Using a 1F supercap, the RTC will operate for approximately two weeks.

A: The FM3130, FM3135, FM3127x, FM31L27x, and FM33xx have a fast charge mode. If a 1F supercap has been discharged to 1.5V, it will take approximately 30 minutes.

See also: AN404 – RTC Backup and UL
 

A: When a battery is first attached to the FM31xx while Vdd is powered off, there are a number of registers inside the chip that come up in an unknown state.  These circuits are connected to the internal power supply. It is possible that the Ibak current is a few microamps or as much as 100 microamps.  It's a random startup condition.  Once Vdd is applied, the Ibak current goes to zero.  All internal circuits are set to known states.  When  Vdd is then powered down, the Ibak current is < 1uA.  This issue has been resolved on the FM3127x, FM31L27x, FM33xx, FM3130, and FM3135 devices.

A: Check that your board layout is clean - do not route switching signals next to the X1/X2 pins and be sure to have a guard-ring around the X1/X2 pins.  Typically if the crystal pins are being disturbed by nearby switching signals, you will observe the CAL output is much lower than 512 Hz. Other things to check: be sure to remove solder flux near crystal pins, replace the crystal, initialize all battery-backed registers to a value within its valid range.

A: Two-wire (I²C) is the most common and many micros implement a two-wire dedicated port.  SPI operates at much higher clock rates, up to 20MHz.  For simplicity and the use of fewer I/O micro pins, two-wire is generally used.

A: The microcontroller could issue a STOP condition but this may be difficult since bus contention will occur. If the micro cannot drive SDA high, there is an alternate method. The second way to abort a pending read operation is to issue clocks until a data bit value of ‘1’ occurs. A ‘1’ data bit will cause the serial FRAM to release the SDA line since it only drives low. To make this method work, the master issues an SCL rising edge, then reads the SDA line while SCL is high. If SDA is ‘0’, then another clock must be issued. If SDA is ‘1’, then the master can force SDA low while SCL is high. This is a START condition. A START condition also will cause the memory to end the read operation immediately.

See also: AN201 Interrupting a Two-Wire Read
 

A: Using a 1F supercap, the RTC will operate for approximately two weeks.

A: A: The FM3130, FM3135, FM3127x, FM31L27x, and FM33xx have a fast charge mode. If a 1F supercap has been discharged to 1.5V, it will take approximately 30 minutes.

See also: AN404 – RTC Backup and UL

A: When a battery is first attached to the FM31xx while Vdd is powered off, there are a number of registers inside the chip that come up in an unknown state.  These circuits are connected to the internal power supply. It is possible that the Ibak current is a few microamps or as much as 100 microamps.  It's a random startup condition.  Once Vdd is applied, the Ibak current goes to zero.  All internal circuits are set to known states.  When  Vdd is then powered down, the Ibak current is < 1uA.  This issue has been resolved on the FM3127x, FM31L27x, FM33xx, FM3130, and FM3135 devices.

A: Check that your board layout is clean - do not route switching signals next to the X1/X2 pins and be sure to have a guard-ring around the X1/X2 pins.  Typically if the crystal pins are being disturbed by nearby switching signals, you will observe the CAL output is much lower than 512 Hz. Other things to check: be sure to remove solder flux near crystal pins, replace the crystal, initialize all battery-backed registers to a value within its valid range.

A: Two-wire (I²C) is the most common and many micros implement a two-wire dedicated port.  SPI operates at much higher clock rates, up to 20MHz.  For simplicity and the use of fewer I/O micro pins, two-wire is generally used.