[size=16 px] recently CY7C68013A chip was adopted to design a product, the inside is actually a 51 MCU, the control of USB communication, found in the testing process after power on the part of the circuit board, the computer can't find the new USB hardware, nor hint, unable to identify the DEVICE manager nor UNKNOW the prompt of the DEVICE, after careful screening doubt internal 51 microcontroller without work,


Screening the crystals, power supply, IO haven't found the problem after foot and so on, finally reset circuit has a problem is suspected,

Based on chip data described: VCC rise time too fast, need at least 200 us, while using passive crystals, reset reset time must be after the VCC is 3.0 V, about 5 ms, if the external clock power-on reset time is approximately 200 us,




We usually use single chip microcomputer, hardware circuit of the reset circuit design as shown in the figure below, the reset circuit is in view of the low level in terms of effective restoration, the diode is play in the case of a power outage can quickly release at the ends of the capacitor voltage, in preparation for the electric reset on next time,





At the ends of the capacitor initial voltage U0 (generally consists of 0 V), T moments on both ends of the capacitor voltage of UT, is set to 3.3 V voltage VCC,

Capacitance by the current I and at the ends of the capacitor voltage variation formula: I=C * dUt/dt

Can get: dU dt=C * I * t

Both sides respectively integral can be got: I * T=?(0-1) C * dUt; The I T=C * * Ut? C * U0

(U0=0 v),

The VCC=UR + UT can get formula: VCC=R1 * * UT/(C T) + UT

Complete reset on capacitance charging 0.9 * when VCC, at this time can be concluded that T=9 * RC, T is the need to reset the time, according to their own in the circuit parameter to calculate the reset time

T=900 ms;

But in the process of actual use LOTO OSCA02 trigger function of oscilloscope, capture the normal work of the USB circuit boards electric moment reset signal waveform condition; As shown in the figure below (yellow as the VCC voltage waveform, turquoise for complex signal waveform) :






Status of waveform, normal USB circuit boards electricity, VCC rise time is about 100 us, RESET RESET signal is 500 us,




USB circuit boards, electricity without reaction caught oscilloscope waveform as shown in figure 5, VCC in 100 us can be increased from 0 V to 3.3 V, and RESET RESET signal rise time lasts 400 ms, and has amounted to less than 3.3 V,








Encounter this phenomenon, the use of the oscilloscope observation on electric RESET delay relations between the waveform and the VCC voltage waveform can be helpful, of course is also easy to use USB oscilloscope operating on the computer, check the RESET pin resistance and capacitance and so on around the presence of virtual welding, welding leakage errors, such as welding fault, the fault should be capacitance in a capacitance is normal, troubleshooting,