UPD6P8 datasheet(39/95 Pages) NEC

39 / 95 page

µPD6P8, 6P8A, 6P8B

Data Sheet U17848EJ3V0DS

8.1 Functions of POC Circuit

The POC circuit has the following functions:

• Generates an internal reset signal when VDD

≤ VPOC.

• Cancels an internal reset signal when VDD > VPOC.

Here, VDD: power supply voltage, VPOC: POC detection voltage.

Notes 1. Actually, oscillation stabilization wait time must elapse before the circuit is switched to operation mode.

The oscillation stabilization wait time is about 534/fX to 918/fX (when about 134 to 230

µs; @ fX = 4 MHz).

2. For the POC circuit to generate an internal reset signal when the power supply voltage has fallen,

it is necessary for the power supply voltage to be kept less than the VPOC for the period of 1 ms or

more. Therefore, in reality, there is the time lag of up to 1 ms until the reset takes effect.

3. The POC detection voltage (VPOC) varies between approximately 1.7 to 1.9 V; thus, the reset may

be canceled at a power supply voltage smaller than the guaranteed range (VDD = 1.9 to 3.6 V).

However, as long as the conditions for operating the POC circuit are met, the actual lowest operating

power supply voltage becomes lower than the POC detection voltage.

Therefore, there is no

malfunction occurring due to a shortage of power supply voltage. However, malfunction for such

reasons as the clock not oscillating due to low power supply voltage may occur (refer to Cautions

3 in 8 POC CIRCUIT).

8.2 Oscillation Check at Low Supply Voltage

A reliable reset operation can be expected of the POC circuit if it satisfies the condition that the clock can oscillate

even at low power supply voltage (the oscillation start voltage of the resonator being even lower than the POC

detection voltage). Whether this condition is met or not can be checked by measuring the oscillation status in a

product that actually includes a POC circuit, as follows.

<1> Connect a storage oscilloscope to the XOUT pin so that the oscillation status can be measured.

<2> Connect a power supply whose output voltage can be varied and then gradually raise the power supply

voltage VDD from 0 V (making sure to avoid VDD > 3.6V).

At first (during VDD < approx. 1.7 V), the XOUT pin is 0 V regardless of the VDD. However, at the point that VDD

reaches the POC detection voltage (VPOC = 1.8 V (TYP.)), the voltage of the XOUT pin jumps to about 0.5VDD. Maintain

this power supply voltage for a while to measure the waveform of the XOUT pin. If by any chance the oscillation

start voltage of the resonator is lower than the POC detection voltage, the growing oscillation of the XOUT pin can

be confirmed within several ms after the VDD has reached the VPOC.

VDD

3.6 V

1.9 V

VPOC

Approx. 1.7 V

0 V

Internal reset signal

Reset

Operating ambient temperature TA = – 40 to + 85

°C

Clock frequency fX = 3.5 to 4.5 MHz

←POC detection voltage VPOC = 1.8 V (TYP.)Note 3

→ t

Operation mode

↑ Reset

Note 2

↑

Note 1

UPD6P8 Datasheet(PDF) 39 Page - NEC

UPD6P8 Datasheet(HTML) 39 Page - NEC

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