Thanks for the long reply.
Oh, I did not recognize the operating voltage problem.
Most servos will just work fine with a 3.3V PWM signal as long as you
supply 5V to the power connection of the servo. I am not sure if that is
risky for the MC ?
But since you have to use another battery / regulator, you will not add
much complexity when adding an additional MC, for example an arduino.
I just thought it would be cool if you could simply plug in an servo to
the altimeter, and did not recognize the problem with the operating voltage.
Best
Thomas
Am 27.05.2013 06:55, schrieb Bdale Garbee:
Thomas Müller <[email protected]> writes:
there is one thing I would really like to see in future versions of
TeleMetrum or TeleMega. It is the possibility to control a
servo.
It is very useful for controlling servo releases for serial dual deploy
like spacetecs SRM2. They are pretty common in Europe.
Interesting .. I've never actually seen someone using an RC-style servo
for deployment before. As you say, there's no conceptual reason we
couldn't do that... but in practice there are some issues I'll address
below.
In principle you need only a analog/digital output which can be freely
controlled.
To drive an RC-style servo, you need to be able to PWM an output pin,
which is easiest to do if that pin is connected to a timer with PWM
support, but of course also possible to do with a GPIO and software PWM
generation. That part isn't hard. The difficult part is that all the
servos I know of run at 5V, and all of our products are 3.3V using
single-cell LiPo batteries with 3.7V nominal and 4.2V fully charged. So
at minimum, you'd need a separate battery / regulator to generate the 5V
that is required, and you'd need a level shifter (transistor) on the PWM
output to enable it to drive a 5V input.
My son and I are actually working right now on a board to control 12
such servos for a robotic arm project using the same STM32L151 part
that's on TeleMega. I haven't loaded the first prototype boards yet,
but once we do, we'll be generating an RC servo PWM driver for AltOS
that could potentially be used in our rocket firmware on boards like
TeleMega using that chip.
Is there some output accessible? As far as I understand the companion
port it is only a SPI interface.
The SPI clock, miso, and mosi pins on the companion connector are shared
with other devices on the board, but the chip select line is a unique
GPIO (P1_2) that could be re-tasked with a firmware modification.
Port 3 and 4 on the debug port seem for me to be regularly unused
digital IO ports. Can they be used?
Yes. We've avoided using them for anything "active" simply because it's
nice to be able to do source-level debugging of the firmware without
having to compete with some active function... but there's no hard
reason you couldn't re-task these. They are P2_1 and P2_2 on the
cc1111.
Any of these three pins would require software PWM generating code, and
the biggest problem with that is that the existing TeleMetrum build uses
almost all of the available memory, and so anyone wanting to create
custom firmware to drive servos would have to choose something to live
without...
It would certainly be easier to implement this in TeleMega where we're
not yet so resource constrained... but even there some solution to the
5V interface problem needs to be devised.
Thoughts?
Bdale
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