AN2839 Freescale Semiconductor / Motorola, AN2839 Datasheet - Page 4

• Manufacturer Part Number: AN2839
• Description: Implementing a Lamp Dimmer with an HC908Q Family MCU
• Manufacturer: Freescale Semiconductor / Motorola

Application Description
Application Description
The HC908Q Family allows the user to choose the MCU clock source. The microcontroller has three clock
source options available. Because this application aims to be low cost, the clock frequency is internally
generated. The internal oscillator circuit is designed to provide a clock source with tolerance less than
±25% untrimmed without external components. An 8-bit trimming register allows adjustment to a
tolerance of less than ±5%.
Other possible choices, although more expensive and definitely not necessary for this sort of application,
would be a built-in RC oscillator module that requires an external resistor connected to the chip. There is
also a built-in XTAL oscillator module designed to operate with an external crystal or ceramic resonator
to provide an accurate clock source.
The software accomplishes the brightness control by adjusting the conduction angle. The circuit does not
require a transformer to supply DC voltage to the MCU. For this reason, the user must use caution during
circuit assembly.
Figure 2
the AC line through the capacitor C1 that provides circuit isolation. C1 must be a 1 µF/ 600 V
non-polarized polyester capacitor. The diodes D1 and D2 enforce the half-wave rectification and the
capacitor C2 implements the ripple filtering. C3 is a ceramic bypass capacitor located as near as possible
to the MCU power pins in order to suppress high-frequency noise. The zener diode helps provide a
reasonable regulated voltage, which reduces the rectifier voltage to the desired supply voltage.
R1 and R2 provide the zero-crossing detection to the MCU to synchronize the triac trigger pulses with the
AC power line, achieving an accurate control. All four diodes are 1n4007, which allows the circuit to be
supplied by 115 V or 240 V AC.
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Flexible I/Os allow direct drive of LEDs and other circuits to eliminate external drivers and help
reduce system cost
System protection features, including watchdog timer and on-chip low-voltage detect/reset to help
reduce cost and increase reliability
Space-sensitive packages — 8 PDIP, 8 SOIC, 16 PDIP, 16 SOIC, and 16 TSSOP — with more to
come as the Family develops
illustrates the complete schematic diagram of the dimmer. The supply voltage is connected to
Implementing a Lamp Dimmer with an HC908Q Family MCU, Rev. 0
Freescale Semiconductor