COP8CBE9 datasheet(19/75 Pages) NSC

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4.0 Functional Description (Continued)

In Case 1, V

CC rises from 0V and the on-chip RESET is

undefined until the supply is greater than approximately

1.0V. At this time the brownout circuit becomes active and

holds the device in RESET. As the supply passes a level of

about 1.8V, a delay of about 3 ms (t

d) is started and the Idle

Timer is preset to a value between 00F0 and 00FF (hex).

Once V

CC is greater than Vbor and td has expired, the Idle

Timer is allowed to count down (t

id).

Case 2 shows a subsequent dip in the supply voltage which

goes below the approximate 1.8V level. As V

CC drops below

bor, the internal RESET signal is asserted. When VCC rises

back above the 1.8V level, t

d is started. Since the power

supply rise time is longer for this case, t

d has expired before

CC rises above Vbor and tid starts immediately when VCC is

greater than V

bor.

Case 3 shows a dip in the supply where V

CC drops below

bor, but not below 1.8V. On-chip RESET is asserted when

CC goes below Vbor and tid starts as soon as the supply

goes back above V

bor.

If the Brownout Reset feature is enabled, the internal reset

will not be turned off until the Idle Timer underflows. The

internal reset will perform the same functions as external

reset. The device is guaranteed to operate at the specified

frequency down to the specified brownout voltage. After the

underflow, the logic is designed such that no additional

internal resets occur as long as V

CC remains above the

brownout voltage.

The device is relatively immune to short duration negative-

going V

CC transients (glitches). It is essential that good

filtering of V

CC be done to ensure that the brownout feature

works correctly. Power supply decoupling is vital even in

battery powered systems.

There are two optional brownout voltages. The part numbers

for the three versions of this device are:

COP8CBE, V

bor = low voltage range

COP8CCE, V

bor = high voltage range

COP8CDE, BOR is disabled.

Refer to the device specifications for the actual V

bor volt-

ages.

High brownout voltage devices are guaranteed to operate at

10MHz down to the high brownout voltage. Low brownout

voltage devices are guaranteed to operate at 3.33MHz down

to the low brownout voltage. Low brownout voltage de-

vices are not guaranteed to operate at 10MHz down to

the low brownout voltage.

Under no circumstances should the RESET pin be allowed

to float. If the on-chip Brownout Reset feature is being used,

the RESET pin should be connected directly to V

CC. The

RESET input may also be connected to an external pull-up

resistor or to other external circuitry. The output of the brown-

out reset detector will always preset the Idle Timer to a value

between 00F0 and 00FF (240 to 256 t

C). At this time, the

internal reset will be generated.

If the BOR feature is disabled, then no internal resets are

generated and the Idle Timer will power-up with an unknown

value. In this case, the external RESET must be used. When

BOR is disabled, this on-chip circuitry is disabled and draws

no DC current.

The contents of data registers and RAM are unknown fol-

lowing the on-chip reset.

4.8 OSCILLATOR CIRCUITS

The device has two crystal oscillators to facilitate low power

operation while maintaining throughput when required. Fur-

ther information on the use of the two oscillators is found in

Section 7.0 Power Saving Features. The low speed oscillator

utilizes the L0 and L1 port pins. References in the following

text to CKI will also apply to L0 and references to G7/CKO

will also apply to L1.

4.8.1 Oscillator

CKI is the clock input while G7/CKO is the clock generator

output to the crystal. An on-chip bias resistor connected

between CKI and CKO is provided to reduce system part

count. The value of the resistor is in the range of 0.5M to 2M

(typically 1.0M).

Table 3 shows the component values re-

quired for various standard crystal values. Resistor R2 is

on-chip, for the high speed oscillator, and is shown for

reference.

Figure 12 shows the crystal oscillator connection

diagram. A ceramic resonator of the required frequency may

be used in place of a crystal if the accuracy requirements are

not quite as strict.

20022514

FIGURE 11. Reset Circuit Using Power-On Reset

COP8CBE9 Datasheet(PDF) 19 Page - National Semiconductor (TI)

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