The slow rising edge
On the MOSFET boards every LOW is a transistor pulling hard and every HIGH is a 10 kΩ resistor catching up. That is perfect for I²C and too slow for fast SPI, and one slider shows where the line falls.
Down is fast, up is slow
Every wire holds a little electric charge — think of it as a tiny bucket. To make the wire LOW, the transistor empties the bucket straight to ground, and that is quick. To make it HIGH, the 10 kΩ resistor has to fill the bucket through a narrow pipe, and that takes time.
How much time depends on the wire. A short jumper is a small bucket. A long wire is a bigger one.
The dashed line is what the 3.3 V side sent. The solid line is what the 5 V side actually sees. A 5 V input only calls it HIGH once it passes 3.5 V.
Where the line falls
With short jumpers, roughly:
| Signal | Shortest HIGH | On a MOSFET board |
|---|---|---|
| Serial at 115200 | 8.7 µs | comfortable |
| I²C, 100 kHz | 5 µs | comfortable |
| I²C, 400 kHz | 1.25 µs | comfortable |
| SPI, 1 MHz | 0.5 µs | marginal |
| WS2812 LED data | 0.4 µs | marginal |
| SPI, 4 MHz and up | 0.125 µs | fails |
The wire figures in the slider are round numbers for three kinds of wiring, not measurements. They show the shape of the problem, not a promise.
Two ways to buy time
- Shorter wires. Less to fill.
- The module's own pull-ups. Most I²C modules have 4.7 kΩ resistors on their lines. Alongside the board's 10 kΩ, they fill the wire about three times faster, which is part of why I²C through these boards is so reliable.
For anything faster, use the TXB or TXS. Their edges are pushed by the chip, not filled by a resistor.
When it does not work
That is this page. The HIGH pulses got shorter than the time the resistor needs to fill the wire. Move the SPI lines to a TXB0108 or TXS0108, or keep the clock down around a few hundred kHz while you test.
Longer wire, more to fill, slower rising edge. Something that was marginal on short jumpers can fail at 30 cm. Shorten the run, or use a chip board for that signal.
It would make the edges faster, but they are tiny 0603 surface-mount parts, and every channel has two. For a fast signal it is much simpler to use a TXB or TXS board, which was built for it.
No. A falling edge is the transistor, or the device on the other side, pulling hard to ground, and it is fast. Only the rising edge depends on the resistor.
Two questions decide it, and a table answers them for the six things people actually connect.
Which board for which job →Edit this page — content/books/llc/the-slow-rising-edge.mdx
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