Re: [Paparazzi-devel] Tiny v2.1 - PCB files and Components list

[nisma]

Hi, i have made similar things.
some comments.

- J17 (IRH) change the 4pin header to 5pin header like ADC0, this eliminates 
the need
         for a 4pin header.
- LPC_RESET     add this signal to the download header.
- 4017  The supply should be connected to 5V in order to make it compatible 
with all servos,
        not only JR and some others.
- GPS_RESET     max voltage is 1,95V . I recommend
        using a open drain pin (output on gnd, otherwise input) to drive it.
        For merging BOOT and GPS_RESET, i suggest a diode in order to protect 
the GPS
        from user download socket. Don´t forget a 10n cap. Don´t make 
programmer errors or
        the gps module become damaged. If you connect the GPS_RESET to the 
LPC_RESET as 
        described below, merging boot and GPS_RESET don´t work.
- GPS   Connection from V_ANT to GND is wrong.  Leave unconnected if not used.
        Connect VCC_REF and V_ANT with a 10 Ohm resistor. 
        Note that this resistor must be 1/4 Watt, the normal 603 resistors 
don´t work.
        This is for active antennas (3.3V, 5V designs don´t work) or it´s 
possible to don´t
        popolate it, or to drive it by SW.
- GPS   add a testpoint or solder jumper to BOOT_INT in order to possibly 
recover from a 
        wrong or failed firmware update.
- GPS/LPC_RXD0  add safety resistors, in case of both GPS and USER drive the 
pins, ie 
        GPS_RESET is not driven low or the low driving signal is too high (gps 
require low <0,65V , LPC <=0.8V).
- GPS EXTINT0  shoud be used, example: LPC214x drives the camera and at the 
same time triggers
        timestamp from gps, that is logged, included the gps position. 
        Maybe isolated with a resistor, that can not be placed or eventually 
desoldered.
- GPS TIMEPULSE should be used, specially when the gyro module with a own 
processor is used,
        in order to timestamp the single ADC readings and as another case to 
check the osc long term stability.
- GPS/LPC  VBAT from LPC and GPS should be connected. This allows holding a 
(checksum protected)
        ram portion as logging (usb memory as example, if usb is not used in 
flight), that
        allows having a history, if somethings goes wrong. At the same time, if 
there are
        problems with the main supply, some spikes or ground bouncing from the 
servos,
        with the VBAT used, one can be sure that the ram contents is ok.
- GPS   GPSMODE should be adjusted, either (12/6/5) 1/1/1 or 1/0/0
- GPS Reset     As Note, reset can be triggered by SW, without the need for a 
HW pin.
- RESET strategy: The GPS has a internal 2.4V voltage supervisor. If the GPS 
module is
        connected to the 3.3V using a 0.3V drop diode, the voltage supervisor 
could be
        used for both, the gps (2.4V) and the LPC (2.7V). Maybe consider 
improving the
        reset line from GPS by pulling a external resistor and cap between the 
reset and
        the vdd18out line. (cap naturally to GND). 
        Note: The LPC reset voltage must be above 2.0V, this implies a sort of 
level converter.
- Powering: It can help, if the analog VCC and the GPS VCC came from a 3V LDO 
connected to the
        3.3V VCC rail. Further the 3.3PSU are located a bit distant from the 
GPS module.
        Using a 3.0V LDO can be the same as using the 0.3V drop diode for the 
voltage monitor.
- GOLDCAP add the possibility to use a recharcable battery instead of the 
goldcap.
        It´s not so easy to find goldcaps rated for higher temperature.
- GPS / GPSMODE design for the possibility to use the cheaper A module.
- SUPPLY: the PSU must be checked, if the VOUT could be at higher voltage than 
VIN.
        Othervise protection diodes must be added to the desing.
- USB-Led add a led after the T3 resistor in order to have a feedback when the 
USB is activated.
        This led don´t need to be populated, but it´s nice to have it.
- USB hotplugging       Protection diodes are needed for hotplugging.
        For the same reason, 100 Ohm resistors should be plugged on the button, 
led3 ... lines.
- supply Analog and digital GND/VCC should be layouted separatly and merged in 
one point. 
- Supply Add a 603 resistor before the inductance. This could limit the 
interference and can
        be substituted with a ferrite bead.
- POWER Seeing the DC/DC converter, use the Inhibit. This can be used to dont´ 
allow power-up
        the thing, if the battery voltage is too low. Further, add a switch to 
the LCP and
        Inhibit with a OR (two diodes) borren from VBAT_MEAS and one line from 
LPC.
        In this case, one can power up the thing like a cellular phone.
        On power up (power line pressed) the LPC sets the own power line high 
in order to
        remain powered. If the button is pressed for a long time, the LPC 
powers off itself.
        Shorter button pressing could be used for takeoff, abort signaling and 
similar.
- LAYOUT: Check the lea integrations guide for dividing the analog section on 
the board with 
        the RF emitting digital section as MCU´s and similar. This increases 
the size of the board,
        but it helps a lot for having a better GPS solution. I prefer having 
the GPS on it´s own board,
        maybe combined with the infrared sensors.
- LPC214x       The ground plane is a bit missing. With some more hand-Layout 
it´s possible
        to have more ground plane below the MCU. 
- Grounds between servos and MCU, especially GPS should be divided. Think of 
current measuring
        of the servos using a resistor between the MCU`s ground and the Servos 
ground (including cap)
        in order to measure the current of servos and automatically reduces a 
bit the ground bouncing.
- TVS   Adds some TVS devices, for the GPS antennas, there exist extra devices. 
Required for EC compliance test,
        because the GPS antenna input is not 20K safe.
        Further,  TVS could help with the ground bouncing problem when using 
the current sensor
        because this behaves like filters on certains situations. Spice 
simulation is suggested.
- Temperature: Add a internal thermistor or temperature sensor (i2c) in order 
to sense the own temperature
        Further, on the power connector, add a pin for a thermistor sensing 
that either can be used as thermistor
        or 1-Wire interface for cheap smart-battery controller.
- Power after adding the power logic in order that the board can be power-on or 
off by SW, add
        the possibility to power only the logic from the USB, excluding the 
servos.
        This allows powering the uav externally from the usb port (modem, gps, 
micro) having
        a sort of standby without using the main battery or for HIL simulations 
without consuming battery.
- AUX   It will be nice, if a high power led or buzzer or something similar can 
be switched
        on indipendently of the payload. This needs only a additional 
transistor connected
        to the 5V rail (not 3.3V) and naturally two resistors.
- I2C mem       Add a I2C memory to the PCB. This allows logging external 
sensors without needing extra payload cpu.
 
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