AN2674

Using an external clock

Doc ID 14204 Rev 2

17/22

5

Using an external clock

Using an external clock source to drive SPEAr3xx is accomplished by using a compatible

clock generator that meets the necessary jitter and waveform specifications, and configuring

the internal clock for an external source.

The external clock signal is applied to oscillator MCLK_XI pin. This is a digital signal with a

nominal voltage swing between 0 V and 2.5 V. If a clock source with a different voltage swing

is used, then a voltage translator must be used. A 3.3 V signal should not be applied to the

MCLK_XI pin.

5.0.1

External clock signal requirements

The clock signal specifications are listed in the table below.

5.0.2

Voltage translation of external clock

If the external clock's output has a signal swing of other than 2.5 V, then some form of level

translation must be used on the signal before it is applied to MCLK_XI. An IC level translator

can be used, or a resistor voltage divider network can be used if the source has a 3.3 V

swing. The level translator must provide a stable, monotonic, and clean waveform to the

clock input.

The resistor network must account for several conflicting requirements. First, the correct

resistor ration must be used to provide the correct levels, and second, because the

connection between the clock source and SPEAr3xx is a trace on a PCB, which is a

transmission line and therefore exhibits transmission line behavior, the resistor values used

in the divider must adequately control reflections. The goal is to provide a clean waveform at

MCLK_XI with minimal overshoot or ringing. Figure 7 depicts the general network

configuration.

Table 5.

External clock signal requirements for oscillator MCLK_XI input

Symbol

Description

Min

Typ

Max

Units

Oscillator power supply voltage

2.5

V

1.

restricted than digital inputs.

V

Low input voltage (1)

0.2

V

Rise time of clock input signal

3

10

ns

Fall time of clock input signal

3

10

ns

Frequency of external source

30.000

MHz