Classic CPU reset circuit (including principle analysis)

Classic CPU reset circuit

Working principle:
1. The purpose of diode D501
If there is no D501, when the power is turned on immediately after power failure, the charge on the capacitor cannot be completely discharged, and it cannot be reset normally. When the system is powered off, the VCC load is heavy, so it drops quickly, while the C502 load is light, so it drops slowly. When D501 is added, VCC drops, the capacitor will drop with it, and the reset terminal voltage will drop rapidly.
2. Why is the input of U502F grounded?
CMOS input cannot be empty, it can be grounded or VCC.
3. Why use 74HC14 instead of ordinary 74HC04?
74HC14 is a non-gate with a Schmitt trigger. The capacitor discharge signal is not a digital signal, but a "continuous and slow" analog signal. A Schmitt trigger is needed to shape the analog signal into a square wave digital signal. This is the same reason why digital signals need to be shaped after long-distance transmission.
4. Why does "Non-come, Non-go"
first reset the peripherals, then reset the CPU chip, to ensure that the CPU and peripherals are reset at the same time, otherwise the CPU may end the reset and start running, while the peripherals are still in the reset. (nRESET input CPU reset pin)