adsp-21990 Analog Devices, Inc., adsp-21990 Datasheet - Page 41

• Manufacturer Part Number: adsp-21990
• Description: Mixed-signal Dsp Controller
• Manufacturer: Analog Devices, Inc.

TEST CONDITIONS
The DSP is tested for output enable, disable, and hold time.
Output Disable Time
Output pins are considered to be disabled when they stop driv-
ing, go into a high impedance state, and start to decay from their
output high or low voltage. The time for the voltage on the bus
to decay by 'V is dependent on the capacitive load, C
load current, I
lowing equation.
The output disable time t
and t
val from when the reference signal switches to when the output
voltage decays 'V from the measured output high or output low
voltage. The t
with 'V equal to 0.5 V.
Figure 20. Equivalent Device Loading for AC Measurements (Includes All
DECAY
REFERENCE
V
V
OH (MEASURED)
OL (MEASURED)
as shown in
SIGNAL
OUTPUT
DECAY
L
. This decay time can be approximated by the fol-
PIN
TO
t
Figure 19. Output Enable/Disable
DIS
is calculated with test loads C
OUTPUT STOPS
50pF
t
DECAY
Figure
DRIVING
DIS
is the difference between t
t
TEST CONDITIONS CAUSE THIS VOLTAGE
Fixtures)
MEASURED
19. The time t
V
V
=
t
DECAY
OH (MEASURED)
OL (MEASURED)
TO BE APPROXIMATELY 1.5V
C L V
-------------- -
I
HIGH IMPEDANCE STATE.
I
OL
OH
I L
'
– V 2.0V
+ V 1.0V
MEASURED
OUTPUT STARTS
1.5V
L
t
ENA
DRIVING
and I
Rev. A | Page 41 of 50 | August 2007
is the inter-
L
, and the
MEASURED
L
, and
Output Enable Time
Output pins are considered to be enabled when they have made
a transition from a high impedance state to when they start driv-
ing. The output enable time t
reference signal reaches a high or low voltage level to when the
output has reached a specified high or low trip point, as shown
in the Output Enable/Disable diagram
pins (such as the data bus) are enabled, the measurement value
is that of the first pin to start driving.
Example System Hold Time Calculation
To determine the data output hold time in a particular system,
first calculate t
on Page
ADSP-21990 output voltage and the input threshold for the
device requiring the hold time. A typical 'V will be 0.4 V. C
the total bus capacitance (per data line), and I
age or three-state current (per data line). The hold time will be
t
write cycle).
DECAY
Figure 21. Voltage Reference Levels for AC Measurements (Except Output
plus the minimum disable time (i.e., t
41. Choose 'V to be the difference between the
OUTPUT
INPUT
OR
DECAY
using the equation at
1.5V
Enable/Disable)
ENA
is the interval from when a
(Figure
1.5V
Output Disable Time
ADSP-21990
DATRWH
L
19). If multiple
is the total leak-
for the
L
is