PD17225_15 Datasheet, PDF (38/86 Pages) Renesas Technology Corp – 4-BIT SINGLE-CHIP MICROCONTROLLER FOR SMALL GENERAL-PURPOSE INFRARED REMOTE CONTROL TRANSMITTER

µPD17225, 17226, 17227, 17228
5.3.2 Countermeasures against noise during transmission (carrier output)
When a signal is transmitted from the transmitter of a remote controller, a peak current of 0.5 to 1 A may flow through
the infrared LED. Since two batteries are usually used as the power source of the transmitter, several Ω of equivalent
resistance (r) exists in the power source as shown in Figure 5-2. This resistance increases to 10 to 20 Ω if the supply
voltage drops to 2 V. While the carrier is output from the REM pin (while the infrared LED lights), therefore, a high-
frequency noise may be generated on the power lines due to the voltage fluctuation that may take place especially during
switching.
To minimize the influence on the microcontroller of this high-frequency noise, take the following measures:
<1> Separate the power lines of the microcontroller from the power lines of the infrared LED with the terminals of the
batteries at the center. Use thick power lines and keep the wiring short.
<2> Locate the resonator as close as possible to the microcontroller and shield it with GND lines (as indicated by the
shaded portion in the figure below).
<3> Locate the capacitor for stabilization of the power supply closely to the power lines of the microcontroller. Also,
use a capacitor to eliminate high-frequency noise.
<4> To prevent data from changing, do not execute an interrupt that requires read/write processing and stack, such
as key scan interrupt, and the CALL/RET instruction, while the carrier is output.
<5> To improve the reliability in case of program hang-up, use the watchdog timer (connect the WDOUT and RESET
pins).
Figure 5-2. Example of Countermeasures against Noise
Infrared LED
0.5 to 1 A
Batteries
r
+
–
REM
VDD
Microcomputer
RESET
WDOUT
VSS
Remark In this figure, the RESET pin is connected to a pull-up resistor by mask option.
36
Data Sheet U12643EJ2V0DS00