The SD transmit control unit is used for writing transfers to transfer data to the card. After the command is issued, the controller waits for a block of data to be available in the block buffer and transfers the data onto the SD DAT lines. Based on the configuration of data lines (1-bit, 4-bit, or 8-bit), the data from the block buffer is appropriately routed. The CRC16 is individually calculated on a per-lane basis and is attached at the end of block transfer before the END bit. In DDR operation, the transmit control unit implements a separate CRC16 for each edge of the clock. At the end of block transfer, it waits for the CRC response on the DAT0 line and reports the result of the CRC check to the register set. The controller also checks for a write busy indication (DAT0 line) before transferring the next block of data. A timeout check is implemented to ensure that the write busy is asserted no more than the required limit.