Two microphones, one wire
The microphone's L/R pin decides which half of each clock cycle it drives DATA in. On the TK101 it is held low by 10 kΩ and a jumper to 3V3 ships open. One microphone needs no change. Two can share one CLK and one DATA wire if one board's jumper is closed, which is the usual way a pair of PDM microphones makes stereo.
Each microphone takes half a cycle
A PDM microphone does not drive DATA for the whole clock cycle. It sends its bit in one half of the cycle and lets go in the other, leaving the wire free. Which half is set by its L/R pin.
So two microphones with opposite settings can share one CLK and one DATA wire: each sends in its own half, and the receiver sorts the bits into two channels. Two with the same setting both drive DATA at the same instant, and the result is noise.
On the TK101
L/R has no header pin. On the board it is:
- held low by a 10 kΩ resistor to GND, and
- joined to 3V3 by a solder jumper, SB1, that ships open.
As it ships, then, L/R is low. Closing the jumper with a blob of solder makes it high.
What to do with it
- One microphone: nothing. Leave the jumper open. The sketches in this book read one microphone in mono, as it ships.
- Two microphones as a pair: wire both CLK pins to one GPIO and both DATA pins to another, and close the jumper on one board only. A sketch then reads the input in stereo, and each microphone is one channel.
The book's sketches read mono and do not cover the pair. It is here so the jumper is not a mystery, and so nobody closes it on a single board expecting it to do something.
When it does not work
No. Leave it as it ships. The book's sketches read one microphone in mono, and they were written for the board with its jumper open.
The datasheet this book works from does not say which clock edge each setting uses, so the book does not name one. With two microphones wired as a pair, tap one of them while recording in stereo and see which channel jumps.
Both microphones now drive DATA in the same half of every clock cycle, and when one sends a 1 and the other a 0 at the same instant what arrives is neither. Open one jumper again: remove the solder bridge with desoldering braid.
It is two small pads side by side near the top right of the board, SB1. Bridge them with a small blob of solder from a clean iron. It ties L/R to 3V3; the 10 kΩ to GND then draws about 0.33 mA, worked out from 3.3 V across 10 kΩ.
The half-millimetre hole every sound has to reach, and what not to do to it.
Where the sound gets in →Edit this page — content/books/pdm-microphone/two-microphones-one-wire.mdx
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