Open Timecard Mini V1 - Dip Switch Guide

Two banks of hardware dip switches are relevant when using the Timecard Mini.  The first is located on the baseboard itself.  Removing the GNSS module and CM4/riser board will reveal a bank of 8 switches directly opposite the SD card slot.  Setting these switches will enable or disable various features the Timecard Mini offers.  The second bank is on the GNSS module itself.

Baseboard switches: 

Switch 1:          Disable eMMC Boot.

Setting the “on” switch turns the Raspberry Pi CM4 into a USB device. 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 1.8v 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. 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.

This will allow the CM4 serial console to be active when set to on. 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 that this feature be disabled if UART 2 on the CM4 (/dev/ttyAMA1) is used for another purpose. To disable it, 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 first to discuss the concept of the PPS Net.  The GNSS module guide details the Net and the pins, but looking at the diagram below could be more straightforward.

As the above diagram suggests, signals run 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 must turn off PPS IN using the dipswitch.

For that, we flip switch number 4 to off:

 This means that the GNSS is the unit's driving input. If you want to have PPS enter the PPS Net and not the GNSS, then you will want to turn switch four back on and also turn switch two off as shown below: