TXDAT: Transmit data buffer. This buffer must be periodically written to with transmit data during the rf transmission. Before the transmission begins, it must contain the first 16 chips to be transmitted. After approximately 8 chips have been transmitted, a TX_RELOAD_IRQn will be generated indicating the buffer should be written to with the next 16 chips. Chips are transmitted out of the buffer MSB first. If the number of chips to be transmitted is not a multiple of 16, then the most-significant bits of the last buffer update are the ones transmitted.