Was this a MEGA8 or MEGA8L ? Do you have the Brown-Out-Detector (BOD) enabled or disabled? If enabled, what BOD level are you using, the higher or the lower level? What is the supply voltage to the AVR? Are the fuses set to same in working and non-working cases? Have you checked if the datasheet states maximum rise time for power supply?
The AVRs might be buggy, although I highly doubt it. Maybe the conditions are just so extreme the AVR will not boot? But seriously, at such VCC rise conditions one would normally use a voltage monitor chip like MAX709 or something to keep the AVR reset until the power supply is over some treshold value.
What happens, if you manually (by switch) put it in reset after the power is UP (4.5V) and hold it there for some seconds? Will it then boot ?
0.25V/s rise time are extreme conditions for an AVR !
What happens if you hold it manually in reset during power up ?
Keep in mind that the Reset Threshold value for Reset is 0.2..0.9 x Vcc !
If Vcc is 1V, then the AVR goes out of reset at 0.2..0.9 V !
If your voltage rise is so slow, that will happen !
With this low voltages, the AVR cannot run without failure !
Try to use a PNP transistor and a Zdiode at the base ( to Vcc) while the transistor is pulling down the AVRs reset until the base voltage is > ~(0.5V+Vz)
Yes, it will re-boot if I tick the reset pin while its in limbo at full voltage.
"0.25V/s rise time are extreme conditions for an AVR ! "
agreed, but that was just an example. its more like .75 to 1v / s
Still, the old lot works perfect and the new lot does not. its that simple.
© 2020 Microchip Technology Inc.