The V_VBAT_BKUP pin is not accessible through the Arduino Expansion Board. Although there is a test point for it, there isn't a header to access it. The test point connected to the V_VBAT_BKUP pin is the test point TP23 which is located at the back side of the Arduino Expansion Board. You could try to solder a wire to get access to the pin, but it could be challenging due to the risk of creating a short circuit between the other test points next to it. I don't recommend you to do it because you could damage the board permanently.
On the other hand, the Arduino Expansion Board uses the LTC4067 to regulate the voltage for the VSYS power input. This IC has a battery input that can be used to power the module (only the module, not the whole expansion board). This battery input is available through the J2 jumper. You could use it to power the board through a battery when the external power supply is off. I recommend you to check the Arduino Expansion Board documentation and the LTC4067 datasheet for further information about powering the module through the J2 jumper:
- Intel® Edison Boards — Intel® Edison Kit for Arduino* Hardware Guide
- Intel® Edison Boards — Arduino* Board Schematic
I connected the battery as shown in the figure 1 below and got the results
- The system time is stored when powered only from the battery
- The voltage on pin 23 is 3V and on pin 2,4,6 is 0V
1. Why the voltage on pin 2,4,6 is 0V (VSYS). The documentation says (Intel® Edison Compute Module - Hardware Guide): "...Intel® Edison uses VSYS (pins 2, 4, and 6) as the only power input path. Internal to Intel® Edison, this signal is also
connected to the VBAT path. " (see figure 2)
2. If I put one super-capacitors (rechargeable) instead of CR1220 (not rechargeable), we need additional external components? The documentation says "...The PMIC has a dedicated charging subsystem for a backup battery supply that could be either a rechargeable coin cell batteries or super-capacitors", but not shown on the scheme application. For example:
First I would like to know how you are accessing the Edison's pins directly in the Arduino Expansion Board, are you using the test points at the back side of the board?
I measured the pins 2, 4, 6 and 14 with a DMM and I got a value of 4.34V when powering the board from the external power supply. These pins are accessible through the jumper J10 (it is just above where the Edison module is placed). If you are not applying any external power source, only using the CR1220, then there is no power at the VSYS line so getting a value of 0V would be the expected value.
Regarding the super-capacitor question, I'm not quite sure. Let me investigate about it to give you a more accurate response.
Thanks for reply.
1. I used the test point (TP23) at the back side of the board. (figure 1)
2. I used the jumper J10 for measure of V_SYSTEM=VSYS
3. Why the VSYS voltage is 0V, if it is closed with VBAT (only using the CR1220). Maybe between them is the diode? (figure 2)
4. Waiting for your reply about the super-capacitor. You will help me
3. Check the schematic for the Arduino board. There is no connection between VSYS and V_VBAT_BKUP. I imagine that the power chip inside of the Edison module has pins for the main power and the backup power, and they are not connected.
4. Try to connect a supercapacitor to VBAT through a 22-50 ohm resistor to limit the current. Maybe, the power chip inside of the Edison module already has a resistor; then you can try to connect a supercapacitor without an external resistor.
I'll try to experiment with a LiPo battery connected to V_BAT_BKUP on the weekend.
Check the schematic for the Arduino board. There is no connection between VSYS and V_VBAT_BKUP.
There are connection between VSYS and V_VBAT_BKUP on the Edison module, not Arduino. (see picture above) I also think that in reality there is no connection. But in the documentation it is written otherwise. So I want to understand. From documentation "Intel® Edison Compute Module Hardware Guide (January 2015 Revision 004)" on page 39
>> 5.1 Main power supply VSYS
>> Intel® Edison uses VSYS (pins 2, 4, and 6) as the only power input path. Internal to Intel® Edison, this signal is also connected to the VBAT path.
Sorry for the delay. It seems there is confusion between VBAT and V_VBAT_BKUP names. VBAT and V_VBAT_BKUP are not the same pins; VBAT is an internal connection on the Edison module while the V_VBAT_BKUP is the external pin available in the 70-pin connector. Consequently, V_VBAT_BKUP in not connected to VSYS so when you use the CR1220, the voltage applied won't be reflected on the VSYS line. Let me know if this topic is clear now?
Regarding the super-capacitor question, there is no need for an external circuit to charge either the rechargeable coin cell battery or the super-capacitor. However, I suggest you to review the section 4.13 of the Edison Compute Module Hardware Guide (Intel® Edison Boards — Compute Module Hardware Guide) to make sure the charging output specifications are in agreement with the specifications of the rechargeable coin cell or the super-capacitor to avoid damaging the PMIC.