1-Wire bus implementation in PIC18F series for Parallax ColorPAL #28380 sensors

Question

I've bought a Parallax ColorPal 28380 color sensor, http://www.parallax.com/Portals/0/Downloads/docs/prod/sens/28380ColorPAL.pdf

In the documentation above, they've used a Basic Stamp microcontroller, and I want to use this sensor with Microchip's 18F (18F2550 or 18F4550) microcontroller series.

The problem that I've been experiencing is, the sensor communicates in 2400-7200 bps range. I can't use PIC's own UART unit because the sensor has 1 pin only. First thing that came into my mind is using 1-wire protocol but it is operating at 15400 bps which is too fast.

Now I'm wondering, if I place a diode in front of TX line and merge it with RX line, is this setup works ? Or is there any way to control the transmission speed of the 1-Wire protocol ?

Thanks in advance,

Answer 1

First of all, you shouldn't use a UART because it's an Open-Collector interface - the pin used for the serial data should never be driven high. It should be set to tri-stated (high-impedance input) to transmit a "one" bit and set to an output driven low to transmit a "zero" bit, and this data should be sent at a prescribed baud rate. I'm pretty confident that any UART by itself will drive the signal both high and low. So if you want to use a UART to implement this interface, you need to introduce some external hardware. The following circuit is taken from an Atmel app-note:

I think the module you've linked to provides the (equivalent of) the 4.7kOhm pullup, but it's hard to say.