Rainbow Electronics MAXQ7667 User Manual

Page 21

MAXQ7667

16-Bit, RISC, Microcontroller-Based,

Ultrasonic Distance-Measuring System

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MAXQ is a registered trademark of Maxim Integrated Products, Inc.

Type 2 Timers/Counters

The MAXQ7667 includes three 16-bit timers/counters
with programmable I/O (Figure 9). Each timer is a Type
2 timer implemented in the MAXQ

family. The Type 2

timer is an autoreload 16-bit timer/counter offering the
following functions:

• 8-bit/16-bit timer/counter

• Up/down autoreload

• Counter function of external pulse

• Capture

• Compare

Clock Sources

The MAXQ7667 oscillator module supplies the system
clock for the µC core and all of the peripheral modules.
The high-frequency oscillator operates with a 1MHz to
16MHz crystal. Use the internal RC oscillator as the
system clock for applications that do not require pre-
cise timing. See Section 15 of the

MAXQ7667 User’s

Guide

The MAXQ7667 supports the following master clock
sources:

• Internal high-frequency oscillator drives an exter-

nal 1MHz–16MHz crystal or ceramic resonator

• Internal, fast-starting, 13.5MHz RC oscillator

(default oscillator at startup and in the event the
external crystal fails)

• External 4MHz–16MHz clock input

Crystal Selection

The MAXQ7667 requires a crystal with the following
specifications:

Frequency: 1MHz–16MHz

LOAD

: 6pF (min)

Drive level: 5µW

Series resonance resistance: 30

Ω (max)

Note: Quartz crystal vendors often specify series reso-
nance resistance (R1). Series resonance resistance is
the resistance observed when the resonator is in the
series resonant condition. When a resonator is used in
the parallel resonant mode with an external load capac-
itance, as is the case with the MAXQ7667 oscillator cir-
cuit, the effective resistance at the loaded frequency of
oscillation is:

R1 x (1 + (CO/C

LOAD

))

For typical shunt capacitance (CO) and load capaci-
tance (C

LOAD

) values, the effective resistance poten-

tially exceeds R1 by a factor of 2.