4 external rc, 5 internal system clock, 4 external rc -15 – Maxim Integrated MAXQ Family User Manual

Page 20: 5 internal system clock generation -15, Figure 2-8. on-chip crystal oscillator -15, Figure 2-9. rc relaxation oscillator -15, Table 2-2. system clock rate control settings -15, Maxq family user’s guide, Table 2-2. system clock rate control settings, 5 internal system clock generation

2-15

resistor to ensure a satisfactory logic level for active clock pulses. To minimize system noise on the clock circuitry, the external clock

source must meet the maximum rise and fall times and the minimum high and low times specified for the clock source. The external

noise can affect clock generation circuit if these parameters do not meet the specification.

2.7.4 External RC

For timing-insensitive applications, the external RC option offers additional cost savings. The RC oscillator frequency is a function of

the supply voltage, external resistor (Rext) and capacitor (Cext) values and tolerances, and the operating temperature. In addition to

this, the oscillator frequency varies from unit to unit due to normal process parameter variation. Figure 2-9 shows how the external RC

combination is connected to the MAXQ microcontroller.

2.7.5 Internal System Clock Generation

The internal system clock is derived from the currently selected oscillator input.

By default, one system clock cycle is generated per oscillator cycle, but the number of oscillator cycles per system clock can also be

increased by setting the Power Management Mode Enable (PMME) bit and the Clock Divide Control (CD[1:0]) register bits per Table 2-2.

Table 2-2. System Clock Rate Control Settings

PMME

CD[1:0]

CYCLES PER CLOCK

1 (default)

256

Figure 2-9. RC Relaxation Oscillator

MAXQ

CLOCK

CIRCUITRY

STOP OR

RGSL

XTAL1

XTAL2

INTERNAL CIRCUIT

EXT

MAXQ Family User’s Guide

Figure 2-8. On-Chip Crystal Oscillator