NSAM265SR Datasheet, PDF (19/58 Pages) National Semiconductor (TI) – CompactSPEECH Digital Speech Processors

2 0 Functional Description (Continued)
Crystal Oscillator
A crystal resonator is connected to the on-chip oscillator
circuit via the X1 and X2 signals as shown in Figure 2-11
TL EE 12378 – 14
FIGURE 2-11 Connections for an
External Crystal Oscillator
Stray capacitance and inductance should be kept as low as
possible in the oscillator circuit The crystal and the external
components should be as close to the X1 PLI and
X2 CLKIN pins as possible to keep the trace lengths in the
printed circuit as to an absolute minimum
Crystals with maximum load capacitance of 20 pF may be
used although the oscillation frequency may differ from the
crystal’s specified value
Table 2-1 defines the components in the crystal oscillator
circuit
2 13 POWER-DOWN MODE
Power-down mode is useful during a power failure when the
power source for the CompactSPEECH is a backup battery
or in battery powered devices while the CompactSPEECH
is idle Note that there is no need to battery backup the
AFLASH devices in the NSAM265SF
In power-down mode the clock frequency of the Compact-
SPEECH is reduced and some of the processor modules
are deactivated As a result the CompactSPEECH con-
sumes much less power than in normal-power mode The
CompactSPEECH does not perform its usual functions in
power-down mode but it still preserves stored messages
and maintains the time of day
The NSAM265SF stores messages and all memory man-
agement information in FLASH memory Thus there is no
need to maintain the power to the processor to preserve
stored messages If the microcontroller’s real-time clock
(and not the NSAM265SF’s real-time clock) is used to main-
tain the time and day neither the FLASH nor the
NSAM265SF require battery backup during power failure In
this case when returning to normal mode the microcontrol-
ler should perform the initialization sequence as described
in Section 1 10 and use the SETD command to set the time
and day
To keep power consumption low in power-down mode the
RESET MWCS MWCLK and MWDIN signals should be
held above V_CC b 0 5V or below V_SS a 0 5V
The PDM (Go To Power-down Mode) command switches
the CompactSPEECH to power-down mode It may only be
issued when the CompactSPEECH is in the IDLE state If it
is necessary to switch to power-down mode from any other
state the controller must first issue an S command to switch
the CompactSPEECH to the IDLE state and then issue the
PDM command Sending any command while in power-
down mode will return the CompactSPEECH to normal op-
eration mode
2 14 POWER AND GROUNDING
The CompactSPEECH processor requires a single 5V pow-
er supply applied on the V_CC pins
The grounding connections are made on the GND pins
For optimal noise immunity the power and ground pins
should be connected to V_CC and ground planes respective-
ly on the printed circuit board If V_CC and ground planes are
not used single conductors should be run directly from
each V_CC pin to a power point and from each GND pin to a
ground point Avoid daisy-chained connections
Use decoupling capacitors to keep the noise level to a mini-
mum Standard 0 1 mF ceramic capacitors can be used for
this purpose They should attach to V_CC GND pins as close
as possible to the CompactSPEECH
During prototype using wire-wrap or similar methods the
capacitors should be soldered directly to the power pins of
the CompactSPEECH socket or as close as possible with
very short leads
Design Notes
When constructing a board using high frequency clocks with
multiple line switching special care should be taken to avoid
resonances on signal lines A separate power and ground
layer is recommended Switching times of under 5 ns are
possible on some lines Resonant frequencies should be
maintained well above the 200 MHz frequency range on
signal paths by keeping traces short and inductance low
Loading capacitance at the end of a transmission line con-
tributes to the resonant frequency and should be minimized
if possible Capacitors should be located as close as possi-
ble across each power and ground pair near the Compact-
SPEECH
Component
Crystal Oscillator
Resistor R1
Capacitor C1
L (Inductance)
TABLE 2-1 Components of a Crystal Oscillator Circuit
Parameters
Values
Resonance Frequency
Third Overtone
Type
Maximum Serial Resistance
Maximum Shunt Capacitance
Maximum Load Capacitance
40 96 MHz
Parallel
AT-Cut
50X
7 pF
12 pF
10 MX
1000 pF
3 9 mH
Tolerance
NA
5%
20%
10%
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