ICM7170

Specification Number ICM7170_IM (IL) REV -

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The new load configuration (Figure 6) allows these two

conditions to be met independently. The two load capacitors,

C1 and C2, provide a fixed load to the oscillator and crystal.

C3 adjusts the frequency that the circuit resonates at by

reducing the effective value of the crystal's motional

capacitance, C0. This minute adjustment does not

appreciably change the load of the overall system, therefore,

stability is no longer affected by tuning. Typical values for

these capacitors are shown in Table 5. C1 and C2 must

always be greater than twice the crystal’s recommended load

capacitance in order for C3 to be able to trim the frequency.

Some experimentation may be necessary to determine the

ideal values of C1 and C2 for a particular crystal.

This three capacitor tuning method will be more stable than

the original design and is mandatory for 32kHz tuning fork

crystals: without it they may leap into an overtone mode

when power is initially applied.

The original two-capacitor circuit (Figure 8) will continue to

work as well as it always has, and may continue to be used

in applications where cost or space is a critical

consideration. It is also easier to tune to frequency since one

end of the trimmer capacitor is fixed at the AC ground of the

between the adjustment tool and the trimmer capacitor. Note

that in both configurations the load capacitors are connected

AC ground.

Layout: Due to the extremely low current (and therefore high

impedance) design of the ICM7170s oscillator, special

attention must be given to the layout of this section. Stray

capacitance should be minimized. Keep the oscillator traces

on a single layer of the PCB. Avoid putting a ground plane

above or below this layer. The traces between the crystal,

the capacitors, and the ICM7170 OSC pins should be as

short as possible. Completely surround the oscillator

with any digital signals. The final assembly must be free from

contaminants such as solder flux, moisture, or any other

potential sources of leakage. A good solder mask will help

keep the traces free of moisture and contamination over

time.

Oscillator Tuning

Trimming the oscillator should be done indirectly. Direct

monitoring of the oscillator frequency by probing OSC IN or

OSC OUT is not accurate due to the capacitive loading of

most probes. One way to accurately trim the ICM7170 is by

turning on the 1 second periodic interrupt and trimming the

oscillator until the interrupt period is exactly one second.

This can be done as follows:

1.Turn on the system. Write a 00H to the Interrupt Mask Register

(location 10H) to clear all interrupts.

2. Set the Command Register (location 11H) for the appropriate

crystal frequency, set the Interrupt Enable and Run/Stop bits to

1, and set the Test bit to 0.

3. Write a 08H to the Interrupt Mask Register to turn on the 1s

interrupt.

4. Write an interrupt handler to read the Interrupt Status Register

after every interrupt. This resets the interrupt and allows it to be

set again. A software loop that reads the Interrupt Status

Register several times each second will accomplish this also.

5. Connect a precision period counter capable of measuring 1s

within the accuracy desired to the interrupt output. If the interrupt

is configured as active low, trigger on the falling edge. If the

interrupt is active high, trigger on the rising edge. Be sure to

measure the period between when the transistor turns ON, and

when the transistor turns ON a second later.

6. Adjust C3 (C2 for the two-capacitor load configuration) for an

interrupt period of exactly 1.000000 seconds.

Application Notes

Digital Input Termination During Backup

To ensure low current drain during battery backup operation,

none of the digital inputs to the ICM7170 should be allowed

to float. This keeps the input logic gates out of their transition

region, and prevents crossover current from flowing which

will shorten battery life. The address, data, CS, and ALE pins

the internal battery switchover circuit is used or not.

IBM/PC Evaluation Circuit

Figure 9 shows the schematic of a board that has been

designed to plug into an IBM PC/XT (Note 1) or compatible

computer. In this example CS is permanently tied low and

access to the chip is controlled by the RD and WR pins.

These signals are generated by U1, which gates the IBM’s

lOR and lOW with a device select signal from U3, which is

TABLE 5. TYPICAL LOAD CAPACITOR VALUES

CRYSTAL

FREQUENCY

LOAD CAPS

(C1, C2)

TRIMMER CAP

(C3)

32kHz

33pF

5 - 50pF

1MHz

33pF

5 - 50pF

2MHz

25pF

5 - 50pF

4MHz

22pF

5 - 100pF

OSC IN

10

ICM7170

OSC OUT

23

C1

C2

X1

9

C1 ≈ 2 x LOAD

C2 ≈ 5pF - 35pF

FIGURE 8. ORIGINAL OSCILLATOR CONFIGURATION

ICM7170