Optimise system energy consumption with MRAM

Rise time: During this phase, I assumed that all of the energy goes into the decoupling capacitor and that the non-volatile memory consumes negligible energy.

Power-up time: Once the voltage on V_DD is above a threshold, a small delay (^tPU) is required for MRAM to become ready but not for EEPROM or Flash. During this phase, I assumed that the MRAM consumes the current shown in the standby specification of the datasheet.

Write time: During this phase, the non-volatile memory consumes the current shown in the active specification of the datasheet.

Assuming a 3.3 V system with a tolerance of ±10%, the lowest voltage on the I/O could be 3.3 V -10% = 2.97 V. This voltage of 2.97 V was used in calculations.

Energy calculations
I started by looking at the energy used by the non-volatile memory during writing.

Energy used by an EEPROM: I looked at a typical 3.3 V EEPROM with a standby current of 1?A, a write time of 5 ms, and a write current of 3 mA. For EEPROM I assumed the following:
1. The EEPROM is ready to begin operation as soon as VDD rises to be within operating limits (a power-up time of zero).
2. The amount of data written fits into one page, and writing takes place using the block write capability.
3. The write time of the EEPROM is only that required to perform the write operation of the EEPROM, and I ignored any processing and communication time of the MCU and SPI interface. (This assumption is the opposite of that used for MRAM. MRAM only requires communication time because the write time is so short that it can be considered to be zero.)
4. The EEPROM is powered directly from the microcontroller I/O and use a small (0.1?F) decoupling capacitor (table 1).

Table 1: Energy consumed during write operations for EEPROM.

Energy used by a serial Flash: Serial flash has much higher write and standby currents. I used a standby current of 50?A, a write time of 3 ms, and a write current of 15 mA.

Since Flash is similar to EEPROM, I made three of the same assumptions: 1) zero power-up time; 2) the data fits into one page; and 3) the write time is so long that I ignored the communication time. I also assumed that the Flash writes are to a pre-erased page (table 2).

Table 2: Energy consumed during write operations for serial flash.

Energy used by an MRAM: For MRAM, only Everspin Technologies has commercially available products, so I used the MR25H256, 256 kbit serial SPI MRAM.

Table 3 shows the energy per data byte is lowest when all of the energy from the decoupling capacitor is used. The decoupling capacitor size should be chosen to match the amount of data that is typically acquired by the system.

Table 3: Serial MRAM energy consumed during write operations.

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