Panasonic KX-FT21BX Service Manual page 90

KX-FT21LA
For these reasons and the software sequence to boot up the unit, if you use an oscilloscope to judge whether a signal is OK or
NG, you must check in the same order as in [List 1]. (If the ASIC (CPU) failed to access the ROM, the ASIC cannot access the
SRAM or DRAM normally.)
The digital circuit actually operates according to the timing combinations of these signals. So, if the timing of these signals is
even slightly off, the circuit will not operate normally. Even of the IC did malfunction, the output voltage level may become
abnormal but the timing is accurate according to the specifications. (If oscillation is provided accurately.)
Accordingly, the problem presented here is whether each IC outputs the correct signal. (See the I/O direction diagram on the next
page.) In other words, is it constantly switching between 5V or 3.3V (H) and 0V (L) as described earlier.
All you have to do is check that the IC repeatedly outputs (H) 5V or 3.3V and (L) 0V.
- 64 -