM21 bit divided by the MCD
register).
(2) Set the MCD register to 12h (no division).
18.4 Output Impedance of Sensor Equivalent Circuit under A/D Conversion
To take full advantage of the A/D converter performance, Internal capacitor (C) charge shown in Figure 18.9
must be completed within the specified period (T) as sampling time. Output impedance of the sensor equivalent
circuit (R0) is determined by the following equation:
VC =
V I N _⎨⎧ 1
–
–
e
-------------1---------------
C(R0 + R)
t_⎫
⎬
⎩
⎭
When t = T,
VC
=
VIN
–
X--- _V
Y
IN
=
VIN_⎝⎛1 –
XY---_⎠⎞
–
e
-------------1--------------- T
C(R0 + R)
=
X---
Y
– _C-----(---R----0-1----+-----R-----₎T =
ln ^X---
Y
R0 = – ------T------- – R
C ln ^X---
Y
where:
VC = Internal capacitor voltage
R = Internal resistance of the MCU
X = Accuracy (error) of the A/D converter
Y = Resolution (1024 in 10-bit mode, and 256 in 8-bit mode)
Figure 18.9 shows a connection example of analog input pin and external sensor equivalent circuit.
In the following example, the impedance R0 is obtained from the equation above when VC changes from 0 to
VIN-(1/1024)VIN within the time (T), if the difference between VIN and VC becomes 1LSB. (1/1024) means
that A/D accuracy drop, due to insufficient capacitor charge, is held to 1LSB at time of A/D conversion in the
10-bit mode. Actual error, however, is the value of absolute accuracy added to 1LSB.
When φAD = 10 MHz, T = 0.3 μs in A/D conversion with the sample and hold function. Output impedance
(R0) enough to complete charging the capacitor (C) within the time (T) is determined by the following
equation:
Using T = 0.3 μs, R = 2.0 kΩ, C = 7.5 pF, X = 1, Y = 1024,
R0 = – --------------0---.--3-----^×----1----0---^–---⁶-------------- – 2.0 × 10³
7.5 × 10^–12 ⋅ ln ------1------
≅
3.8 × 10³Î©
1024
Thus, the allowable output impedance R0 of the sensor equivalent circuit, making the accuracy (error) 1LSB or
less, is approximately 3.8 kΩ maximum.
Rev.1.00 Jul 15, 2007 Page 270 of 352
REJ09B0385-0100