diff --git a/rpi_spi/2023-12-29_pi-spi-rates.md b/rpi_spi/2023-12-29_pi-spi-rates.md
index 6ee0fa1618df5a295155f203e10e727b26f622e6..78300c7d5b4609cf567c514e35c0880e57eab339 100644
--- a/rpi_spi/2023-12-29_pi-spi-rates.md
+++ b/rpi_spi/2023-12-29_pi-spi-rates.md
@@ -101,4 +101,8 @@ There's another oddity in here... the RP2040 only uses SPI framing where the CS
 - do the RP2040-to-RP2040 UART PIO test 
 - finish the thought, blog it up (1 day, max) 
 - take i.e. [notes](https://abyz.me.uk/rpi/pigpio/pigs.html), right ? 
-    - incl. the maybe-likely-change to use PIO spi-slave, not this bs 
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+    - incl. the maybe-likely-change to use PIO spi-slave, not this bs 
+- i.e. go-notes/ref/rp2040 ? 
+    - https://cec-code-lab.aps.edu/robotics/resources/pico-c-api/index.html 
+    - https://github.com/raspberrypi/pico-examples/tree/master 
+    - https://www.circuitstate.com/tutorials/making-two-raspberry-pi-pico-boards-communicate-through-spi-using-c-cpp-sdk/ 
\ No newline at end of file