ADSP-21065L_15 Datasheet, PDF (36/44 Pages) Analog Devices – DSP Microcomputer

ADSP-21065L
OUTPUT DRIVE CURRENT
80
60
3.6V, –40؇C
V_OH
3.1V, +85؇C
40
3.3V, +25؇C
20
3.1V, +100؇C
0
–20
3.1V, +100؇C
–40
3.3V, +25؇C _{3.6V, –40؇C}
–60
–80
–100
3.1V, +85؇C
V_OL
–120
0
0.50 1.00 1.50 2.00 2.50 3.00 3.50
SOURCE VOLTAGE – V
Figure 24. Typical Drive Currents
TEST CONDITIONS
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 DV is dependent on the capacitive load, C_L and
the load current, I_L. This decay time can be approximated by
the following equation:
t_DECAY = ^{CL ¥ DV}
I_L
The output disable time t_DIS is the difference between t_MEASURED
and t_DECAY as shown in Figure 26. The time t_MEASURED is the
interval from when the reference signal switches to when the
output voltage decays DV from the measured output high or
output low voltage. t_DECAY is calculated with test loads C_L and
I_L, and with DV equal to 0.5 V.
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
driving. The output enable time t_ENA is the interval from when a
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. If multiple 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_DECAY using the equation given above. Choose DV
to be the difference between the ADSP-21065L’s output voltage
and the input threshold for the device requiring the hold time. A
typical DV will be 0.4 V. C_L is the total bus capacitance (per
data line), and I_L is the total leakage or three-state current (per
data line). The hold time will be t_DECAY plus the minimum
disable time (i.e., t_DATRWH for the write cycle).
REFERENCE
SIGNAL
t _DIS
V_{OH (MEASURED)}
OUTPUT
V_{OL (MEASURED)}
t _MEASURED
t _ENA
V_{OH (MEASURED)} – ⌬V
V_{OL (MEASURED)} + ⌬V
t _DECAY
2.0V
1.0V
V_{OH (MEASURED)}
V_{OL (MEASURED)}
OUTPUT STOPS
DRIVING
OUTPUT STARTS
DRIVING
HIGH-IMPEDANCE STATE.
TEST CONDITIONS CAUSE
THIS VOLTAGE TO BE
APPROXIMATELY 1.5V
Figure 25. Output Enable
I_OL
TO
OUTPUT
PIN
50pF
+1.5V
I_OH
Figure 26. Equivalent Device Loading for AC Measure-
ments (Includes All Fixtures)
INPUT OR
OUTPUT
1.5V
1.5V
Figure 27. Voltage Reference Levels for AC Measure-
ments (Except Output Enable/Disable)
–36–
REV. C