I’ve been working on a project that uses I2C for Temps, Humidity, and 16x2 RGB LCD display,
(3) 1-Wire DS18B20’s and reads 2- analog voltages. Everything is working using a small breadboard.
I decided to use the Shield Shield since it provides a small prototyping area and the Adafruit (714) Display Shield seemed like a good choice. I discovered the when the Photon is plugged into the Shield Shield that it interferes with the Display Shield unless you use stacking headers, this raises the Display Shield by 3/4". A low profile pin connector by Mill-Max might may eliminate needing to use stacking headers.
All components are wired to the Shield Shield and everything is working except the 1-Wire devices, trying D2, D3, D4, D5, D6 and they all behave the same. About 1.5 seconds or so so after the firmware is loaded the pin controlling the DQ of the 1-Wire bus starts oscillating. The only difference I can find in the two setups is the TXB0108PWR Level Translators. Since the Breadboard setup up does not have level translators I’m wondering if I should remove them from the SS PCB. A note on the SS data sheet,… excessive capacitive loading and cause the auto direction detection to fail. The 1-Wire signal is plugged directly into D6 on the Photon header using a 5" breadboard patch wire. And that is on the 3.3V side of the translator. The breadboard setup also used a 5" patch wire for the 1-Wire bus and it is functioning properly… I don’t see for my application the need for the Level Translators but maybe I missing something.
Last night I tried connecting 1-wire bus on the 5V side so DQ went through the translator and the DQ signal stays about at a 2V level. On page 16 of the translator data sheet it says OE should be tied low through a pull down restore at power up to put the lines in a high Z state and "
• An external pulldown or pullup resistor decreases the output VOH and VOL.”
I can’t find any pull down resistor on the Shield data sheet, just a 10K pull up resistor.
Photos when DQ is connected to D5 or D6 and not working.
IMG_0525.JPG
In a few seconds trace changes to:
IMG_0523.JPG
I stripped my program down to just 1-wire devices and turned Off all I2C code. Then I moved DQ to (D0) right at the µcontroller. This line goes through a dedicated MOSFET based voltage translation on the I2C lines, since D0 & D1 are used for I2C, this is shown on the Shield datasheet, page SHIELD SHIELD SCHEMATIC - MOSFET I2C.
The 1-wire now works
.
Scope settings were not changed from those shown on this photo. The number 1 on the Left side of the screen is
Ground level for the Yellow trace.
IMG_0521.JPGIMG_0526.JPG
DO YOU THINK I SHOULD USE A HTTP://WWW.LINEAR.COM/PRODUCT/LTC4315 after removing
the TXB0108 IC’s?
I’ve decided to add a mosfet in the prototyping area like the dedicated mosfets used for the I2C (Do & D1) to do the level translating for the 1-wire bus. Does anyone know the part number for Q1 & Q2 on the Shield Shield?
