Datasheet

ADSP-BF538/ADSP-BF538F
Rev. E | Page 49 of 60 | November 2013
TEST CONDITIONS
All timing parameters appearing in this data sheet were mea-
sured under the conditions described in this section. Figure 43
shows the measurement point for ac measurements (except out-
put enable/disable). The measurement point V
MEAS
is 1.5 V for
V
DDEXT
(nominal) = 3.0 V/3.3 V.
Output Enable Time Measurement
Output pins are considered to be enabled when they have made
a transition from a high impedance state to the point when they
start driving.
The output enable time t
ENA
is the interval from the point when
a reference signal reaches a high or low voltage level to the point
when the output starts driving as shown on the right side of Fig-
ure 44, “Output Enable/Disable,” on page 49.
The time t
ENA_MEASURED
is the interval from when the reference
signal switches to when the output voltage reaches V
TRIP
(high)
or V
TRIP
(low). V
TRIP
(high) is 2.0 V and V
TRIP
(low) is 1.0 V for
V
DDEXT
(nominal) = 3.0 V/3.3 V. Time t
TRIP
is the interval from
when the output starts driving to when the output reaches the
V
TRIP
(high) or V
TRIP
(low) trip voltage.
Time t
ENA
is calculated as shown in the equation:
If multiple pins (such as the data bus) are enabled, the measure-
ment value is that of the first pin to start driving.
Output Disable Time Measurement
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 output disable time t
DIS
is the
difference between t
DIS_MEASURED
and t
DECAY
as shown on the left
side of Figure 44.
The time for the voltage on the bus to decay by V is dependent
on the capacitive load C
L
and the load current I
L
. This decay
time can be approximated by the equation:
The time t
DECAY
is calculated with test loads C
L
and I
L
, and with
V equal to 0.5 V for V
DDEXT
(nominal) = 3.0 V/3.3 V.
The time t
DIS_MEASURED
is the interval from when the reference
signal switches, to when the output voltage decays V from the
measured output high or output low voltage.
Figure 41. Drive Current E (Low V
DDEXT
)
Figure 42. Drive Current E (High V
DDEXT
)
Figure 43. Voltage Reference Levels for AC
Measurements (Except Output Enable/Disable)
- 40
SO
U
R
C
E
C
U
R
R
E
N
T
(
m
A)
SOURCE VOLTAGE (V)
0 0.5 1.0 1.5 2.0 2.5 3.0
- 60
0
-
10
V
OL
- 20
- 30
-
50
V
DDEXT
=2.75V
- 40
S
O
U
RC
E
C
U
R
R
E
N
T
(m
A
)
SOURCE VOLTAGE (V)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0
- 10
-
20
- 80
-
70
V
OL
4.0
- 60
- 50
- 30
V
DDEXT
=3.0V
V
DDEXT
=3.3V
V
DDEXT
=3.6V
INPUT
OR
OUTPUT
V
MEAS
V
MEAS
Figure 44. Output Enable/Disable
t
ENA
t
ENA_MEASURED
t
TRIP
–=
t
DIS
t
DIS_MEASURED
t
DECAY
–=
t
DECAY
C
L
VI
L
=
REFERENCE
SIGNAL
t
DIS
OUTPUT STARTS DRIVING
V
OH
(MEASURED) ⴚ ⌬V
V
OL
(MEASURED) + ⌬V
t
DIS_MEASURED
V
OH
(MEASURED)
V
OL
(MEASURED)
V
TRIP
(HIGH)
V
OH
(MEASURED
)
V
OL
(MEASURED)
HIGH IMPEDANCE STATE
OUTPUT STOPS DRIVING
t
ENA
t
DECAY
t
ENA_MEASURED
t
TRIP
V
TRIP
(LOW)