MCP795W21 datasheet(33/54 Pages) MICROCHIP

33 / 54 page

 2011-2012 Microchip Technology Inc.

Preliminary

DS22280C-page 33

MCP795W1X/MCP795W2X

9.1.7

POWER-FAIL TIME-STAMP

The MCP795WXX family of RTCC devices feature a

power-fail time-stamp feature. This feature will save the

time at which VCC crosses the VTRIP voltage and is

shown in Figure 9-4. To use this feature, a VBAT supply

must be present and the oscillator must also be

running. There are two separate sets of registers that

are used to record this information:

• The first set located at 0x18h through 0x1Bh are

loaded at the time when VCC falls below VTRIP

and the RTCC operates on the VBAT. The VBAT

(register 0x03h bit 4) bit is also set at this time.

• The second set of registers, located at 0x1Ch

through 0x1Fh, are loaded at the time when VCC

is restored and the RTCC switches to VCC.

The power-fail time-stamp registers are cleared when

the VBAT bit is cleared in software.

FIGURE 9-4:

POWER-FAIL GRAPH

9.1.8

READ STATUS REGISTER

(SRREAD)

The Read Status Register (SRREAD) instruction

provides access to the STATUS register. The STATUS

cycle. The STATUS register is formatted as follows:

The Write-In-Process (WIP) bit indicates whether the

MCP795WXX is busy with a nonvolatile memory write

operation. When set to a ‘1’, a write is in progress,

when set to a ‘0’, no write is in progress. This bit is

read-only.

The Write Enable Latch (WEL) bit indicates the

status of the write enable latch. When set to a ‘1’, the

latch allows writes to the nonvolatile memory, when

set to a ‘0’, the latch prohibits writes to the nonvolatile

memory. The state of this bit can always be updated

via the WREN or WRDI commands, regardless of the

state of write protection on the STATUS register. This

bit is read-only.

The Block Protection (BP0 and BP1) bits indicate

which blocks are currently write-protected. These bits

are set by the user issuing the WRSR instruction. These

bits are nonvolatile.

See Figure 9-5 for the RDSR timing sequence.

Note:

It is strongly recommended that the time-

saver function only be used when the

oscillator is running. This will ensure

accurate functionality.

VCC

VTRIP(max)

VTRIP(min)

Power-Down

Time-Stamp

Power-Up

Time-Stamp

654

—

——— R/W

R/W

XXX

BP1

BP0

WEL

WIP

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