I figure that the explanation regarding rise/fall times warrants an extension because it's omitting one thing that's not immediately obvious: why is it a problem when the signal is distorted due to dampened harmonics? If you add suitable termination and/or signal conditioning at the receiver end you can extend the length, for example. SPI is usually limited because of the unterminated lines with sharp edges as the clock rate is usually in the MHz range and thus requires short rise/fall times.
If you'd try sending a high bit too quickly after a low bit, the bit would be messed up because the receiver would still read a low bit. The two I2C pull-up resistors pose a limit on bandwidth because after each time a line was pulled low, it takes some time until it reaches high level again due to line impedance and stray capacitance in the circuit elements. Generally can think of it this way: fast clock rate -> short rise/fall times -> high number of harmonics -> high frequency spectrum -> transmission line has to be suitable or harmonics will be dampened and the signal will be distorted) (rise/fall times are important because they relate to the bandwidth required to transmit the signal in its original form - a long wire is effectively a low pass, limiting the bandwidth. 10Mbaud, the 15m max length doesn't apply anymore, so keep in mind that "UART" isn't "UART".
115kbaud is relatively slow and usually uses slow rise/fall times as a result, so long wires are okay. Imposing length limits are one way to help make sure that the individual bits don't become a mush because high/low levels can't be distinguished anymore or because reflections overlap bit n-1 with bit n. ), resistance (voltage drop across the transmission line) and also capacity/inductance (U/I phase shift, coupling, time-variable impedance). You have issues like reflections (MOSFET inputs are like open line ends unless there's termination), you have interference (depending on transmission scheme, voltage level, rise/fall time. Multiple reasons but it boils down to the basic properties of a transmission line:
0 kommentar(er)
