Re: Diodes on SMPSU module outputs (FIRE RISK).
Posted: Wed Feb 10, 2021 10:28 pm
In the motorola 68060 manual. The problem statement...
Even then you will be fine provided there is at least a 10 ohm input resistor in series per IO to limit the current draw and prevent damage. Just like the Xilinx.
The proposed example solution by motorola...
Break a problem down and separate it from the solution. Decide is it actually a problem in your use case window. If you are building a VME machine with a 10 amp backplane power rail then yeah it could be an issue. If you are putting an accelerator in an Amiga which has a Choke on the input and caps that will take time to charge up then probably not. Don't be fooled into thinking that because X is true and Y follows right after it Y is also true.
All that Motorola has said here is that "it is possible to add three diodes in series from the 5-V supply to the 3.3-V plane". They haven't considered the wider implications of doing this in a consumer piece of electronics where users might accidentally short things.
The actual problem here is we cannot let input signals to the 060 be more than 4V above VDD. Well i'm telling you right now there is always a drop between the motherboard and the accelerator. Firstly its rare to see an Amiga running at true 5v. They're usually around 4.8v. Then there are drops across series resistors and contact resistance drops at the socket. So even if the PSU is missing it probably wont be much more than 4v difference.
Sure you can get guys like Jens who think the chip will explode if you go 0.1v over.. but thats not going to happen and honestly... in terms of outcomes. A flaming house or needing to replace an 060.. its not much of a choice is it?
This is exactly the same as the XIlinx and will be for the same reasons.. As stated above ... in order for the the SMPSU output to be less than 1V when the 5V rail is completely up would need a startup current of 4amps or higher (without sagging the 5V line). So the diodes are completely unnecessary on an Amiga.11.2.1.2 INPUT SIGNALS DURING POWER-UP REQUIREMENT. The second issue
involves the requirement that during power-up, input signals to the MC68060 not exceed Vdd
by more than 4 V. This is achieved by ensuring that the 5-V supply not exceed the 3.3-V
supply by more than 4 V.
Even then you will be fine provided there is at least a 10 ohm input resistor in series per IO to limit the current draw and prevent damage. Just like the Xilinx.
The proposed example solution by motorola...
I disagree with this solution and i think i was cobbled together in a hurry. I think it's something that has been thrown into the docs without enough thought and are better solutions than this. It did not address the issue of the power rail being shorted or the safety needed in that instance.In any of the previously discussed DC-to-DC conversion solutions,
it is possible to add three diodes in series from the 5-V supply to the 3.3-V plane. During
power-up, the diodes forward bias and thus provide a current path between the 5-V source
and the 3.3-V plane. This solution provides no more than (0.7 * 3) = 2.1 V drop between the
5-V input and the 3.3-V plane. When the voltage regulator stabilizes, the difference of (5 –
3.3) = 1.7 V is insufficient to forward bias the three diodes, hence not dissipating any energy.
Both Motorola and Linear Technologies have indicated that the three diode shunt does not
adversely affect operation.
Break a problem down and separate it from the solution. Decide is it actually a problem in your use case window. If you are building a VME machine with a 10 amp backplane power rail then yeah it could be an issue. If you are putting an accelerator in an Amiga which has a Choke on the input and caps that will take time to charge up then probably not. Don't be fooled into thinking that because X is true and Y follows right after it Y is also true.
All that Motorola has said here is that "it is possible to add three diodes in series from the 5-V supply to the 3.3-V plane". They haven't considered the wider implications of doing this in a consumer piece of electronics where users might accidentally short things.
The actual problem here is we cannot let input signals to the 060 be more than 4V above VDD. Well i'm telling you right now there is always a drop between the motherboard and the accelerator. Firstly its rare to see an Amiga running at true 5v. They're usually around 4.8v. Then there are drops across series resistors and contact resistance drops at the socket. So even if the PSU is missing it probably wont be much more than 4v difference.
Sure you can get guys like Jens who think the chip will explode if you go 0.1v over.. but thats not going to happen and honestly... in terms of outcomes. A flaming house or needing to replace an 060.. its not much of a choice is it?
