Dynamixel & ARDUINO

[gr33nhorn]

hi jon,
as stated from your past year reply.regarding the ATmega UART can selectively switch off the transmit and receive pins while it is operating, which is what the code my brother wrote does. In general, the Tx pin is disabled, and its in receive mode. When it needs to transmit an answer back again, it disables the Rx pin, enables the Tx, and then sends the response, and switches them back again.

where can i learn how to change the uart code?any gd website to recommend?i really want to learn from the very basic and pick up one by one
thanks in advance

[JonHylands]

No idea - the only example I have is the code my brother wrote...

- Jon

[StuartL]

I can't help with the Arduino programming side of it but if you're using the AVR directly...

The key things you're interested in are the DDR and PORT registers for each of the IO pins. If your TX pin was PD4 and your RX pin was PE0 the registers you would be interested in are:

DDRD bit 4 (data direction for TX pin)
PORTD bit 4 (output data for TX pin)

and

DDRE bit 0 (data direction for RX pin)
PORTE bit 0 (output data for RX pin)

Note that this is all documented in the AVR PDFs.

If DDRx is set to zero the pin is an input pin. This is the default. With the pin as an input pin the PORTx register sets the pull-up for the pin, so bit zero of the PORTE register being zero would mean no pull-up (floating pin, what you want for an input) and bit zero of the PORTE register being one would mean weak pull-up. Pull-up is useful for shared buses but isn't really necessary for the bioloid bus. You can enable it without consequence.

If DDRx is set to one the pin is an output pin and the PORTx bit is used as the value for the output pin.

In all cases you probably want the RX input to be set, although you may wish to disable the receiver so that you don't see your own transmissions.

Code: Select all

cbi(DDRE, 0); sbi(PORTE, 0);  // Set the RX pin to input with weak pull-up.  You probably don't want pull-up on anything but the controller.


And your transmit routine looks like this:

Code: Select all

sbi(DDRD, 4); // Set DDR for Pin D4 to output.
UARTD0 = byte;
while (!txcomplete());
cbi(DDRD, 4);  cbi(PORTD, 4);  // DDR for Pin D4 to input.

[StuartL]

i didn't know that you had modules working this way. This issue was discussed before here with relation to the Pepper board and somehow I got the impression that there was some kind of electrical or signal problem with shorting the RX and TX on the Atmega.

If you set both TX and RX as output (i.e. if(DDRtx & _BV(TXpin) && DDRrx & _BV(RXpin)) then you run a severe risk of blowing up the AVR.

We solve this problem by ensuring that the only place we modify those two bits is in the same routine and the current output is always set to zero (making it an input) before the new output is set. This way we don't short out the PSU even for a microsecond.

It's code discipline

[billyzelsnack]

StuartL.. Do you put a small delay in between? I wonder if those pins can still be high for a short amount time. Seems unlikely that the uC would block on that instruction waiting for it to go low. Maybe it does though or maybe it doesn't really matter.