Hardware Reference
In-Depth Information
The datasheet for this part from various manufacturers shows the
V
IL
and
V
IH
levels
when
V
DD
= +5
V
(and higher). The values shown for
V
OH
for each
V
DD
are all listed at a
value of
V
DD
− 0
.
5
V.
Extrapolating from that, I have assumed
V
OH
= 3
.
3 - 0
.
5 = 2
.
8
V
in the
following table. The
V
OL
is listed as 0
.
05
V
for all
V
DD
values listed, so we'll assume the
same for 3
.
3
V
.
The following table compares the Raspberry Pi GPIO logic levels with those of the
CD4013 chip operating at +3.3 V.
Raspberry Pi, GPIO
CD4013,
V
DD
= +3.3
V
Parameter
Volts
Volts
Parameter
V
IL
£ 0
.
8
V
£0
.
05
V
V
OL
V
IH
³ 1
.
3
V
³ 2
.
8
V
†
V
OH
†Derived from a National Semiconductor datasheet
From Figure
8-1
, recall that we are using the flip-flop output Q to drive a GPIO input.
The flip-flop's
V
OL
is much lower than the maximum value for
V
IL
, so that works well.
Additionally, from the table, notice that the
V
OH
level of the CD4013 output is well above
the minimum required for
V
IH
for the GPIO input as well. From this signal comparison,
we can conclude that the CD4013 part should play very nice with the Pi when powered
from 3.3 V.
■
Unused CmoS
inputs
should not be left unconnected. If an unused input has
no contribution to your design, ground it. If you must have the input in a high state, wire
it directly to the +3.3 V supply. no limiting resistor is required, since a CmoS input draws
no current. likewise, do not omit
R
1
and
R
2
, shown in Figure
8-1
. Unused CmoS
outputs
,
however, can be left unconnected.
Caution
Unused CMOS inputs should not be left to float. In the presented flip-flop circuit, the
following unused pins will be grounded:
Pin
Function
Wired to
Notes
3
Clock 1
Ground
Not used
5
Data 1
Ground
Not used
8
Set 2
Ground
If FF2 not used
9
Data 2
Ground
Not used
10
Reset 2
Ground
If FF2 not used
11
Clock 2
Ground
Not used
