iMac G3 computers still refusing to power up? Here is what I've found out!
iMac G3 computers still refusing to power up? Even with a new PRAM battery? Even seconds after a PMU reset? Here is what I have found out!
I read in these discussion forums so many iMac G3 users have failed to fix the no-power-up issue. They have purchased a new battery and applied the procedures to reset the Power Management Unit (PMU). To no avail! Then, some of them have noticed the iMac can start without the battery, but provided the PMU is reset while the mains is supplied to the computer. But the time and date are again reset respectively to 12:00 AM and 01/01/1904.
Actually, no matter if a battery is inserted in the holder, powering up the iMac G3 can only happen within two seconds after the PMU reset button has been released.
Why two seconds?
In a faulty iMac DV, I have made some observations by means of an oscilloscope on pin 9 (XCOUT) of the logic-board U34 chip, which is the PMU microcontroller (Mitsubishi M30624FGLGP, now Renesas M30624FGAGP). I noticed the 32.768-kHz oscillations vanish two seconds after the iMac has been completely turned off (the hard disk stops spinning and the green light in the front power button goes out) and also after the active-low reset pulse on pin 10 (RESET) has elapsed. Without the low-frequency sub-clock in the low-power-consumption mode, the PMU cannot execute any firmware inside, and hence it can neither react to a power button signal on pin 73 (P15/D13/INT3) nor perform any timekeeping. Actually, its firmware interpretes a power-off command from the G3 microprocessor through the SPI serial bus, and mistakenly sets bit 1 of the CM1 register (system clock control register 1) to 1 and/or bits 4 and 5 of the CM0 register respectively to 0 and 1, after two seconds of routine execution. In both situations, these binary settings force the on-chip oscillators to shut down, and the microcontroller enters the STOP mode.
Neither the operating system nor Open Firmware offer any command to directly gain access to these register settings for correction. The problem instead is that the firmware portion which reacts to the shutdown Open-Firmware method, has to be fixed in the FLASH memory of the PMU microcontroller.
The owners of an early 2001 iMac or of a summer 2001 iMac may succesfully fix the issue by applying the Power Management Update 1.1, which is downloadable under this link:
http://docs.info.apple.com/article.html?artnum=120048 .
The original iMac, iMac DV and iMac DV Special Edition don't let this update proceed and an abort message indicates "the iMac Power Management update is not compatible with this Macintosh".
In the latter case, it would be great if a software developper at Apple Inc. could volunteer to fix the firmware and post the patch on the Apple website. There are perhaps thousands of owners all over who would enjoy their iMac like in the early days of this marvellous Apple experience.
/* User's Manual of the PMU microcontroller */
http://documentation.renesas.com/eng/products/mpumcu/62aeum.pdf
Aegidius_2
------------------------
Knowledge is not valuable without the science of problematics.
I read in these discussion forums so many iMac G3 users have failed to fix the no-power-up issue. They have purchased a new battery and applied the procedures to reset the Power Management Unit (PMU). To no avail! Then, some of them have noticed the iMac can start without the battery, but provided the PMU is reset while the mains is supplied to the computer. But the time and date are again reset respectively to 12:00 AM and 01/01/1904.
Actually, no matter if a battery is inserted in the holder, powering up the iMac G3 can only happen within two seconds after the PMU reset button has been released.
Why two seconds?
In a faulty iMac DV, I have made some observations by means of an oscilloscope on pin 9 (XCOUT) of the logic-board U34 chip, which is the PMU microcontroller (Mitsubishi M30624FGLGP, now Renesas M30624FGAGP). I noticed the 32.768-kHz oscillations vanish two seconds after the iMac has been completely turned off (the hard disk stops spinning and the green light in the front power button goes out) and also after the active-low reset pulse on pin 10 (RESET) has elapsed. Without the low-frequency sub-clock in the low-power-consumption mode, the PMU cannot execute any firmware inside, and hence it can neither react to a power button signal on pin 73 (P15/D13/INT3) nor perform any timekeeping. Actually, its firmware interpretes a power-off command from the G3 microprocessor through the SPI serial bus, and mistakenly sets bit 1 of the CM1 register (system clock control register 1) to 1 and/or bits 4 and 5 of the CM0 register respectively to 0 and 1, after two seconds of routine execution. In both situations, these binary settings force the on-chip oscillators to shut down, and the microcontroller enters the STOP mode.
Neither the operating system nor Open Firmware offer any command to directly gain access to these register settings for correction. The problem instead is that the firmware portion which reacts to the shutdown Open-Firmware method, has to be fixed in the FLASH memory of the PMU microcontroller.
The owners of an early 2001 iMac or of a summer 2001 iMac may succesfully fix the issue by applying the Power Management Update 1.1, which is downloadable under this link:
http://docs.info.apple.com/article.html?artnum=120048 .
The original iMac, iMac DV and iMac DV Special Edition don't let this update proceed and an abort message indicates "the iMac Power Management update is not compatible with this Macintosh".
In the latter case, it would be great if a software developper at Apple Inc. could volunteer to fix the firmware and post the patch on the Apple website. There are perhaps thousands of owners all over who would enjoy their iMac like in the early days of this marvellous Apple experience.
/* User's Manual of the PMU microcontroller */
http://documentation.renesas.com/eng/products/mpumcu/62aeum.pdf
Aegidius_2
------------------------
Knowledge is not valuable without the science of problematics.
