Note from the lecture by Mr.Yamamoto (skilled electrician) at Fablab Kamakura.
Good power source to avoid multiple causes.
if not you may encounter problem such as : too much noises, current drop if powering with USB
Do not trust jumper wires and bread boards.
Use experienced wire.
Reliable measuring instruments
Oscilloscope / Logic analyzer / protocol analyzer
Spectrum analyzer (few mega Hz accurancy)
Apporx 60MHz resolution oscilloscope. Commonly used among electronics students. It has multiple functions in it:
The frequency of the ociloscope should be 3 times bigger than the signal you want to observe.
E.g.) If micro controler runs in 20M you will need 60MHz.
The board and oscilloscope both should be connected to PC.
attach prove of channel 1 to RX and 2 to TX in FTDI pins. (We made an extension board in order to make it easy to connect prove to designated pins with out any unintended contact.) Both GND electrodes should be connected with GND pin in FTDI as well.
Download Waveforms app from here
Once you have done setting oscilloscope, hit “Run” button in upper left corner in the window to start observing. Then hit “Single” button right next to “Run” button to pause and capture the wave pattern.
Here is what we got from our oscilloscope
and this is the result of running program.
Looking at bule Channel2 wave pattern (which is monitoring TX of FTDI pin), the wave consist of two electrical state (2 different voltage) HIGH(1) and LOW(0).
If you try to read it from the very beginning to the end, its says…
Omitting start-bit( a bit to tell the program that the data block starts here which is
0) and stop-bit( bits terminate the data block which is
11), data inside can be read as…
LSB first block of data is read form right to left, so…
7A in hexdecimal.
According to ASCII code list, 7A is encoded as “z”.
“z” is typed in the program window.
We also analyzed RX signal and wave pattern and the characters shown on the program matched.
When we look into the C file (hello.ftdi.44.echo.c), respond from the board should start with “hello.ftdi.44.echo.c:”, but it doesn’t.
put_string(&serial_port, serial_pin_out, "hello.ftdi.44.echo.c: you typed \"");
Each time we receive data, first few characters changes randomly.
As the wave pattern matches the result we get, I suspect that it is a problem of the software. However, we couldn’t dig in any further.
By using protocol analyzer or logic analyzer, it is no longer necessary to stare at raw signal pattern and ASCII code list back and forward to analyze the data in the signal.
Parity: none (bit for error detecting)