BMC block handles BMC encoding and decoding. It also handles CC line activity detection, discharging the

external MCU of such operations.

The default state of the BMC block is to listen to the line (RX mode). TX mode is enabled only by assertion of the

TX_EN signal via the external MCU.

3.2.2

TX mode

When the TX_EN signal is asserted via MCU, BMC block goes to the TX state:

•

NSS signal is driven low, indicating to the SPI/MSP slave interface of the MCU that data are being

transmitted on the CC line. MCU provides the data

•

The STUSB1702 drives the NSS signal low, informing SPI/MSP slave interface of the MCU that data are

requested on the MISO line

•

The STUSB1702 clocks the SCLK signal

•

MCU presents data to be transmitted on the MISO line and data are sampled on the rising edge of SCLK

(data must be stable on this edge)

•

Sampled data (from MISO line) are encoded by the BMC, and the resulting values drive the CC line

according to USB PD standard

When all data are transmitted, MCU drives the TX_EN pin low, and lists the end of transmission. The STUSB1702

ends transmission with a corresponding trailing edge termination. It then goes back into to default state and

releases the CC line from the BMC driver to the pull-up/pull-down CC line interfaces.

3.2.3

RX mode

RX mode is the default state of the BMC interface.

In this mode, the receiver listens to the connected CC line. It does not interface with the CC line interfaces or the

CC control logic.

When all data are detected and received on the CC line, according to the activity described in the USB Power

Delivery Standard, the BMC interface:

•

Drives NSS signal low

•

Outputs the clock on the SCLK signal which is recovered from the BMC signal

•

Outputs recovered data (from the BMC signal) on the MOSI line to the connected MCU. Data are valid on

the SCLK falling edge and are sampled on this edge by the SPI/MSP interface of MCU

When no more data are detected on the CC line, the NSS goes back to “high” which is its default state. This

informs the MCU that no more activity is present on the bus.

3.3

VBUS power path control

3.3.1

VBUS monitoring

The VBUS monitoring block supervises from the VBUS_SENSE pin the VBUS voltage on the USB Type-C

receptacle side.

It is used to check that the VBUS is within a valid voltage range:

•

To establish a valid source-to-sink connection according to USB Type-C standard specifications

•

To safely enable the VBUS power path through the VBUS_EN_SRC pin

It allows detection of unexpected VBUS voltage conditions such as undervoltage or overvoltage relative to the

valid VBUS voltage range. When such conditions occur, the STUSB1702 reacts as follows:

•

At attachment, it prevents the source-to-sink connection and the VBUS power path assertion

•

After attachment, it deactivates the source-to-sink connection and disables the VBUS power path. In source

power role, the device goes into error recovery state.

The VBUS voltage value is adjusted automatically at attachment (vSafe5V) and via MCU at each PDO transition.

Monitoring is then disabled during T_PDO_transition (i.e. the default value of 300 ms is changed through NVM

programming). Additionally, if a transition occurs to a lower voltage, the discharge path is activated during this

time.

STUSB1702

VBUS power path control

DS12664 - Rev 1

page 8/38