Equinox Systems ISPnano I Series User Manual page 82

Distributor of Equinox Technologies: Excellent Integrated System Limited
Datasheet of ISPNANO S3 KIT - ISP PORTABLE PROGRAMMER
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Discharging the Target Vcc to <200 mV also forces most target microcontrollers to perform a
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'Power-on-RESET' which they would not perform if power had not been fully removed.
User-programmable 'discharge timeout' and 'discharge voltage'
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Please note:
Many microcontroller based target systems feature low-power microcontrollers and do not have a
resistive path to ground capable of discharging the reservoir capacitors on the board. This effectively
means that after the power is removed from the Target System, the Target Vcc rail may not decay for
minutes, hours and sometimes days. This has the effect of keeping power on the microcontroller so it
may keep running even after the power has been removed. It also means that there is a potential
ESD problem when the Target System is removed from the programmer.
For many target microcontrollers, the target voltage must drop below e.g. 400 mV before the on-chip
reset circuitry detects that power has been removed. If the power is not completely removed, then the
target device can either remain in programming mode after the next power-up or it may end up getting
stuck in a brown-out state where it never comes of reset. Using the 'Target Discharge Circuit'
should prevent these problems from happening.
Scope trace showing the voltage decay with the 'Target Discharge Circuit' DISABLED....
The 'Target Voltage' decays from 3.0V to 0V in approximately 20ms.