document planned simplifications

diff --git a/Notebook b/Notebook
index 0fa5c4e395d14e08d50543626fc9d88f1eb13c76..7fc252b64277b75d0b391c01e521e7c0e8800951 100644 (file)
--- a/Notebook
+++ b/Notebook
@@ -349,3 +349,31 @@
   a resistor suitable for 3mV/V, and use a screw terminal strip to attach it
   so customers don't have to scare us with their (lack of) soldering skills.
 
+2025.04.19
+- let's stop pretending writing an AD7997 emulation for the LPC is useful. I
+  see no way to get sample timestamps in userspace from the ADS8688 without
+  modifying the driver (they're stored at "interrupt" time, but not exposed
+  to userspace?).  After some discussion with Keith, the only reason we have
+  both sense_batt and sense_pyro is to determine state of the SAFE/ARM
+  physical switch.  But with a spare comparator, we could do that differently
+  and feed an RPI GPIO that state info.  It's also obvious that R11 and R17
+  should not both be there, that doubles the continuity current just because
+  of a thoughtless copy/paste from telefireeight.  So ..
+
+       - remove R11
+       - change the value of R15 to something slightly higher
+       - connect sense_batt to U13 pin 2, the comparator - input
+       - connect sense_pyro to U13 pin 3, the comparator + input
+       - connect U13 pin 1, comparator output, to an unused RPI GPIO
+       - connect sense_batt to an ADS8688 input
+       - connect sense_1 to an ADS8688 input
+
+  This means we're using 4 of the 8 ADC inputs for 
+       chamber pressure
+       thrust
+       battery voltage
+       igniter continuity
+  leaving 4 available inputs for daughter cards, like a strand burner
+  interface board with another pressure sensor and 3 burn wires.
+
+