In the following article, we distinguish between an internal RTC (on the Apalis/Colibri/Verdin module) and an external RTC (on the carrier board).
For the Apalis and Colibri families, Toradex recommends the use of an external RTC because it often consumes more than 10 times less current than an internal RTC allowing a much smaller backup battery to be used. On the other hand, use of an internal RTC may simplify the carrier board design and may allow wake up using the alarm capabilities.
For the Verdin iMX8M Mini module, an external RTC is not necessary, since the module has an internal low power RX8130CE real-time clock from Epson.
For information regarding RTC use in Linux, see How to use the Real-Time Clock in Linux.
For information regarding RTC use in Win CE, see RTCSync.
All Apalis, Colibri, and Verdin modules provide an internal RTC. Most modules (except some PXA models) provide an internal RTC which can be powered by a battery when other power is absent. Some modules also support the charging of the battery when other power is applied. The use of a battery is optional. When a battery is not utilized, the RTC is simply cleared when power is removed.
Some modules possess two RTCs - one in the SoC and one in the PMIC. In this case, the PMIC RTC is what is powered by the external battery when no other power is applied.
The following Colibri modules do have an on-module RTC:
Note: On WinCE T20/T30 use the settings described here to use the on-module RTC.
The Apalis TK1 features an RTC in the PMIC.
The internal RTC on modules that have i.MX 6, i.MX 6ULL, i.MX 7, i.MX 8 and i.MX 8X family processors is provided by the SoC.
The module has an internal low power RX8130CE real-time clock from [Epson] plus an internal RTC provided by the SoC.
Vybrid-based modules have an internal RTC that can be powered through VBATT and will not be influenced by changes of state of the main processor.
It is possible to use an external RTC and the RTCSync tool (already included in the standard Windows CE images) to synchronize it with the internal RTC at each boot.
We have added an external RTC (M41T0 by ST Microelectronics) on all our carrier boards. This is also recommended in many cases for custom carrier boards. Several factors lead the SoC manufacturers to not use ultra low-power RTCs: it may be a result of the silicon process selected for a specific SoC, while optimizing it for performance and speed; or due to the fact that some SoCs are designed for battery-operated devices like phones, a configuration in which the battery would be large enough that the current consumption of the RTC doesn't matter. The external RTC M41T0 has a very low operating current of about 1.7µA.
Evaluation Board V1.0
Evaluation Board V3.1/V3.2
Evaluation Board V2.1
To increase the accuracy of the external RTC on the Evaluation board V2.1, you can remove the two capacitors C100 and C101 on the Evaluation Board V2.1 (see Evaluation board V2.1 schematics).
The following picture shows the location of the two capacitors which could be removed.
The RTC circuit components are not assembled by default. Please refer to the Real Time Clock (RTC) section of the Viola datasheet for detailed information.
Viola Plus V1.2
The Viola Plus V1.2 comes with the RTC circuit components assembled.
RTC drift measurements were made from each device's respective Linux image and are presented as absolute values.
|Ext/Int||CoM Family||Device||Ver||RTC||Current (uA @ 3.1V)||Drift (ppm)|
|Ext||Apalis||Apalis Eval Board||1.0a||Ext||1.35||20|
|Ext||Colibri||Colibri Eval Board||3.1a||Ext||0.93||17|