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ben-wpan/atusd/ERRATA
Werner Almesberger 6e726d1fb9 Improved clock stability by using a capacitative divider and found more minor
issues.

- atusd/ERRATA: work around the clock instability by replacing the
  resistive divider with a capacitative divider
- atusd/ERRATA: a ground plane under the clock circuit would also be good to
  have
- atusd/ERRATA: via near pin 1 is too close to the chip if we need to cut
  wires (in DIY boards)
- atusd/sim/cdiv.sch: simulation of the capacitative divider
2010-09-09 12:35:47 -03:00

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Version 20100903:
- changed C7 to 1 nF to debug a signal attenuation problem. Turned out to be
a bad trace. According to simulations, 22 pF should be more than enough.
- added wire connecting uSD-side ground plane to ground plane at outer edge,
to improve CLK signal return. (Probably unnecessary, too.)
- the footprint of the transistor (Q1) is reversed :-( It works after
converting the chip from SOT to PLCC.
- not an erratum, but with experiments showing power-on reset to be
reliable, we can consider removing the hardware reset circuit. This will
also simplify the layout.
Version 20100908:
- SPI activity causes the PLL to unlock. Specifically, toggling nSEL does
this.
- work-around on second 20100908 board: replace the resistive divider with
a capacitative divider. See sim/cdiv.sch. This is a simple BOM change:
C7 -> 0 R
R3 -> 33 pF
R4 -> 220 pF
- considering that the clock input has a Vpp of only 400-500 mV, we should
have a ground plane also under as much of the the clock circuit as
possible.
- via between pins 1 and 32 is too close to the chip for DIY PCBs