plz help WITH ERROR I9100

Selected device model: Samsung GT-I9100
Connecting. Please wait…
Checking data…
VREF = 0.0V is less than required level. Please check target device power supply.
Medusa JTAG Firmware version 1.1.0.
Medusa JTAG Hardware S256 revision D.
ERROR[030]: TDO is constant high. Please check target device connection.
MI: Medusa JTAG init failed.