Richi's blog

Tag: raspberrypi

  • sniffing i2c with the BusPirate

    I received my BusPirate v4 a while ago, but didn’t really use it so far. That’s a cool analysis/debug tool for serial buses such as uart, spi, i2c and the like. For me i2c is the most interesting. From time to time, the communication doesn’t work as it should, and so far, I worked it out with trial and error. I hope the BusPirate can be of help in such situations in the future. So, here is my first test run.

    The BusPirate is controlled through an uart textual interface:

    minicom -D /dev/ttyACM1 -b 115200

    When you connect to it, it performs a self test, and then you can choose the mode by entering m. In my case, that’s 4 for i2c. Next I get to choose between hardware and software. I don’t know the implications yet, but what I see is that hardware offers higher speeds, and locks up more often. Then I get to choose the bus speed. 100KHz is the standard. With ? you can always get a list of possible commands. (0) shows a list of available macros. (1) scans all possible addresses for connected devices, just like i2cdetect would do it on the computer. (2) finally is what I was after, that’s the i2c sniffer.

    I was actually hoping it could find out why I’m having problems reading back a simple value from an AtMega8 to a RaspberyPi. The AtMega8 is at address 0x11 and the command to read the value is 0xA1. I verified with a serial connection to the AtMega8 that it has a proper value, but on the RaspberryPi I always get a 0. At least the command was received on the AVR as I could verify with the UART, but writing the value back is the problem. So here is what the sniffer outputs for the attempted read:

    [[[][]][[[0x01+][0x04-[][[0x20+][[[[[][0x20-[][0x4C+][0x04-[][0x24+][0x20-][]]]

    Let’s decipher those numbers. Plus means ACK and minus means NACK. Opening square bracked means start bit, and closing square bracket means stop bit. The expected sequence would be 0x22 (the address for sending to the AVR) 0xA1 (send back which value) 0x23 (the address for receiving from the AVR) 0x08 (or whatever value was stored on the AVR). But the above output doesn’t look like this at all. So, lets try to communicate from the BusPirate to the AVR directly. Here we go: (more…)

  • RaspberryPi reading analog input using an AtTiny through i2c

    The raspberrypi has some GPIO (General Purpose Input Output) pins. That’s great for experimenting with electronics for example sensors and actuators. It’s totally different than an Arduino in many respects, but that’s something they have in common. Some of the pins have special functions. For example SPI, I2C, UART …

    There is a breakboard adapter for all the GPIO pins with a ribbon cable that you can order from the US. That’s cool, but ordering stuff from abroad can be expensive. And the pins look somehow like good old IDE. So I soldered an adapter myself and bought an IDE cable. Well, some pins worked, and some didn’t… Enough for the first round of experimenting, but it took a while to find out what’s going on. I just assumed that all the wires of the IDE cable were connected which for some reason was not the case.

    But something is missing that the arduino offers: analog. Before I really needed analog sensing capabilities, I found an article, describing a hack to read analog input by measuring the time it takes to discharge a capacitor through the resistance you want to measure. Immediately, I tried it myself with a photo resistor. The author warned, that the timings with the python script are not really accurate, and that the correct values for the components would have to be calculated. The Values I got were fluctuating wildly, and I couldn’t really see a difference with the brightness in all that noise.

    So I looked for something more accurate. I still have some AtTiny’s and they have analog inputs. But SPI is the only means of communication they support in hardware. Last week, I implemented uart receiving capabilities in software, but this time I was looking for i2c. (more…)