OSD335x RTC Use Cases

Published On: October, 1, 2020 By: Eshtaartha Basu

Introduction

This document will explain different Real-Time Clock (RTC) use cases for the OSD335x Family of Devices. It will also provide design recommendations on each use case.

OSD335x RTC Use Cases

The Real-Time Clock (RTC) peripheral within the AM335x can provide time keeping functions. However, if the RTC is not needed for a given application, there are a couple of different ways that the RTC peripheral can be configured. The sections below walk through each of the four supported use cases for the RTC and provides design / hookup guidelines as well as any caveats or other design considerations.

Use Case – 1: RTC Enabled

In this use case, the RTC is enabled and available for use to provide time keeping functions. The connections for this use case are shown in PMIC-RTC Connections, RTC_KALDO_ENN, RTC External Power, OSC1 RTC Circuitry Figures and Use Case 1 Table. The OSD3358-SM-RED reference design implements the Use Case -1 RTC connections.

Use Case - 2: RTC Enabled, No Oscillator

In this use case, OSC1 clock circuitry hardware is removed. Therefore, the RTC peripheral will not be able to keep time, but all of the registers will still be powered and accessible. From a connection point of view, this use case implements PMIC-RTC Connections, RTC_KALDO_ENN, and RTC External Power Figures. However, instead of implementing the circuitry in the OSC1 RTC Circuitry Figure, the OSC1_IN, OSC1_OUT, and OSC1_GND connections are left floating as seen in the below figure. PocketBeagle implements the Use Case -2 RTC connections.

Use Case – 3: RTC Externally Powered, No Oscillator

In this use case, the RTC’s internal LDO is disabled and the RTC is powered using the VDD_CORE power input. Disabling the internal RTC LDO (by pulling RTC_KALDO_ENN high) can allow the application to achieve lower power consumption in low power modes. This use case implements the connections in the PMIC-RTC Connections and the Floating Connections Figures. However, for RTC_KALDO_ENN Figure , RTC_KALDO_ENN is pulled high to SYS_RTC_1P8V and in RTC External Power Figure, CAP_VDD_RTC is connected directly to VDD_CORE, instead of being left unconnected.

Use Case – 4*: RTC Disabled

In this use case, the RTC peripheral is powered but held in reset. In this use case the RTC is not accessible by software. See NOTE below before choosing this Use Case. To implement this use case, please follow the RTC disabled connection figures in the SiP datasheet.

*NOTE: Linux images that rely on mainline U-Boot, such as the OSD3358-SM-RED Custom Embedded Linux Image and the BeagleBoard.org® Debian Linux Images, interact with the RTC during boot. If the RTC is held in reset (as in Use Case-4), the boot sequence may halt with no error messages/output on UART boot console due to bus accesses to the RTC not properly completing. To fix this, it is up to the customer to modify their SPL, U-Boot and Linux Device Tree to eliminate any interaction with the RTC.

References

• OSD335x Datasheet: https://octavosystems.com/docs/osd335x-datasheet/
• OSD335x SM Datasheet: https://octavosystems.com/docs/osd335x-sm-datasheet/
• OSD335x C-SiP Datasheet: https://octavosystems.com/docs/osd335x-c-sip-datasheet/
• AM335x Datasheet: https://www.ti.com/lit/pdf/sprs717
• AM335x Technical Reference Manual: https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf

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