FS26 datasheet(126/156 Pages) NXP

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NXP Semiconductors

FS26

Safety system basis chip with low power for ASIL D/ASIL B

19.4.1 LDT characteristics

In normal operation, the timer can count up to 194 days, with a 1 second resolution. In short count mode, the

prescaler 2 is bypassed and the timer can count up to 2.28 hours, with a 488 µs resolution.

Oscillator

frequency

Oscillator

period

Prescaler

Counter

resolution

Max count

Long count

32.768 kHz

30.52 µs

16 x 2048

1 s

4660 hours

194 days

Short count

32.768 kHz

30.52 µs

488 µs

8192 s

2.28 hours

Table 124. long duration timer characteristics

The LDT has two modes of operation, based on the prescaler used during the count:

• When LDT_MODE = 0, the LDT is set in Long Count mode.

• When LDT_MODE = 1, the LDT is set in Short Count mode, suitable for calibration.

The LDT_AFTER_RUN[15:0] bits is used to set or to read the after run target value in normal mode.

The LDT_WUP_H[7:0] and the LDT_WUP_L[15:0] bits is used to set or to read the wake-up target value, in

combination with the LDT_SEL bit:

• LDT_WUP_H[7:0] contains the 8 most significant bits of the wake-up target value.

• LDT_WUP_L[15:0] contains the 16 least significant bits of the wake-up target value.

The LDT_SEL bit allows the MCU to either set/read the wake-up target value or to read the current value of the

24 bit LDT counter in the LDT_WUP_H[7:0] and the LDT_WUP_L[15:0] bits.

• When LDT_SEL = 0, the MCU can read or write the wake-up target value in the LDT_WUP_H[7:0] and the

LDT_WUP_L[15:0] bits.

• When LDT_SEL = 1, the MCU can read the counter current value (running or not).

The LDT_EN bit is provided to start the LDT timer operation:

• When LDT_EN = 0, the LDT is disabled.

• When LDT_EN = 1, the LDT starts counting as defined in the M_LDT_CTRL and M_LDT_CFGx registers.

The LDT_LPSEL[7:0] bits indicate the appropriate Low-power mode. The LDT_LPSEL[7:0] bits reset to 0 every

time the device enters the normal mode. When timer function 2 or 3 is selected, the MCU must first write the

safe key value on the LDT_LPSEL[7:0] bits to confirm that the device will go into the corresponding Low-power

mode after the run timer has expired.

• When LDT_LPSEL[7:0] = AAh, the device goes into standby mode after the run timer expires.

• When LDT_LPSEL[7:0] = A5h, the device goes into LPOFF mode after the run timer expires.

• When timer function 4 or 5 is selected and LDT_EN = 1, the LDT does not start any count until the device

enters the corresponding Low-power mode.

FS26_SDS

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

Product short data sheet

Rev. 6.0 — 12 December 2025

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