ZED-F9R-Integration manual
Figure 18: I2C write access
3.6.3 SPI interface
ZED-F9R has an SPI slave interface that can be selected by setting D_SEL = 0. The SPI slave
interface is shared with UART1 and I2C port, the physical pins are same. The SPI pins available are:
• SPI_MISO (TXD)
• SPI_MOSI (RXD)
• SPI_CS_N
• SPI_CLK
See more information about communication interface selection from the D_SEL section.
The SPI interface is designed to allow communication to a host CPU. The interface can be operated
in slave mode only.
3.6.3.1 Read access
As the register mode is not implemented for the SPI port, only the UBX/NMEA message stream is
provided. This stream is accessed using the back-to-back read and write access (see section Back-
to-back read and write access below). When no data is available to be written to the receiver, MOSI
should be held logic high, i.e. all bytes written to the receiver are set to 0xFF.
To prevent the receiver from being busy parsing incoming data, the parsing process is stopped after
50 subsequent bytes containing 0xFF. The parsing process is re-enabled with the first byte not equal
to 0xFF.
If the receiver has no more data to send, it sets MISO to logic high, i.e. all bytes transmitted decode
to 0xFF. An efficient parser in the host will ignore all 0xFF bytes which are not part of a message and
will resume data processing as soon as the first byte not equal to 0xFF is received.
3.6.3.2 Back-to-back read and write access
The receiver does not provide any write access except for writing UBX and NMEA messages to
the receiver, such as configuration or aiding data. For every byte written to the receiver, a byte will
simultaneously be read from the receiver. While the master writes to MOSI, at the same time it needs
to read from MISO, as any pending data will be output by the receiver with this access. The data
on MISO represents the results from a current address read, returning 0xFF when no more data is
available.
UBX-20039643 - R06
3 Receiver functionality Page 54 of 119
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