Understanding RTC Issues in Embedded Linux Systems

Introduction

The RTC (Real Time Clock) is an electronic device used to keep track of time, widely applied in various embedded systems. This article analyzes issues encountered with RTC in practical applications through real case studies and provides practical suggestions, such as time “resetting” and excessive RTC time errors.

RTC (Real Time Clock): A standalone timer that keeps working as long as the power voltage remains within the operational range, regardless of whether the device is in running mode, low power mode, or reset state, fulfilling the timing function.

Although the RTC circuit is simple, some issues may arise in practical applications, such as time “resetting” and excessive RTC time errors, which involve both hardware and software aspects. Below, we will discuss some solutions and suggestions based on actual problems encountered.

Additionally, on 32-bit Linux systems, one inevitably encounters the Year 2038 problem, which will also be addressed here.

Understanding RTC Issues in Embedded Linux Systems Why is the RTC time 1970 for a brand new product shipped from the warehouse?

For a brand new product, if the system time is 1970 upon startup, there are two possibilities:

The RTC battery is not installed correctly, or the isolation plastic spacer between the lithium battery and the battery compartment has not been removed, causing the RTC to not function due to lack of power.
The RTC battery is installed but has already been depleted, which generally occurs in products that have been stored for a long time.

Note: For products powered independently by RTC, the RTC battery must be installed correctly before use, and the RTC time must be set correctly during the first boot. For stored products, do not install the RTC battery or ensure power isolation during battery installation.

Understanding RTC Issues in Embedded Linux Systems

RTC turns to 1970 after running for a while, causing abnormal program operation

In a normally running system, after some time, the kernel displays the following message:

rtc-pcf8563 0-0051: low voltage detected, date/time is not reliable.

This indicates that the RTC battery voltage has dropped too low. This can lead to unreliable RTC time, thereby affecting the accuracy of the Linux system time.

The PCF8563 chip has a battery voltage detection function, which cannot guarantee the accuracy of clock information when the voltage drops below 0.9V (see Figure 1), leading to abnormal system operation.

Understanding RTC Issues in Embedded Linux Systems

Figure 1: Low Voltage Detection Function of PCF8563 RTC Chip

To avoid low voltage issues with the RTC battery, the following considerations can be made:

Consider using rechargeable RTC batteries and designing a charging circuit;
Disable the CLKOUT function of the RTC chip to reduce RTC power consumption and extend RTC battery life;
Add a supercapacitor, powered by the system power during normal operation, and by the supercapacitor after the system is powered off, with the lithium battery as backup when the supercapacitor is depleted.

Figure 2 shows a reference schematic for RTC multi-power supply.

Understanding RTC Issues in Embedded Linux Systems

Figure 2: Reference Schematic for RTC Multi-Power Supply

During design, pay attention to the following points:

RTC_VDD only powers the VDD pin of the clock chip;
Use low forward voltage drop and low leakage current diodes on the RTC_VDD power supply line;
Use system power for I²C pull-up resistors;

Reserve a CLKOUT signal test point for debugging clock accuracy, and disable this function after debugging to reduce power consumption.

The Troublesome “Year 2038” Problem

First, let’s introduce the concept of time in the UNIX world. UNIX epoch time starts from Coordinated Universal Time (UTC) at 00:00 on January 1, 1970, counting the total seconds to the present. This time is also known as POSIX time. Linux inherits UNIX time, and in 32-bit processor Linux systems, the timer is 32 bits, with a maximum count of 0xFFFFFFFF, while in the Linux system, rtc_time is defined as signed integers for seconds, minutes, hours, etc.

<uapi/linux/rtc.h>

struct rtc_time {        int tm_sec;        int tm_min;        int tm_hour;        int tm_mday;        int tm_mon;        int tm_year;        int tm_wday;        int tm_yday;        int tm_isdst;};

The maximum count for a 32-bit timer as a signed integer is 0x7FFFFFF, which converts to 2147483647 in decimal.

2147483647 seconds, when converted to years, is approximately 68.09625973490614 years, or about 68 years and 18 days. Therefore, UNIX time will overflow after 68 years and 18 days (specifically at 11:14:07 on January 19, 2038), causing the RTC to malfunction. To solve this problem on 32-bit systems, it is necessary to upgrade the Linux kernel to a higher version and also upgrade glibc, which is a significant workload. For some older processors, the original manufacturer may not provide a new version of the kernel, which can be quite troublesome. However, if a 64-bit processor is used with a 64-bit Linux system, this issue does not occur.

After upgrading to a 64-bit Linux system, the maximum value of the RTC counter becomes 0x7FFFFFFFFFFFFFFF, which converts to 9223372036854775807 in decimal. 9223372036854775807 seconds, when converted to years, is approximately 29.2 billion years, completely eliminating concerns about RTC time overflow.