prop talking to bs2

bigirv

What would be the best way to have a prop talk to a bs2 and vice versa? Figured I would mess around with this since i have the BS2 HW board. I already tried serin on the bs2 and the simple serial object on the prop, but the only thing that showed up on the bs2 debug terminal was a p then i changed the values and it was a q but there should have been at least 2 characters. Should I use a different com method?

Mike Green

You can connect a Stamp I/O pin to a Propeller I/O pin if you put a 2.2K resistor in series with the connection. This protects the Propeller I/O pin (3.3V logic levels) from the 5V logic levels of the Stamp I/O pin. On the BS2, you'd use SERIN or SEROUT. On the Propeller, you'd use FullDuplexSerial or Simple_Serial. Make sure the Bauds are the same. I'd suggest using 2400 Baud at first. You could use one I/O pin in each direction on each device for a full duplex connection.

Remember that the BS2 and Simple_Serial on the Propeller are unbuffered. The receiving end has to be waiting for a character when the transmitting end actually sends something or the receiving end will miss it. FullDuplexSerial is buffered and doesn't have this problem when it receives. The BS2's serial I/O is never buffered.

pedward

FWIW, this 5v tolerant issue came up the other day and the proper value has been revised to 3.3k. The protection diodes on the prop are good for 500ua and require a 3.3k resistor to be 5v tolerant without stressing the protection diodes. Just thought I'd pass this along.

Mike Green

Let's see ... The diodes start conducting at about 0.6V (one diode drop) above Vdd which is normally 3.3V. That's 3.9V. The Stamp's Vdd is 5V. Maybe a Stamp I/O pin will reach Vdd, but more typically it will reach about 0.3V below Vdd which is about 4.7V. So 4.7V - 3.9V = 0.8V. By Ohm's Law, 0.8V / 0.5mA = 1600 Ohms. A 2K or 2.2K resistor will give plenty of leeway. A 3.3K resistor will give lots and lots of protection. Most digital power supplies are expected to be correct to within 10%. Worst case would have the Propeller's Vdd be 3.0V (3.3V - 10%) and the Stamp's Vdd be 5.5V (5.0V + 10%). That would give 5.5V - 0.3V - 0.6V - 3.0V = 1.6V. By Ohm's Law, you'd have 1.6V / 0.5mA = 3.2K. The next standard value is 3.3K.

pedward

Yeah, we had that discussion here: http://forums.parallax.com/showthread.php?136429-Ping-resistor

Something I was thinking of the other day: If it only takes a 3.3k resistor on an input to make the Prop 5v tolerant, then why wasn't the input impedance made higher to begin with? I mean, in "input" mode it's supposed to be High-Z, which implies at least 100k. Based on the 3.3k requirement, it would imply that the protection diodes are place nearer to the I/O pins than the input circuitry. :frown: