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A few small schematic improvements

Posted: Fri Feb 15, 2019 1:29 am
by robertbaruch
I've been looking at the schematic (rev D3_E) and there are a few small improvements that I've noticed might be made. Obviously if it works, it works, these are just things I noticed:

The power supply section:
  • Holy war material: C3, C4, C5, C6 are specified as tantalum, but MLCC capacitors would work just as well if not better -- these are linear regulators, not switching regulators -- and they won't fail short :)
  • PWR_JACK says 7-9V, and the manual (rev 1.2) says not to connect more than 9VDC, but it seems that D2 would block any higher voltage, and the LM1117 can take up to 15V.
  • Z1 - not sure why this is needed, considering the above.
  • The USB 2.0 spec doesn't want more than 10u capacitance on VBUS. Higher than that and you're going to have higher inrush current for more time, which could cause a well-behaved USB host to not connect (again, if it works, it works, but still...). You have 2.2u after the fuse, then a diode, then 10u on the LM1117 input. And if the 5V_VR jumper is selected you have an additional 47u. You could instead use a TPS2112, for example. This serves also as a switch between two supplies, so you could also eliminate D5 and reduce C10 to 100n. It also has a current limiter, so you could also eliminate F1.
  • C12 could probably stand being bumped up to 4.7u. This is the example value given in the AT91SAM7S datasheet section 5.3.
  • It looks like R6 and R7 are there to provide the minimum load current required by the LM1117-ADJ. However, the fixed-reference versions don't need such a minimum. You could eliminate R6, R7, R5, R10, R37, R4, and R9 by using the LM1117-5.0 and LM1117-3.3.
  • It's likely that D1 and D4 aren't necessary. The LM1117 datasheet says that these are needed when the output capacitance is extremely high (>1000uF).
The MCU section:
  • R19 doesn't need to be there. The AT91SAM7S datasheet (section 6.2) says that there's already an integrated 15k pulldown on the TST pin.
  • Likewise with R17: section 6.3 says there's already a pullup.
  • ADVREF and AD4-AD7 should probably be connected to ground -- I'm assuming the ADC isn't being used.
  • R26 could be increased to 27k and R32 to 47k. This would match what's in the AT91SAM7S manual (section 35.4) and also reduce the load drawn from VBUS -- not by much, but still :)
  • C19: Holy wars have broken out over this. My take is that if you do separate system ground from shield ground, then the shield ground should connect to a surrounding metal enclosure. There's no such thing here, so you may as well connect shield to system ground.
Anyway, just a few thoughts. I'm going to experiment with a few of these ideas.


Re: A few small schematic improvements

Posted: Fri Feb 15, 2019 7:40 am
by ZrX
Don't know who's responsible for the revisioning or if there's going to be more to come as the next evolution of KF has been in the planning and that would utilise a completely different hardware.