USB Type-C™ / USB Power Delivery interface (UCPD) RM0440
2060/2126 RM0440 Rev 4
This re-iterates until the entire payload of data is transmitted.
Upon sending the CRC packet, the TXMSGSENT flag is set to indicate the completion of
the message transmission.
Hard Reset transmission
As soon as it is known that a Hard Reset needs to be transmitted, setting the TXHRST bit of
the UCPD_CR register forces the internal state machine to generate the correct sequence.
The value of UCPD_TX_ORDSET does not require update in this precise case (the correct
code for Hard Reset is sent by UCPD).
The USB Power Delivery specification requires that in the case of an ongoing message
transmission, the Hard Reset takes precedence. In this case, for example, UCPD truncates
the payload of the current message, appending EOP to the end. No notification is available
via the registers (for example through the TXMSGSEND flag). This is justified by the fact
that the Hard Reset takes precedence over any previous activity (for which it is therefore no
longer important to know if it is completed).
Use of DMA for transmission
DMA (Direct Memory Access) can be enabled for transmission by setting the TXDMAEN bit
in the UCPD_CR register.
For each message:
• Prepare the whole message in memory (starting with two header bytes)
• Program the DMA operation with a length corresponding to the two header bytes plus a
number of data bytes corresponding to the number of data words multiplied by four
• Write TXSEND to initiate the message transfer
• If TXMSGDISC then go back to previous line (TXSEND)
• Wait for DMA transfer complete interrupt (that is, when last Tx byte written to UCPD)
• Double-check subsequent TXMSGSENT interrupt appears
USB PD receive
Notification of start of the receive message sequence is triggered by an interrupt on
UCPD_SR (bit RXORDDET).
The information is recovered by reading:
• UCPD_RX_SOP (on interrupt RXORDDET)
• UCPD_RXDR (on interrupt RXNE, repeats for each byte)
• UCPD_RXPAYSZ (on interrupt RXMSGEND)
The data previously read from UCPD_RXDR above must be discarded at this point if the
RXERR flag is set.
If the CRC is valid, the received data is transferred to the protocol layer.
For debug purposes, it may be desirable to track statistics of the number of incorrect
K-codes received (this is done only when 3/4 K-codes were valid as defined in the
specification). This is facilitated through:
• RXSOP3OF4 bit indicating the presence of at least one invalid K-code
• RXSOPKINVALID bitfield identifying the order of invalid K-code in the ordered set
Use of DMA for reception
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