Questions on the Rev F1 schematic

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unebonnevie
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Joined: Sat Sep 19, 2009 7:46 am

Questions on the Rev F1 schematic

Post by unebonnevie »

Hi,

I am reading the Rev F1 schematic (http://belogic.com/uzebox/schematics/Uzebox-rev-F1.pdf), have some questions:

1. The led that is connected to PD4 is the power indicator? That is, when the power button connected to PD2 is toggled, the led will turn on or off? If in OFF state, how the MCU is turned off, since I don't any switch of turn off the incoming power?

2. Isn't a decouple cap of 0.1uF sufficient for pins 11 and 10 and other VCC-GND pairs?

3. The voltage divider is a neat trick to make sure the SD card will get only 3.3-3.6V for the MISO/MOSI pins. What I don't understand is how the SD card is immune from signals when the MISO/MOSI are active during programming the MCU? And signals for programming the MCU will be flaky? That is, how this voltage divider can really help?

Thanks.
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uze6666
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Re: Questions on the Rev F1 schematic

Post by uze6666 »

Hi,
1. The led that is connected to PD4 is the power indicator? That is, when the power button connected to PD2 is toggled, the led will turn on or off? If in OFF state, how the MCU is turned off, since I don't any switch of turn off the incoming power?
Well, it's more of a "user LED" and the power switch, a "user switch". Currently, the kernel doesn't use it, but it could in the future. The trick would that a pin change interrupt on the "power" switch would have the MCU stop the LED and enter sleep mode. Another toggle would wake the MCU.
2. Isn't a decouple cap of 0.1uF sufficient for pins 11 and 10 and other VCC-GND pairs?
100nf is good enough when the MCU talk to other digital devices. However the resistor DAC can require sudden burst of current, which can be quite "big" when switching from black to white pixels for instance. Small caps doens't hold enough current for those burst. I recall doing lots of testing for this cap and I ended up with a tantalum vs. a electrolytic. Tantalum seemed to much better handle ripple current and gave a better picture. A picture is worth a thousand words, check out these comparisons: http://forums.adafruit.com/viewtopic.ph ... b0b#p41260.
3. The voltage divider is a neat trick to make sure the SD card will get only 3.3-3.6V for the MISO/MOSI pins. What I don't understand is how the SD card is immune from signals when the MISO/MOSI are active during programming the MCU? And signals for programming the MCU will be flaky? That is, how this voltage divider can really help?
Hmm, good question. What I can say is that the MCU programming is not flaky and all work fine!

-Uze
unebonnevie
Posts: 36
Joined: Sat Sep 19, 2009 7:46 am

Re: Questions on the Rev F1 schematic

Post by unebonnevie »

Thanks for the answers! Wow! I certainly learned something there about using the 10uF tantalum cap rather than the plain old 0.1uF because of the anlog DAC potential power swing from black to white in color.
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