diff --git a/README.md b/README.md
index aba8ef4..82e84cd 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,24 @@ RTC
 
 The onboard Real Time Clock (RTC) is used to track the uptime a periodically start querying all electricity meter for their measurement value.
 The time interval between the periodic queries is hard coded in `main.c`.
+By connecting a (coin cell) battery to VBAT the RTC keeps running, even if the board is not powered anymore.
+
+DDM100TC
+--------
+
+This 3-phase 4-wire electricity meter provides an S0 interface.
+
+On my model this would output 1600 impulse/kWh, thus each impulse corresponds to a power consumption of 0.625 Wh).
+To record these impulses a timer input capture is used.
+The number of impulses determines the energy used.
+It is stored in a backup register, keeping the value in memory even when the board is powered down as long as a (coin cell) battery is connected to VBAT.
+The interval between impulses allow to calculate the current power consumption.
+The time has a resolution of 0.91 ms (leading to a power calculation error of <1%).
+
+Connections DDM100TC <-> board:
+
+- S0+, pin 8; +3.3V or +5V
+- S0-, pin 7; TIM4_CH1, PB6 (use pull-down resistor)
 
 ESP8266
 -------