6MPS5 datasheet(39/101 Pages) TI1

39 / 101 page

www.national.com

The crystal oscillator you choose may require external com-

ponents different from the ones specified above. In that case,

consult with National Semiconductor for the component

specifications.

The crystals and other oscillator components should be

placed close to the X1CKI/X1CKO and X2CKI/X2CKO de-

vice input pins to keep the printed trace lengths to an abso-

lute minimum.

Choose capacitor component values in the tables to obtain

the specified load capacitance for the crystal when combined

with the parasitic capacitance of the trace, socket, and pack-

age (which can vary from 0 to 8 pF). As a guideline, the load

capacitance is:

CL = (C1 * C2)/(C1+C2) + Cparasitic

C2 > C1

C1 can be trimmed to obtain the desired load capacitance.

The start-up time of the 32.768 kHz oscillator can vary from

one to six seconds. The long start-up time is due to the high

“Q” value and high serial resistance of the crystal necessary

to minimize power consumption in Power Save mode.

11.2

MAIN SYSTEM CLOCK

The main system clock is generated by the main oscillator. It

can be stopped by the Power Management Module to reduce

power consumption during periods of reduced activity. When

the main clock is restarted, a 14-bit timer generates a “Good

Main Clk” signal after a start-up delay of 32,768 clock cycles.

This signal is an indicator that the main clock oscillator is sta-

ble.

The “Stop Main Osc” signal from the Power Management

Module stops and starts the main oscillator. When this signal

is asserted, it presets the 14-bit timer to 3FFF hex and stops

the main oscillator. When the signal goes inactive, the main

oscillator starts and the 14-bit timer counts down from its pre-

set value. When the timer reaches zero, it stops counting and

asserts the “Good Main Clk” signal.

11.3

SLOW SYSTEM CLOCK

The slow (32.768 kHz) clock is necessary for operating the

device in Power Save modes and to provide a clock source

for modules such as the Timing and Watchdog Module.

The slow clock operates in a manner similar to the main

clock. The “Stop Slow Osc” signal from the Power Manage-

ment Module stops and starts the slow oscillator. When this

signal is asserted, it presets a 6-bit timer to 3F hex and dis-

ables the slow oscillator. When the signal goes inactive, the

slow oscillator starts and the 6-bit timer counts down from its

preset value. When the timer reaches zero, it stops counting

and asserts the “Good Low Speed Clk” signal, thus indicating

that the slow clock is stable.

For systems that do not require a reduced power consump-

tion mode, the external crystal network may be omitted for

the slow clock. In that case, the slow clock can be created by

dividing the main clock by a prescaler factor. The prescaler

circuit consists of a fixed divide-by-2 counter and a program-

mable 8-bit prescaler register. This allows a choice of clock

divisors ranging from 2 to 512. The resulting slow clock fre-

quency must not exceed 100 KHz.

A software-programmable multiplexer selects either the

prescaled main clock or the 32.768 kHz oscillator as the slow

clock. Upon reset, the prescaled main clock is selected, en-

suring that the slow clock is always present initially. Selection

of the 32.768 kHz oscillator as the slow clock disables the

clock prescaler, which allows the CLK1 oscillator to be turned

Table 13

Component Values of the High Frequency Crystal Circuit

Component

Parameters

Values

Tolerance

Oscillator

Resonance Frequency

Type

Max. Serial Resistance

Max. Shunt Capacitance

Load Capacitance

4 MHz

AT-Cut

Ω

4 pF

12 pF

12 MHz

AT-Cut

Ω

4 pF

15 pF

16 MHz

AT-Cut

Ω

4 pF

15 pF

20 MHz

AT-Cut

Ω

4 pF

20 pF

N/A

Crystal

Resistor R1

1 M

Ω

1 M

Ω

1 M

Ω

1 M

Ω

Resistor R2

Ω

Capacitor C1, C2

22 pF

20 pF

20%

Table 14

Component Values of the Low Frequency Crystal Circuit

Component

Parameters

Values

Tolerance

Oscillator

Resonance Frequency

Type

Maximum Serial Resistance

Maximum Shunt Capacitance

Load Capacitance

32.768kHz

Parallel

N-Cut or XY-bar

40 k

Ω

2 pF

9-13 pF

N/A

Crystal

Resistor R1

10-20 M

Ω

Resistor R2

4.7 k

Ω

Capacitor C1, C2

20 pF

20%

CR16MNS5, CR16MFS5, and CR16MPS5 are Obsolete Devices

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