M41ST85W datasheet(17/43 Pages) STMICROELECTRONICS

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M41ST85W

Operating modes

2.3

Write mode

In this mode the master transmitter transmits to the M41ST85W slave receiver. Bus protocol

is shown in Figure 12. Following the START condition and slave address, a logic '0' (R/W=0)

is placed on the bus and indicates to the addressed device that word address An will follow

and is to be written to the on-chip address pointer. The data word to be written to the

memory is strobed in next and the internal address pointer is incremented to the next

memory location within the RAM on the reception of an acknowledge clock. The

M41ST85W slave receiver will send an acknowledge clock to the master transmitter after it

has received the slave address and again after it has received the word address and each

data byte.

Figure 12. Write mode sequence

2.4

Data retention mode

With valid VCC applied, the M41ST85W can be accessed as described above with READ or

WRITE cycles. Should the supply voltage decay, the M41ST85W will automatically deselect,

write protecting itself (and any external SRAM) when VCC falls between VPFD(max) and

VPFD(min). This is accomplished by internally inhibiting access to the clock registers. At this

time, the reset pin (RST) is driven active and will remain active until VCC returns to nominal

levels. External RAM access is inhibited in a similar manner by forcing ECON to a high level.

This level is within 0.2 volts of the VBAT. ECON will remain at this level as long as VCC

remains at an out-of-tolerance condition. When VCC falls below the battery backup

switchover voltage (VSO), power input is switched from the VCC pin to the SNAPHAT

battery, and the clock registers and external SRAM are maintained from the attached battery

supply.

All outputs become high impedance. The VOUT pin is capable of supplying 100 µA of current

to the attached memory with less than 0.3 volts drop under this condition. On power-up,

when VCC returns to a nominal value, write protection continues for trec by inhibiting ECON.

The RST signal also remains active during this time (see Figure 20 on page 33).

Note:

Most low power SRAMs on the market today can be used with the M41ST85W RTC

SUPERVISOR. There are, however some criteria which should be used in making the final

choice of an SRAM to use. The SRAM must be designed in a way where the chip enable

input disables all other inputs to the SRAM. This allows inputs to the M41ST85W and

SRAMs to be “Don’t Care” once VCC falls below VPFD(min). The SRAM should also

guarantee data retention down to VCC = 2.0 volts. The chip enable access time must be

sufficient to meet the system needs with the chip enable output propagation delays

AI00591

BUS ACTIVITY:

SDA LINE

BUS ACTIVITY:

MASTER

DATA n

DATA n+1

DATA n+X

WORD

ADDRESS (An)

SLAVE

ADDRESS

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