32182 Datasheet, PDF (737/755 Pages) Renesas Technology Corp – 32-BIT RISC SINGLE-CHIP MICROCOMPUTER M32R FAMILY / M32R/ECU SERIES

Appendix 4
SUMMARY OF PRECAUTIONS
Appendix 4.9 Precautions about the A-D Converter
Appendix 4.9 Precautions about the A-D Converter
• Forcible termination during scan operation
If A-D conversion is forcibly terminated by setting the A-D conversion stop bit (AD0CSTP) to "1" during scan
mode operation and the A-D data register for the channel that was in the middle of conversion is accessed for
read, the read value shows the last conversion result that had been transferred to the data register before the
conversion was forcibly terminated.
• Modification of the A-D converter related registers
If the content of any register—A-D Conversion Interrupt Control Register, Single or Scan Mode Registers or A-D
Successive Approximation Register, except the A-D conversion stop bit—is modified in the middle of A-D conver-
sion, the conversion result cannot be guaranteed. Therefore, do not modify the contents of these registers while A-
D conversion is in progress, or be sure to restart A-D conversion if register contents have been modified.
• Handling of analog input signals
When using the A-D Converter with its sample-and-hold function disabled, make sure the analog input level is
fixed during A-D conversion.
• A-D conversion completed bit read timing
To read the A-D conversion completed bit (Single Mode Register 0 bit 5 or Scan Mode Register 0 bit 5) immediately after A-D
conversion has started, be sure to adjust the timing 2 BCLK periods by, for example, inserting an NOP instruction before read.
• Regarding the analog input pins
Figure 4.9.1 shows the internal equivalent circuit of the A-D Converter’s analog input part. To obtain accurate A-
D conversion results, make sure the internal capacitor C2 of the A-D conversion circuit is charged up within a
predetermined time (sampling time). To meet this sampling time requirement, it is recommended that a stabiliz-
ing capacitor C1 be connected external to the chip.
The method for determining the necessary value of this external stabilizing capacitor with respect to the output
impedance of an analog output device is described below. Also, an explanation is made of the case where the
output impedance of an analog output device is low and the external stabilizing capacitor C1 is unnecessary.
• Rated value of the absolute accuracy
The rated value of the absolute accuracy is the actual performance value of the microcomputer alone, with
influences of the power supply wiring and noise on the board not taken into account. When designing the
application system, use caution for the board layout by, for example, separating the analog circuit power supply
and ground (AVCC0, AVSS0 and VREF0) from those of the digital circuit and incorporating measures to prevent
the analog input pins from being affected by noise, etc. from other digital signals.
Inside the microcomputer
10-bit A-D Successive
Approximation Register (ADiSAR)
Analog output device
R1
E
VREF
i→
C1
ADIN n
i1 ^i2→
Cin
↓
10-bit D-A Converter
R2
Selector
V2
C2
Comparator
C1 : parasitic capacitance of the board
+ stabilizing capacitance
R2 : parasitic resistance of the
selector (1-2 KΩ)
R1 : resistance of analog output device C2 : comparator capacitance
Cin : input pin capacitance (approx. 10 pF)
(approx. 2.9 pF)
VREF : analog reference voltage
V2 : voltage across C2
E : voltage of analog output device
Figure 4.9.1 Internal Equivalent Circuit of the Analog Input Part
Appendix 4-12
32182 Group User's Manual (Rev.1.0)