TWI_CWGR
at91twi_writereg(sc, TWI_CWGR, (ckdiv << 16) | (cxdiv << 8) | cxdiv);
printf("%s: ckdiv=%d cxdiv=%d CWGR=0x%08X SR=0x%08X\n", device_xname(self), ckdiv, cxdiv, at91twi_readreg(sc, TWI_CWGR), at91twi_readreg(sc, TWI_SR));