PCF2131 datasheet(37/85 Pages) NXP

37 / 85 page

NXP Semiconductors

PCF2131

Nano-power highly accurate RTC with integrated quartz crystal

Bits labeled as X are undefined at power-on and unchanged by subsequent resets.

Bit

Symbol

Value

Description

where n is the timer value (n > 1)

Write Only

Table 58. Watchdg_tim_val - watchdog timer value register (address 36h) bit description

TF[1:0]

Timer source clock

frequency

Units

Minimum timer period

(n = 2)

Units

Maximum timer period

(n = 255)

Units

15.625

3.984

250

63.744

1⁄4

1020

1⁄64

16320

Table 59. Programmable watchdog timer

7.11.3 Watchdog timer function

The watchdog timer interrupt function is enabled or disabled by the WD_CD bit of the register Watchdg_tim_ctl

(see Table 56).

The 2 bits TF[1:0] in register Watchdg_tim_ctl determine one of the four source clock frequencies for the

watchdog timer: 64 Hz, 4 Hz, 1⁄4 Hz or 1⁄64 Hz (see Table 59).

When the watchdog timer function is enabled, the 8-bit timer in register Watchdg_tim_val determines the

watchdog timer period (see Table 59).

The watchdog timer counts down from the software programmed 8-bit binary value n in register

Watchdg_tim_val. When the counter reaches 1, the watchdog timer flag WDTF (register Control_2) is set logic 1

and an interrupt is generated. The period accuracy corresponds to n +/- 0.5.

The register Watchdg_tim_val is write only and not readable after set.

The counter does not automatically reload.

When WD_CD is logic 1/0 (watchdog timer interrupt enabled/disabled) and the Micro-controller Unit (MCU)

loads a watchdog timer value n:

• the flag WDTF is reset

• INTA/B is cleared

• the watchdog timer starts again

Loading the counter with 0 or 1 will:

• reset the flag WDTF

• clear INTA/B

• stop the watchdog timer

Remark: WDTF can be cleared by:

• loading a value in register Watchdg_tim_val

• writing a logic 0 to WDTF

Writing a logic 1 to WDTF has no effect.

PCF2131

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 2.0 — 5 October 2023

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