Dear ST team ,
We are currently finalizing our hardware design using the L9678P, with a focus only on deployment functionality and functional safety compliance according to IEC 61508.
I’ve gone through the datasheet and functional descriptions in detail — I’d like to summarize the key points I’ve understood and kindly request your confirmation on the following items.
Our use case does not include external sensors (i.e., no sensor data read in Safing State), and we are not using the Remote Sensor Interface and ISO9141 features.
Pins Required for MCU Connection
Below is the list of pins we believe must be connected to the MCU for SPI communication, control logic, arming, and optional monitoring:
Pin Function
| SPI_MOSI | SPI Master Out – data from MCU to L9678P |
| SPI_MISO | SPI Master In – data from L9678P to MCU |
| SPI_SCK | SPI Clock |
| SPI_CS | SPI Chip Select |
| RESET | System Reset – issued by MCU to initialize L9678P |
| WAKEUP | Wake-up input – required only if low-power or sleep modes are used |
| FENH |
External arming enable for high-side drivers (active high) – used when there is no use sensors, and safing State ARM and VSF won’t be valid as seen in figure. 9) 
|
| FENL | External arming enable for low-side drivers (active low) – also used when no sensor input is available, for the same reason as above |
| ACL | Additional Communication Line – required to enter Arming State, as per ACLGOOD = 3 condition in Figure 9 |
| uC_FLEN |
In the reference schematic, uC_FLEN appears to be used as a GPIO from the MCU to control a FET in the VSF-powered safing path — is this correct? Could this function alternatively be driven by one of the L9678P’s GPO drivers to reduce MCU pin usage? (typically near U1 as shown in Figure 58)
|
| ARM | Arming status output – acts as a “ready-to-deploy” flag; can be monitored by the MCU or read via SPI |
| VDDQ | Logic I/O reference – should be connected to MCU's I/O voltage supply to match logic levels |
Please confirm if the above list is sufficient and minimal for a deployment-only application using without sensors ( not receiving any sensor data in Safing State) .
If needed, I can follow up with the external hardware-supported pins (power, filtering, energy reserve, etc.).
Specific Clarifications Requested:
-
ARM: Can this be left unconnected if there is any register to read the status by MCU?
-
WDT/TM: Marked "Not for application" – should it be pulled low, left floating, or tied to MCU?
-
SS01 / SS23: Are external components (e.g., resistors, capacitors, TVS) needed for these?
-
COVRACT: Required only for external crossover FETs, or also with internal ER switch? Is MCU control needed?
- uC_FLEN:In the reference application diagram, I noticed that the uC_FLEN signal from the MCU appears to drive the gate of an N-channel FET in the safing path (related to the VSF-regulated supply). I would like to confirm:
Is my understanding correct that uC_FLEN is a control signal from the MCU used to enable or disable the safing FET path (typically by pulling the gate low)?
What is the recommended driving strategy from the MCU for this pin?
Thank you for your support.
Best regards,
Umesh
