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Missing pullup on the RESET pin #7

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matthijskooijman opened this issue Sep 19, 2014 · 3 comments
Open

Missing pullup on the RESET pin #7

matthijskooijman opened this issue Sep 19, 2014 · 3 comments

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@matthijskooijman
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The RESET pin on 256rfr2 contains an internal pullup resistor. However, IIRC Atmel recommends to use an external pullup on the pin as well, especially in "noisy environments". For maximum reliability, we should probably add a pullup here in future versions.
@amcjen
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amcjen commented Sep 19, 2014

I remember going back and forth on this decision during the initial board design. At the time, I decided to leave it off, per the Atmel 2521 Design doc, because there are all sorts of protections one could add on there if you really want to be noise immune and safe--zener diode, ESD protection diode, a cap for noise immunity, and a series resistor for an external reset switch. Since the rest of the pins were not protected to this level, I left off this stuff as well.

Although I do think an external pull-up should work fine. We'll add it to the next revision.

@matthijskooijman
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Right, now I remember that document as well. Note I'm not really sure if it's really needed or worth the extra cost /effort etc, but it wouldn't hurt of course.

@amcjen
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amcjen commented Dec 11, 2014

Referred to in #6

@amcjen amcjen mentioned this issue Dec 12, 2014
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21 tasks
amcjen pushed a commit that referenced this issue Dec 16, 2014
Hardware updates to Scout v1.1
e90d9a4 
- Add 10k pull-up to reset line (#7)
- Add diode in parallel to 10k pull-up reset resistor (#6)
- Tie second pad on 16U2 crystal to ground (was not connected before)
- Replace LDO with lower-ripple alternative (#8) (STLQ015XX33)
- Add ground tie (0Ω) to pin 2 of balun
- Change all caps in RF signal path to RF-rated C0G/NPO (Murata GRM series)
- Reduce C7 and C8 to 100nF, and make C0G/NPO or X7R (<-Atmel engineer, could be 1.0µF instead)
- Consider adding the SE2438T Front-end module (FEM) amplifier
- Consider ferrite bead between digital and analog VDD lines
- Consider decoupling of 0.1µF on all VDD/GND pin pairs
- Move to a four layer board (GND/VDD in the middle two layers, signals on outer two)
- Add VBUS voltage check (with voltage divider to kick 5v -> 3.3v) into RFR2 and 16U2 input
- Change 16U2 to be powered via VBAT always, instead of off of VUSB
- Tie 16U2 pin to RFR2 pin, to notify RFR2 when serial port is opened
- Add series resistor to 16U2->RST line, removing existing reset circuit
- Remove load switch from VUSB, as it's now unneeded
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@amcjen @matthijskooijman