There are two banks of hardware dipswitches that are relevant when using the Timecard Mini. The first is located on the base board itself. Carefully removing the GNSS module and/or CM4/riser board will reveal a bank of 8 switches situated directly opposite the SD card slot. Setting these switches will enable or disable various features offered by the Timecard Mini. The second bank is on the GNSS module itself.
Switch 1: Disable eMMC Boot.
Setting this switch to the “on” position turns the Raspberry Pi CM4 into a USB device (when present) When connected to a “host” device via the micro USB port the CM4 will be available on the host device as a USB device. This is useful for flashing the CM4 storage with an OS, or as the guide here goes into setting up the eeprom.
Switches 2 – 5 Disable CM4 if present
A quick explanation: The GNSS module requires 1.8v and 3.3v in addition to 5v and we can supply them from either the CM4 or the carrier board but both at the same time is a bad idea. Switches 2 and 3 represent the voltages 1.8 and 3.3v and they should always be set to off if a CM4 is present.
Similarly, access to the GNSS UART serial port from the carrier board is controlled with switches 4 and 5. Actually it's safe to read from two places at once but not to write so it's safe to leave switch 5 (RX) on if desired. Switch 4 (TX) should always be off if the CM4 is present.
Switches 6 – 7 Enable CM4 serial console
When set to on, this will allow the CM4 serial console to be active. This allows the host device to use serial to connect to the CM4.
If UART2 on the CM4 (/dex/ttyAMA1) is used for an alternative purpose on the CM4
It is recommended to disable this feature if UART 2 on the CM4 (/dev/ttyAMA1) is used for another purpose. To disable set the dip switch to the off position.
GNSS module switches:
The second bank of switches is located on the GNSS module itself and to understand how to configure them it is necessary to first discuss the concept of the PPS Net. The GNSS module guide goes into detail about the Net and the pins but it could be simpler to look at the diagram below.
As the above diagram suggests you have signals running both into and out of the PPS Net. GNSS IN and PPS IN are mutually exclusive so it follows that one of the two must be set to off.
As our units ship with GNSS IN configured we have to turn off PPS IN using the dipswitch.
For that we flip switch number 4 to off:
This means that the GNSS is the driving input of the unit. If you want to have PPS enter the PPS Net and not the GNSS then you will want to turn switch 4 back on and also turn switch 2 off as shown below: