H838532H datasheet(115/508 Pages) RENESAS

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Section 4 Clock Pulse Generators

Rev. 1.00 May 30, 2008 Page 95 of 484

REJ09B0436-0100

The oscillation stabilization time in the case of these state transitions is the same as the oscillation

stabilization time at power-on (the time from the point at which the power supply voltage reaches

the prescribed level until the oscillation stabilizes), specified by "oscillation stabilization time trc"

in the AC characteristics.

Meanwhile, once the system clock has halted, a wait time of at least 8 states is necessary in order

for the CPU and peripheral functions to operate normally.

Thus, the time required from interrupt generation until operation of the CPU and peripheral

functions is the sum of the above described oscillation stabilization time and wait time. This total

time is called the oscillation stabilization wait time, and is expressed by equation (1) below.

Oscillation stabilization wait time = oscillation stabilization time + wait time

= t

+ (8 to 131,072 states)

................. (1)

Therefore, when a transition is made from standby mode, watch mode, or subactive mode, to

active (high-speed/medium-speed) mode, with an oscillator element connected to the system clock

oscillator, careful evaluation must be carried out on the installation circuit before deciding on the

oscillation stabilization wait time. In particular, since the oscillation stabilization time is affected

by installation circuit constants, stray capacitance, and so forth, suitable constants should be

determined in consultation with the oscillator element manufacturer.

4.5.2

Notes on Use of Crystal Oscillator Element (Excluding Ceramic Oscillator

Element)

When a microcomputer operates, the internal power supply potential fluctuates slightly in

synchronization with the system clock. Depending on the individual crystal oscillator element

characteristics, the oscillation waveform amplitude may not be sufficiently large immediately after

the oscillation stabilization wait time, making the oscillation waveform susceptible to influence by

fluctuations in the power supply potential. In this state, the oscillation waveform may be disrupted,

leading to an unstable system clock and erroneous operation of the microcomputer.

If erroneous operation occurs, change the setting of standby timer select bits 2 to 0 (STS2 to

STS0) (bits 6 to 4 in system control register 1 (SYSCR1)) to give a longer wait time.

For example, if erroneous operation occurs with a wait time setting of 16 states, check the

operation with a wait time setting of 8,192 states or more.

If the same kind of erroneous operation occurs after a reset as after a state transition, hold the

RES

pin low for a longer period.

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