SYSTEM_MODE_REG
si21_writeregs(state, SYSTEM_MODE_REG, ®1, 0x01);
si21_readregs(state, SYSTEM_MODE_REG, ®data, 0x01);
si21_writeregs(state, SYSTEM_MODE_REG, ®data, 0x01);
id = si21_readreg(state, SYSTEM_MODE_REG);
si21_writereg(state, SYSTEM_MODE_REG, id | 0x40); /* standby off */