Datasheet

368
7682C–AUTO–04/08
AT90CAN32/64/128
27.4 Maximum Speed vs. V
CC
Maximum frequency is depending on V
CC
. As shown in Figure 27-2., the Maximum Frequency
vs. V
CC
curve is linear between 1.8V < V
CC
< 4.5V. To calculate the maximum frequency at a
given voltage in this interval, use this equation:
To calculate required voltage for a given frequency, use this equation:
At 3 Volt, this gives:
Thus, when V
CC
= 3V, maximum frequency will be 9.33 MHz.
At 8 MHz this gives:
Thus, a maximum frequency of 8 MHz requires V
CC
= 2.7V.
Table 27-1. External Clock Drive
Symbol Parameter
V
CC
= 2.7 - 5.5V V
CC
= 4.5 - 5.5V
Units
Min. Max. Min. Max.
1/t
CLCL
Oscillator Frequency 0 8 0 16 MHz
t
CLCL
Clock Period 125 62.5 ns
t
CHCX
High Time 50 25 ns
t
CLCX
Low Time 50 25 ns
t
CLCH
Rise Time 1.6 0.5 µs
t
CHCL
Fall Time 1.6 0.5 µs
∆t
CLCL
Change in period from one clock cycle
to the next
2 2 %
Table 27-2. Constants used to calculate maximum speed vs. V
CC
Voltage and Frequency range a b Vx Fy
2.7 < VCC < 4.5 or 8 < Frequency < 16 8/1.8 1.8/8 2.7 8
Frequency a V Vx–( ) Fy+•=
Voltage b F Fy–( ) Vx+•=
Frequency
8
1.8
-------- 3 2.7–( ) 8+• 9.33= =
Voltage
1.8
8
-------- 8 8–( ) 2.7+• 2.7= =