{"id":4271,"date":"2020-02-05T20:00:59","date_gmt":"2020-02-05T19:00:59","guid":{"rendered":"https:\/\/msalamon.pl\/?p=4271"},"modified":"2025-12-27T17:36:46","modified_gmt":"2025-12-27T16:36:46","slug":"built-in-rtc-in-stm32f1","status":"publish","type":"post","link":"https:\/\/msalamon.pl\/en\/built-in-rtc-in-stm32f1\/","title":{"rendered":"Built-in RTC in STM32F1"},"content":{"rendered":"\n

STM32 microcontrollers undoubtedly have many killer features. Of course, compared to the ancient AVRs that are still used in the most popular Arduino Uno. One of those things is that STMs have a built-in real-time clock<\/strong>, RTC for short. Let me show you in detail what working with such a clock looks like using the HAL library generated by CubeMX.<\/p>\n\n\n\n\n\n\n\n

Some time ago I described external RTC chips (DS3231<\/a>, DS1307 and PCF8563<\/a>). They were connected via the I\u00b2C interface and generated interrupts every second. Depending on the chip, they had either battery-backed SRAM memory or, for example, oscillator signal correction, making them extremely accurate. They also had hardware date handling inside.<\/p>\n\n\n\n

First of all, I will focus on the RTC built into the STM32F103C8T6 chip, i.e. the popular BluePill<\/a><\/strong>. More or less consciously, this is the first choice for beginners.<\/p>\n\n\n\n

You have to remember that this is one of the first RTC implementations used in STM32. It doesn\u2019t have many functions or extensive registers<\/strong>, as you\u2019ll see in a moment.<\/p>\n\n\n\n

\"\"<\/a><\/figure>RTC in the STM32F103<\/h1>\n\n\n\n

The clock circuit in its basic purpose is supposed to measure time, the human, watch-like one. In the F103, only one counter is used to count time\/date<\/strong>. It is a 32-bit register that counts only the number of seconds.<\/strong> I told you it\u2019s a simple RTC, right?<\/p>\n\n\n\n

Alright, but what do we need seconds alone for?! We want hours, minutes, and even the date like we had with dedicated external chips. What a piece of junk\u2026<\/p>\n\n\n\n

It\u2019s true that this single counter may look idiotic at first glance. However, it\u2019s not as stupid as it might seem. I don\u2019t know if you know, but in Linux-family systems time and date are also counted this way.<\/p>\n\n\n\n

Because it\u2019s implemented on a single counter, the programmer is free to decide what format they want to work with RTC in<\/strong>. You can use the time.h<\/em> library and work with UNIX time or WinCE.<\/p>\n\n\n\n

Of course you can write your own RTC handling if you feel up to it. I highly recommend doing something like that. You can learn a lot along the way \ud83d\ude42<\/p>\n\n\n\n

You can also use ST\u2019s library, which is included in the project generated by CubeMX.<\/strong> It works a bit differently than the UNIX standard. That\u2019s the library I\u2019ll focus on in the near future.<\/p>\n\n\n\n

Battery backup<\/h3>\n\n\n\n

An interesting feature built into the RTC are the backup registers<\/strong>. In the F103 there are (as many as!) 10 of them. All are 16-bit, so you can store 20 bytes of data. Seems like little, right?<\/p>\n\n\n\n

These registers are battery-backed when the supply voltage disappears or the microcontroller enters low-power modes. Due to special battery backup, access to them is controlled by a special bit.<\/p>\n\n\n\n

In normal power mode, these registers operate from the voltage applied to VDD. The microcontroller reset circuit detects VDD loss and switches to VBAT power<\/strong> for the backup registers and also several special blocks. In total these are:<\/strong><\/p>\n\n\n\n