7. MCU Start-up Problems
7.1 VDD Ramp Rate
Problem: 1: Intermittent start-up issues during power up or some devices will not start-up at all.
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Failing to meet the data sheet VDD ramp rate can cause lock-up issues or improper POR and BOR (i.e., BOD).
VDD ramp rates are specified in volts per unit time, meaning that throughout the entire power-up process from
0v to nominal VDD, the VDD voltage must comply with the ramp rate specification.
Note: The following VDD waveform contains (2) VDD ramp rate spec violations and can result in start-up issues. It
is not correct to assume that as long as the VDD reaches its nominal threshold within the ramp time that they comply
with the specification. This is an incorrect assumption and can be seen in the following VDD ramp rate figure below.
During two periods of time where the VDD plateaus, the VDD ramp rate is essentially 0v/ms and therefore does not
meet the ramp rate specification. This can cause issues because internal circuits have thresholds that when triggered
start various time delays that assert either internal reset and/or “Not Ready” signals to the internal logic for a limited
period of time. Some trigger logic is sensitive to rise times and therefore the noise riding on the VDD can also cause
the trigger logic output to chatter. This chatter that gates the enable/disable signals and resets the logic and state
machines, can cause logic metastability issues. The VDD does not start from 0V, and for some products may exceed
the minimum VPOR specification to guarantee a proper Power-on-Reset (POR). Therefore all MCUs have VDD ramp
rate specifications, sometimes including even ramp down rates to cover partial transitory power loss due to power
grid fluctuations.
Figure 7-1. VDD Ramp Rate Violation Example
MCU Start-up Problems
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