Using DS1307 RTC Module with NuttX

Kids, always read the datasheet before starting wiring your module. I always said it to other people, but sometimes I’m anxious to get things working and fail to follow my own advice.

It happened again with DS1307 Module that I got from Aliexpress:

I connected it to STM32F4Discovery board this way:

----------------------------------
STM32F4Discovery |  DS1307 Module
-----------------+----------------
GND              |   GND
3V               |   VCC
SDA              |   PB9
SCL              |   PB7
-----------------+----------------

But when NuttX was starting it was failing to communicate with the module, returning error -6 (= ENXIO = “No such device or address”) :

ABCDF
 stm32_ds1307_init: Initialize I2C1
 stm32_ds1307_init: Bind the DS1307 RTC driver to I2C1
 up_rtc_getdatetime: ERROR: I2C_TRANSFER failed: -6

I used the Logic Analyzer and saw that the address was sent correctly, but the board wasn’t responding. After some time I decided to look the datasheet to confirm that it could work with 3V (I assumed it because the module uses a 3V battery) and to my surprise it only works from 4.5V to 5V.

Then connecting the VCC from the module to 5V worked:

ABCDF
 stm32_ds1307_init: Initialize I2C1
 stm32_ds1307_init: Bind the DS1307 RTC driver to I2C1
 rtc_dumptime: Returning:
 rtc_dumptime:    tm_sec: 00000026
 rtc_dumptime:    tm_min: 00000024
 rtc_dumptime:   tm_hour: 00000004
 rtc_dumptime:   tm_mday: 00000019
 rtc_dumptime:    tm_mon: 00000009
 rtc_dumptime:   tm_year: 00000068

nsh> date
 Oct 25 04:36:56 2004

 nsh> date -s "Sep 22 08:04:00 2019"
 up_rtc_settime: Setting time tp=(1569139440,0)
 rtc_dumptime: New time:
 rtc_dumptime:    tm_sec: 00000000
 rtc_dumptime:    tm_min: 00000004
 rtc_dumptime:   tm_hour: 00000008
 rtc_dumptime:   tm_mday: 00000016
 rtc_dumptime:    tm_mon: 00000008
 rtc_dumptime:   tm_year: 00000077
 nsh>

I turned the board OFF and let it alone some minutes and when I connected it again the time was correct:

NuttShell (NSH)
nsh> date
Sep 22 08:23:41 2019
nsh>

These are the main options I enabled on NuttX to get it working:

 STM32_I2C1
 STM32_I2C
 RTC
 RTC_DATETIME
 RTC_DS1307
 RTC_DSXXXX
 RTC_EXTERNAL

• BrainDamage