DRM081, Multi-Button IR Remote Control using the MC9RS08KA2 ...

5.3.2 Key Decoding
Multi-Button IR Remote Control using the MC9RS08KA2, Rev. 0
Transmitter Software Implementation
The flow chart in Figure 5-1 shows that when a button is pressed, the system will wake up from stop mode.
After that, key decoding is performed. The detail operation of the key decoding is shown in the flow chart
of Figure 5-2. When the system wakes up, the keyboard interrupt will be disabled and key debounce
performed to eliminate the noise that may trigger a wrong key pressed. After key debounce, key decoding
is performed to determine which button is pressed. This reference design uses an emulated ADC to find
out which key is pressed. When a key is pressed, both comparator pins (ACMP+ and ACMP–) and the
timer will be enabled and timer count starts. The voltage on the ACMP– pin is dependent on the resistor
divider that is connected to the keys on the keypad. The timer counts the period for the voltage on the
ACMP+ pin to charge up until it is equal to the voltage on the ACMP– pin. When the two voltages are
equal, an interrupt will occur and the timer count is recorded. As each button has a different voltage and
hence, different timer counts, the button pressed can be determined.
When the key is identified, the control frame will be updated according the function of the key. The frame
is then transmitted out through the IR transmitter diode with 38kHz, 1/3 duty, cycle carrier.
5.3.3 Transmission Control Frame Update
When the key is identified, the transmission control frame is updated based on the assigned function of
the key. The definition of the transmission control frame is shown in Table 3-1 and the definition of the
key function are described in 4.2.2 Keypad Scanning.
Freescale Semiconductor 23