ADSP-BF531/ADSP-BF532/ADSP-BF533
The time tDECAY is calculated with test loads CL and IL, and with
V equal to 0.1 V for VDDEXT (nominal) = 1.8 V or 0.5 V for
TEST CONDITIONS
All timing parameters appearing in this data sheet were mea-
sured under the conditions described in this section. Figure 44
shows the measurement point for ac measurements (except out-
put enable/disable). The measurement point VMEAS is 0.95 V for
V
DDEXT (nominal) = 2.5 V/3.3 V.
The time tDIS_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.
V
DDEXT (nominal) = 1.8 V or 1.5 V for VDDEXT (nominal) = 2.5 V/
3.3 V.
REFERENCE
SIGNAL
INPUT
OR
OUTPUT
V
V
MEAS
MEAS
tDIS_MEASURED
tENA_MEASURED
tDIS
tENA
V
OH
V
(MEASURED)
Figure 44. Voltage Reference Levels for AC
Measurements (Except Output Enable/Disable)
OH
(MEASURED)
V
(MEASURED) ؊ ⌬V
(MEASURED) + ⌬V
OH
V
(HIGH)
TRIP
V
(LOW)
V
V
TRIP
OL
V
OL
(MEASURED)
OL
(MEASURED)
Output Enable Time Measurement
tDECAY
tTRIP
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 tENA 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
Figure 45.
OUTPUT STOPS DRIVING
OUTPUT STARTS DRIVING
HIGH IMPEDANCE STATE
Figure 45. Output Enable/Disable
Example System Hold Time Calculation
The time tENA_MEASURED is the interval, from when the reference
signal switches, to when the output voltage reaches VTRIP(high)
or VTRIP (low).
For VDDEXT (nominal) = 1.8 V—VTRIP (high) is 1.3 V and VTRIP
(low) is 0.7 V.
To determine the data output hold time in a particular system,
first calculate tDECAY using the equation given above. Choose V
to be the difference between the processor’s output voltage and
the input threshold for the device requiring the hold time. CLis
the total bus capacitance (per data line), and IL is the total leak-
age or three-state current (per data line). The hold time is tDECAY
plus the various output disable times as specified in the Timing
Specifications on Page 28 (for example tDSDAT for an SDRAM
write cycle as shown in SDRAM Interface Timing on Page 31).
For VDDEXT (nominal) = 2.5 V/3.3 V—VTRIP (high) is 2.0 V and
VTRIP (low) is 1.0 V.
Time tTRIP is the interval from when the output starts driving to
when the output reaches the VTRIP (high) or VTRIP (low) trip
voltage.
Time tENA is calculated as shown in the equation:
tENA = tENA_MEASURED – tTRIP
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 tDIS is the
difference between tDIS_MEASURED and tDECAY as shown on the left
side of Figure 44.
tDIS = tDIS_MEASURED – tDECAY
The time for the voltage on the bus to decay by V is dependent
on the capacitive load CL and the load current II. This decay time
can be approximated by the equation:
tDECAY = CLV IL
Rev. H
| Page 46 of 64 | January 2011
ROCHESTER [ Rochester Electronics ]
