Case study of power-on reset fault of microcontroller using oscilloscope

Recently, a product was designed using the CY7C68013A chip. In fact, there is a 51 microcontroller inside to control USB communication. During the test, it was found that after some circuit boards were powered on, the computer could not discover the new USB hardware, nor did it prompt that it could not be recognized. There was no UNKNOW DEVICE prompt in the device manager. After careful investigation, it is suspected that the internal 51 microcontroller is not working.

After checking the crystal oscillator, power supply, IO pins, etc., no problems were found. Finally, I suspected that there was a problem with the circuit reset.

According to the chip data: the VCC rise time cannot be too fast, it needs at least 200us. When using a passive crystal oscillator, the reset time must be approximately 5MS after VCC reaches 3.0V. If the external clock is used, the power-on reset time is about 200us.

When we generally use a microcontroller, the reset circuit design in the hardware circuit is as shown in the figure below. This reset circuit is for low-level active reset, in which the diode can quickly release the voltage across the capacitor in case of power outage to prepare for the next power-on reset.

The initial voltage across the capacitor is U0 (usually set to 0V), and the voltage across the capacitor at time T is UT. The 3.3V voltage is set to VCC.

The relationship between the current I flowing through the capacitor and the voltage change across the capacitor is: I=C*dUt/dt

It can be obtained: I*dt=C*dU t

By integrating both sides separately, we can get: I*T=∫(0-1)C*dUt; that is, I*T=C*Ut−C*U0

(where U0=0V),

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

The reset is completed when the capacitor is charged to 0.9*VCC. At this time, it can be concluded that T=9*RC, and T is the required reset time. Calculate the reset time based on the parameters in your own circuit

T=900MS;

However, in the actual process, the trigger function of LOTO's OSCA02 oscilloscope is used to capture the waveform state of the reset signal when the USB circuit board is powered on during normal operation; as shown in the figure below (yellow is the voltage VCC waveform, blue-green is the reset signal waveform):

Observing the waveform status, when the USB circuit board is powered on normally, the VCC rise time is about 100US, and the RESET reset signal is 500US.

When the unresponsive USB circuit board is powered on, the waveform captured by the oscilloscope is shown in Figure 5. VCC can rise from 0V to 3.3V at 100US, while the rise time of the RESET signal lasts for 400MS and never reaches 3.3V.

When encountering this phenomenon, it is helpful to use an oscilloscope to observe the delay relationship between the power-on reset waveform and the VCC voltage waveform. Of course, it is also very convenient to use a USB oscilloscope to operate on a computer. Check the resistors and capacitors around the RESET pin for defects such as weak soldering or missing soldering. This time the fault should be caused by the capacitor being soldered incorrectly. Replace the capacitor and it will be normal. The fault is eliminated.