diff --git a/Doorbells.md b/Doorbells.md
index 7fa166c..683d680 100644
--- a/Doorbells.md
+++ b/Doorbells.md
@@ -1 +1,10 @@
-Used for notifying the Firmware of new data in the ringbuffers.
\ No newline at end of file
+Used for notifying the Firmware of new data in the ringbuffers.
+
+ISP register 0x41020. Known bits:
+
+7 - Doorbell 3
+6 - Doorbell 2
+5 - Doorbell 1
+4 - Doorbell 0
+
+