PSoC 6 MCU

[StNu_1610991]

A system is being designed based on CY8PROTO-063-BLE and using PSoC Creator 4.3 for development.  The system is unpowered most of he time but needs the RTC for getting the date and time when powered up.  A 3V battery is attached to VBACKUP for maintaining the power to the RTC.  In the PSoC Creaor 4.3, The DWR System window has vBackup Source set to Dedicated supply and VBACKUP (V) set to 3.0V.

As far as I know these are the proper settings for keeping the RTC powred.  However, the VBACKUP voltage is appearing on the VDD pin and therefore powering the PSoC and all the peripherals on the VDD circuit.  When the PSoC is unplugged and unpowered, an ohmmeter shows that VBACKUP and VDD are connected.

Are there other configuration switches involved that I am missing?

[ShipingW_81 · Accepted solution]

StNu_1610991

On CY8CPROTO-063-BLE board, the VBACKUP and VDD are connected through 0ohm resistor(R29), just as shown in board schem -

If you plan to power VBACKUP rail separately, R29 should be removed firstly.

[StNu_1610991]

Thanks.  That is what I was missing.  I was not aware of the 0 ohm resistor shorting the two pins.

[StNu_1610991]

By the way, I checked the documentation again for "VACKUP" and there is nothing that says that R29 needs to be removed to use a backup battery for the CY8PROTO-063-BLE unit. 

[Len_CONSULTRON]

StNu,

If you leave R29 connected, two problems could exist:

• When you lose the primary source of power supplying P6_VDD, the supercap on VBACKUP will supply all circuits connected to P_VDD.  This will deplete the energy stored in the cap quicker.  This makes a cap on VBACKUP less useful.
• With the cap on VBACKUP connected to P6_VDD, the CPU will try to continue to operate as long as the voltage on the cap of VBACKUP is high enough.  The CPU load will diminish the energy in the cap even faster.

Using a system with a separate VBACKUP voltage domain has an advantage that the in the loss of primary power (to P6_VDD) a fully charged cap on VBACKUP can power certain PSoC6 resources such as the RTC much longer as long as the VBACKUP is disconnected from the primary voltage domain.

On the CY8PROTO-063-BLE, VBACKUP is connected to P6_VDD as a convenience and should be separated (R29 removed) if VBACKUP is needed to preserve the power to the RTC.  Removing R29 and placing a large cap on VBACKUP can allow your RTC to keep time for days.  With R29 connected, a few hours might be as long as the cap energy lasts.

Len