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Re: [Paparazzi-devel] Umarim hardware question

From: Karoly Molnar
Subject: Re: [Paparazzi-devel] Umarim hardware question
Date: Mon, 5 Mar 2012 11:47:54 +0100

Hi All,

Appears that no one could answer my questions in my first mail. In the meantime I have found a number of other design flaws with the Umarim hardware:

1. Apart from the ITG3200 solder pad issue that is discussed in a separate thread, there is another pad size issue with the switchmode power supply IC, even if this is not that serious. The gap between the center pad and the signal pads is just too small. I had short-circuit here with one of the boards that I tried to solder. Still it is not a serious issue and easy to fix in the gerber file directly (actually you do not need Protel to edit a gerber file).

2. The 4017 IC that distributes the servo signals is powered from 3.3V which might be adequate to drive the 5V servos but certainly not in the guaranteed limits and could be a source of noise related issues if longer servo wires are used (most servos have CMOS input not TTL). In case the servos are driven from 6V which is also common in certain applications, the 3.3V HI level is just unacceptable.

From the list of hardware issues I shall conclude that Umarim needs a V2.0 review; the 1.0 is just not reliable enough, even if it is a nice try in this small form factor. I will certainly not finish the manufacturing of the 3 PCBs that I have purchased but look for another hardware solution.

Karoly Molnar

From: address@hidden
To: address@hidden
Subject: Umarim hardware question
Date: Thu, 1 Mar 2012 20:32:43 +0100

Dear All,

I have a number of questions to the Umarim hardware design. I really appreciate the effort that someone has put in designing and testing this circuit so please do not interpret my questions offensive. There might be a certain motivation behind the design decisions that I do not see yet.

1. The pressure sensor is powered from a voltage reference. The guaranteed output current of this reference is 5mA while the average current consumption of the pressure sensor is 6mA. I guess the designer wanted to keep the voltage as precise as possible in order to have a stable output value but the reference circuit is apparently operating beyond its normal working conditions so this goal is hardly achieved.
2. Also for the pressure sensor, I see here a 16bit ADC connected to it that is powered from 5V and then connected an I2C level translator. Couldn't have been simpler to power the ADC from 3.3 V and match the voltages on the ADC input?
3. Also there are I2C pressure sensors available, why wasn't that used?

I am just guessing that the complicated design has the aim to provide an extremely precious pressure reference. I am still beginner in autopilots so it could be the case that such accuracy is really needed, but at first glance it seems just too complicated to use 5 integrated circuits to get a pressure value.

What do you think?

Karoly Molnar

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