25 v signals bor 221 svsl 222 svsh 223 and vddon 209 supply ports_on control logic 231 which determines ports_on signal 214 a low state in ports_on signal 214 forces i o circuitry into a hiz condition both supplies vdd 201 and hvdd 210 are supervised by power supply safety system psss 229 psss 229 includes a pssb circuits 232 generating bor 221 pssl circuit 236 generating svsl 222 and pssh circuit 234 generating svsh 223 the following unsafe power supply conditions are of interest for a brownout decaying vdd brownout reset circuit pssb 232 senses vdd low supply supervisor pssl 232 generates bor and pssb 232 is powered by vdd this brownout feature is always active when vdd gt vth where vth is nmos threshold voltage 1 0 v nom for a short circuit on vdd having a medium to high rate ramp down low supply supervisor pssl 236 vdd senses and generates an active svsl 222 pssl 236 uses precision comparators powered by vdd pssl 236 and internal reference voltage source 235 are powered by hvdd for pssl 236 to produce an active signal svsl 222 it must have hvddmin gt 2 0 v for a decaying hvdd with a medium to high rate ramp down high supply supervisor pssh 234 senses hvdd and generates svsh 223 pssh 234 uses precision comparators powered by hvdd pssh 234 employs an internal reference voltage source 235 and is powered by hvdd pssh 234 must.