I have a strange problem which does not seem quite the same as previous posts. I would appreciate any pointers.
Situation: Nano-clone with CH340 USB driver, Deek-Robot combined DS1307 RTC and microSD shield (ID8122), running a datalogger sketch. I have not been able to find any documentation for Deek-Robot shield, but refer to https://forum.arduino.cc/index.php?topic=446133.msg3069884#msg3069884
My sketch runs fine while Nano is connected to PC, and also if I supply external 5V via USB connector. But when I power with 9V at Vin, the SD card creates the data file, but fails to write any data into it.
I tested my sketch on an Uno r3 with an adafruit-clone RTC/SD shield - all fine.
I have also connected the Deek-Robot RTC/SD to the Uno and it also works fine with either PC power or 9V at Vin. It is only when I use the Nano with 9v to Vin there is a problem.
The external power supply I am using is a 12V 7Ah sealed lead acid battery - 12.5V via a 120ohm dropping resistor to Vin. My Vin supply is "stronger" than either USB source.
Other posts have suggested the tiny 662K 3V3 regulator on the Deek-Robot shield is not adequately rated at 200mA, but it seems enough to establish the file, and to write data under some power supply configurations. The alternative 3V3 supply from the Nano's CH340 chip has only 25mA rating.
According to the Nano schematic, the 5V power to the board is sourced either from Vin via 5V regulator or from 5V pin of USB socket via a Schottky diode (Vfwd = 0.2V). Thus actual supply voltage to nano powered from USB is 4.8V. It seems strange to me that the problem arises with the slightly higher supply voltage. 3V3 supply on SDcard measures on spec for all power sources
Can anyone suggest a solution?
I can of course work-around with an external 5V regulator on my battery and feed power via USB.
Further tests with variable PSU showed that Vin must exceed 10V for correct operation. Appears Nano on-board 5V regulator may be under-rated. I simply removed 100ohn dropping resistor and supply 12+ V from battery direct to Vin.