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ADSP-2185

–16–

REV. 0

CAPACITIVE LOADING

Figures 9 and 10 show the capacitive loading characteristics of

the ADSP-2185.

CL – pF

300

100

150

200

250

T = +85

°C

VDD = 4.5V

Figure 9. Typical Output Rise Time vs. Load Capacitance,

CL (at Maximum Ambient Operating Temperature)

CL – pF

100

150

250

200

–2

NOMINAL

–4

–6

Figure 10. Typical Output Valid Delay or Hold vs. Load

Capacitance, CL (at Maximum Ambient Operating

Temperature)

TEST CONDITIONS

Output Disable Time

Output pins are considered to be disabled when they have

stopped driving and started a transition from the measured

output high or low voltage to a high impedance state. The out-

put disable time (tDIS) is the difference of tMEASURED and tDECAY,

as shown in the Output Enable/Disable diagram. The time is the

interval from when a reference signal reaches a high or low

voltage level to when the output voltages have changed by 0.5 V

from the measured output high or low voltage. The decay time,

tDECAY, is dependent on the capacitive load, CL, and the current

load, iL, on the output pin. It can be approximated by the fol-

lowing equation:

tDECAY =

CL × 0.5V

from which

tDIS = tMEASURED – tDECAY

is calculated. If multiple pins (such as the data bus) are dis-

abled, the measurement value is that of the last pin to stop

driving.

1.5V

2.0V

1.5V

0.3V

INPUT

OUTPUT

Figure 11. Voltage Reference Levels for AC Measure-

ments (Except Output Enable/Disable)

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 (tENA) 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.

2.0V

1.0V

tENA

REFERENCE

SIGNAL

OUTPUT

tDECAY

VOH

(MEASURED)

OUTPUT STOPS

DRIVING

OUTPUT STARTS

DRIVING

tDIS

tMEASURED

VOL

(MEASURED)

VOH (MEASURED) – 0.5V

VOL (MEASURED) +0.5V

HIGH-IMPEDANCE STATE. TEST CONDITIONS CAUSE

THIS VOLTAGE LEVEL TO BE APPROXIMATELY 1.5V.

VOH

(MEASURED)

VOL

(MEASURED)

Figure 12. Output Enable/Disable

OUTPUT

50pF

+1.5V

IOH

IOL

Figure 13. Equivalent Device Loading for AC Measure-

ments (Including All Fixtures)

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