diff --git a/avr/devices/n21/README b/avr/devices/n21/README
index 98c2591..55a3d2c 100644
--- a/avr/devices/n21/README
+++ b/avr/devices/n21/README
@@ -5,6 +5,7 @@ N21
 - Role: Door sensor with temp and motion detection
 - MCU: AtTiny84
 - Connection: RJ45
+- Predecessor: N16
 - Periphery:
   - PIR sensor (AMN31112)
   - door sensor (TCRT1000)