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Power Supply
The power supply of the PS1 is an all-in-one provider for the following:
- Power outlet hookup through a non-standard C7 power cord.
- Rectifies, stabilizes and transforms the AC domestic voltage levels to roughly 8v and 3.3v.
- Handles power on and off by a dual pole switch on the 8v and 3.3v lines.
- Handles reset through a momentary switch.
- Delays and monitors the reset logic through a supervisor chip.
- Provides power and reset logic to the motherboard through a 5-pin cable.
The final pinout of the power plug ends to be always the same on all versions of the console:
| 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|
| 8V | GND | 3.3V | GND | !RESET |
The motherboard then breakouts the power plug into the following subrails:
- "MOT_+8V", with a 0.8A fuse, a 1uH inductance, and a 47uF 16V polarized capacitor.
- "EXT_+8V", with a 0.8A fuse
- "DIG_+3.3V", with a 2A fuse, a 1k pull up resistor to !RESET, and a 220uF 4V polarized capacitor.
- "CTR_3.3V", with a 0.6A fuse.
The datasheets are claiming that the second power rail is 3.5v, but it magically becomes 3.3v on the motherboard itself, without any component in between. Doing some investigation shows that using a plain 3.3v power line will work fine.
The 8v power rail has two main consumers. The first one is the cd-rom and its motors. The second one is a bit more peculiar: there are two LDO regulators that are chained to each other, behind a 0.8A fuse.
The first regulator is a 5v regulator, that brings power to the audio amplifier and video d/a converter, both behind their individual inductance and capacitors, and provides a "DIG_+5V" power rail, also on its individual inductance, used by the audio dram chip.
Then the second regulator is a 3.5v regulator, breaking out to its own power rails of "AUD_+3.5V", with an inductance and capacitor and "SER_+3.5V", with only a capacitor, both feeding into the analog power rails of the SPU.
As for the 3.3v power rail from the PSU, the CTR_3.3V rail is used exclusively to power on the controllers, while the DIG+3.3V rail powers on every chip of the motherboard. In fact, the SPU uses a total of 3 power lines: DIG_+3.3V, used on many different pins, and also the AUD_3.5V and SER_3.5V rails, used on fewer, dedicated pins.
It seems fairly clear that the 8v power rail from the PSU is used mainly to handle the higher voltage motors, and anything that's analog signal related - with the exception of providing 5v to the SPU DRAM chip - while the 3.3v power rail from the PSU is used exclusively for anything digital and logic-related.
The reset circuitry is mostly handled through the PSU itself, active low as typical for chips, with the exception of a single 1k pull up resistor on the motherboard itself. This means that the motherboard can be powered on with simply the first 4 pins of its power plug, and the reset signal will be naturally asserted high, meaning all of the logic will boot and hopefully work.
The reset controller chip on the PSU will do 3 things:
- Monitor the power levels of the 8v and 3.3v power rails
- Listen for the user button for issuing a reset on the console
- Debounce and ensure a minimum reset delay on power up and user reset
During testing, not providing a proper reset signal will sometime result in bootup glitches, up to complete system freeze.