A wireless power system has a wireless power transmitting device and a wireless power receiving device. The devices in the wireless power system may communicate using in-band communication. The wireless power transmitting device may transmit data to the wireless power receiving device using frequency-shift keying (FSK) modulation. The wireless power receiving device may transmit data to the wireless power transmitting device using amplitude-shift keying (ASK) modulation. While transmitting data to the wireless power receiving device using FSK modulation, the wireless power transmitting device may monitor for ASK modulation from the wireless power receiving device. In response to detecting the ASK modulation from the wireless power receiving device, the wireless power transmitting device may abort the FSK data transmission and process the detected ASK modulation to receive data from the wireless power receiving device.