EE-103: Performing Level Conversion Between 5v and 3.3v IC's PDF

Engineer To Engineer Note
EE-103
Technical Notes on using Analog Devices’ DSP components and development tools
Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: [email protected], FTP: ftp.analog.com, WEB: www.analog.com/dsp
Performing Level Conversion
Between 5v and 3.3v ICs
Q: I am currently using the ADSP-2187L in my
system and I want to interface a 5v FPGA to
the DSP. However, I have concerns about
potential interface issues caused by differing
logic standards. Do you have any information
on how to interface ICs that operate at different
voltage/logic levels?
A: There are a number of processors produced by
Analog Devices that operate at 3.3v. The 3.3v
fixed point processors include the ADSP-2183,
ADSP-2184L, ADSP-2185L, ADSP-2186L and
ADSP-2187L. Our 3.3v floating point processors
are the ADSP-21060L, ADSP-21061L, ADSP21062L, and the ADSP-21065L. Additionally, all
of our “M” family processors are capable of
supporting 3.3v I/O standards. However, none of
these devices can tolerate greater than Vcc+0.5v on
their inputs. Therefore, additional logic is necessary
to connect our 3.3v and “M” family DSPs to 5v
peripherals. One simple way of solving this
problem is by adding a Bus Switch (QuickSwitch)
to the system, which adds voltage tolerance
between two ICs.
Bus Switches are essentially bus voltage level
translators that are used to shunt the maximum
voltage that is received by an IC. A typical transfer
function of a Bus Switch is given below (Figure 1):
V out
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5
4
3
2
1
0
0
1.65
3.3
5
Vin
Figure 1: Voltage I/O Characteristic of a Bus Switch
(Vcc = 4.3v)
The transfer function of a Bus Switch is as follows:
Vout = Vin when Vin<Vcc-1
Vout = Vcc-1 when Vin>Vcc-1
(1)
(2)
In this example, with Vcc set to 4.3v, the Bus Switch
will pass Vin to Vout until Vin equals 3.3v. Once Vin
rises higher than 3.3v, Vout continues to hold at
3.3v. Therefore, placing a Bus Switch between a
5v and 3.3v device will effectively remove voltage
incompatibility issues.
Bus Switches are bi-directional ICs; therefore, no
additional routing logic is needed for the connection
of bi-directional buses. When the 3.3v DSP
transmits information to the 5v device, its output
voltage is not shunted or reduced by the Bus
Switch.
A simple way of providing 4.3v to the Bus Switch is
to place a diode between the 5v system voltage and
the Vcc pin on the Bus Switch. The voltage drop
across the diode will force Vcc to approximately
4.3v.
a
Once concern when adding glue logic to a system is
the potential added propagation delay. Bus
Switches have very low propagation delay, on the
order of 0.25ns. Therefore, they can safely be used
in a wide range of timing sensitive applications and
systems.
Bus Switches are manufactured by a number of
semiconductor companies, such as Pericom and
Quality Semiconductor. For more information on
the operation of these devices, please refer to the
following resources:
www.pericom.com
www.qualitysemi.com
Bus Switches typically have a part number of 3384.
Therefore, searching each companies’ web site for
this part number brings up datasheets and
application notes fully describing their proper usage.
For more information on the electrical
characteristics of the DSPs manufactured by
Analog Devices, please refer to the appropriate
DSP data sheet.
EE-103
Page 2
Technical Notes on using Analog Devices’ DSP components and development tools
Phone: (800) ANALOG-D, FAX: (781) 461-3010, EMAIL: [email protected], FTP: ftp.analog.com, WEB: www.analog.com/dsp
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