Datasheet

Rev. E | Page 50 of 60 | November 2013
ADSP-BF538/ADSP-BF538F
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 V
to be the difference between the ADSP-BF538/ADSP-BF538F
processor’s output voltage and the input threshold for the
device requiring the hold time. 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 various out-
put disable times as specified in the Timing Specifications on
Page 28 (for example, t
DSDAT
for an SDRAM write cycle as
shown in Table 27 on Page 33).
Capacitive Loading
Output delays and holds are based on standard capacitive loads:
30 pF on all pins (see Figure 45). V
LOAD
is 1.5 V for V
DDEXT
(nominal) = 3.0 V/3.3 V. Figure 46 through Figure 55 on
Page 52 show how output rise and fall times vary with capaci-
tance. The delay and hold specifications given should be derated
by a factor derived from these figures. The graphs in these fig-
ures may not be linear outside the ranges shown.
Figure 45. Equivalent Device Loading for AC Measurements
(Includes All Fixtures)
T1
ZO = 50Ω (impedance)
TD = 4.04 ± 1.18 ns
2pF
TESTER PIN ELECTRONICS
50Ω
0.5pF
70Ω
400Ω
45Ω
4pF
NOTES:
THE WORST CASE TRANSMISSION LINE DELAY IS SHOWN AND CAN BE USED
FOR THE OUTPUT TIMING ANALYSIS TO REFELECT THE TRANSMISSION LINE
EFFECT AND MUST BE CONSIDERED. THE TRANSMISSION LINE (TD) IS FOR
LOAD ONLY AND DOES NOT AFFECT THE DATA SHEET TIMING SPECIFICATIONS.
ANALOG DEVICES RECOMMENDS USING THE IBIS MODEL TIMING FOR A GIVEN
SYSTEM REQUIREMENT. IF NECESSARY, A SYSTEM MAY INCORPORATE
EXTERNAL DRIVERS TO COMPENSATE FOR ANY TIMING DIFFERENCES.
V
LOAD
DUT
OUTPUT
50Ω
Figure 46. Typical Rise and Fall Times (10% to 90%) vs. Load Capacitance for
Driver A at V
DDEXT
= 2.7 V (Min)
Figure 47. Typical Rise and Fall Times (10% to 90%) vs. Load Capacitance for
Driver A at V
DDEXT
= 3.6 V (Max)
LOAD CAPACITANCE (pF)
RISE TIME
RISE AND FALL TIME ns (10%
to 90%)
14
12
10
8
6
4
2
0
0 50 100 150 200 250
FALL TIME
LOAD CAPACITANCE (pF)
RISE TIME
RISE AND FALL TIME ns (10% to
90%)
12
10
8
6
4
2
0
0 50 100 150 200 250
FALL TIME