EVENT_IN, EVENT_OUT and PPS Signal Protection

The following schematic shows appropriate signal conditioning for EVENT_IN (MKI), EVENT_OUT (VARF) and PPS (TIMEMARK) signals.

The buffers (U1, U2) are chosen to provide additional drive strength for long cables. The series terminations (R1, R2, R3, R4) are present to allow for signal integrity adjustments (to reduce overshoot/undershoot, etc.). The passive component values may require tuning, depending on the application.

The TVS (U3) provides protection from ESD/EFT events and some induced power line surges. There are two implementations shown for the EVENT_IN signals. EVENT_IN1 is shown with basic ESD/EFT protection, while EVENT_IN2 is shown with additional surge protection for harsh environments (see D1, R7, D2, D3 and R9). If the additional surge protection is to be used on a design, the buffer must be included as well. This design has been used to protect EVENT_IN signals from EN61000-4-5 induced surges up to 2kV on several NovAtel enclosure products. If nanosecond-level timing is critical to the application, this protection circuit may be unsuitable, as it causes a timing shift of a few microseconds to the EVENT signal (heavily dependent on temperature).

The 10 kΩ pullups (R5, R6) are required if the buffers are used on the EVENT_IN lines. If the buffers are not used, the OEM7 cards have built-in 10 kΩ pullup resistors – external pull resistors are not required.

EVENT_IN, EVENT_OUT and PPS Pin Designation

	OEM719	OEM729	OEM7600	OEM7700	OEM7720
Connector	P1701	P1802	P1701	P2001	P1901
EVENT_IN1	91 This pin is multiplexed with the COM3 transmit signal and is not available if COM3 is enabled.	8	30	30	30
EVENT_IN2	72 This pin is multiplexed with the CAN1 transmit signal and is not available if CAN1 is enabled.	73 This pin is multiplexed with the COM3 receive signal and is not available when COM3 is enabled. See P1802 Main Connector 24-Pin Header for more information.	29	29	29
EVENT_IN3	–	–	32	32	32
EVENT_IN4	–	–	31	31	31
PPS	19	4	24	24	24
EVENT_OUT1/VARF	64 This pin is multiplexed with the CAN1 receive signal and is not available if CAN1 is enabled.	3	17	17	17
EVENT_OUT2	–	–	26	26	26
EVENT_OUT3	–	–	23	23	23
EVENT_OUT4	–	–	25	25	25

Bill of Materials (critical components)

Designator	Manufacturer	Manufacturer Part Number
D1	On Semiconductor	MMSZ5227BT1G
D2	Comchip Technology	CDBU00340
U1, U2	Fairchild Semiconductor	NC7WZ241L8X
U3	Semtech	SRV05-4A.TCT