Describe dual board setup

Signed-off-by: Marc Zyngier <maz@kernel.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/README.txt b/README.txt
index 7afa616..79c1d1d 100644
--- a/README.txt
+++ b/README.txt
@@ -73,6 +73,29 @@ assembled device, you must first solder a Pico to it. Make sure that:
     many variants with a creative pinout... It may work, it may
     not. Be cheap, don't use anything fancy.
 
+* Dual board configuration:
+
+So you have a pair of Apple machines, two CS boards, but only a single
+Pico? Guess what, that's everything you need. Each CS board can use
+two different configurations that can work together:
+
+- the default configuration is to use UART0, I2C0, and a defined set
+  of GPIOs,
+
+- and there is an alternate setup using UART1, I2C1, and another set
+  of GPIOs.
+
+v0 and v1 use a set of 0R resistors that need to be painfully moved
+(see the schematics to identify the resistors and their landing spot).
+v2+ use a set of PCB jumpers that are easier to modify: for each of
+JP1 to JP8, cut the trace between pads 1 and 2, and place a blob of
+solder across pads 2 and 3.
+
+To assemble the whole thing, I use long wrapping connectors that allow
+all three boards (one Pico and 2 CS) to be stacked. The SW will
+automatically detect which board is present and use the right
+configuration.
+
 * Software:
 
 For the SW that runs on the RPi Pico and how to use the damn thing,