TB073

TB073
Selecting an MCP21XX Device for IrDA® Applications
Author:
Table 1 compares some of the key features of the
MCP2120 and the MCP2122, while Table 2 compares
some of the key features of the MCP2140, MCP2150
and the MCP2155.
Mark Palmer
Microchip Technology Inc.
INTRODUCTION
WORKING THROUGH THE DECISION
TREE
With a growing number of device options within the
MCP21XX family, this Technical Brief serves as a
guide in helping to select the device for your
application.
Figure 1 and Figure 2 provide a decision tree to assist
in selecting the MCP21XX device for your application.
Figure 1 helps to determine if the device should have
the IrCOMM application layer protocol handler with
encoder/decoder implemented (MCP2140, MCP2150,
and MCP2155) or if only the encoder/decoder device
(MCP2120 and MCP2122) is needed. Figure 2 helps to
determine which of the encoder/decoder devices to
use. Other factors that could influence your device
selection decision are also discussed.
All of the MCP21XX devices interface to the Host
Controller with a UART. The MCP2120 and MCP2122
devices use only the TX and RX signals. The
MCP215X and MCP2140 devices also use the TX and
RX signals, as well as up to 6 other non-data signals.
For additional information on this Host UART interface
for the MCP215X (also valid for the MCP2140), please
refer to Application Note 858, “Interfacing The
MCP215X to a Host Controller“, DS00858.
TABLE 1:
Device
ENCODER/DECODER DEVICES
MCP2120 2,400 - 312,500
14
Crystal
MCP2122 2,400 - 115,200
8
16XCLK
Note 1:
TABLE 2:
IDD
(Max)
IDD Low
Power (Max)
7 mA (1)
8 µA
-40°C to Use when the board does not have
+85°C
a 16x clock source
2 mA
4 µA
-40°C to Pinout compatible with HSDL-7000
+125°C
#
Clock
Pins Source
Baud Rate
Temp.
Range
Comment
IDD was measured at 3.0V and 10 MHz. IDD will increase for a higher voltage.
IRCOMM PROTOCOL HANDLER DEVICES(1)
Baud Rate
Device
Host UART
IR
IDD
(Max)
IDD Low
Power
(Max)
2.2 mA
60 µA
Yes (3)
DCE
Easily interfaces to a modem’s
serial port
IR Wake- Host UART
Up-onFlow
Receive
Control
Comment
MCP2140
9,600
9,600
MCP2150
9,600 —
115,200 (2)
9,600 —
115,200 (2)
7 mA
9 µA
No
DTE
Easily interfaces to a PC’s
serial port
MCP2155
9,600 —
115,200 (2)
9,600 —
115,200 (2)
7 mA
9 µA
No
DCE
Easily interfaces to a modem’s
serial port
Note 1:
2:
3:
Supports the 9-wire “cooked” service class of the IrCOMM Application Layer Protocol.
The Host UART baud rate and the IR baud rate operate independent of each other.
The MCP2140 will automatically enter Low-power mode once no IR activity has been detected for approximately 10 seconds. Once an IR pulse is detected, the MCP2140 will return to normal operation. Even
though the MCP2140 low-power current is higher than the MCP215X low-power current, the MCP215X
devices will typically spend a lot more of their time in the normal operating mode, which would mean that
the average current would be higher.
 2004 Microchip Technology Inc.
DS91073B-page 1
TB073
IrCOMM Application Layer Protocol or
Encoder/Decoder Determination
The first thing to determine is if the application should
have one of the devices with the IrCOMM application
layer protocol handler with encoder/decoder
implemented or if only the encoder/decoder device is
needed. Thus, the first question that needs to be asked
is:
“Does your application require support
for the IrDA® standard protocol?”
The application will use an IrDA standard protocol if the
purpose is to communicate with commonly available IR
devices, such as a notebook PC (with IR ports) or PDA
(Palm® and PocketPC devices). The MCP215X and
MCP2140 devices implement the IrCOMM protocol.
If the application does not require the IrDA standard
protocol, it should be determined whether a simple
UART to IR encoder/decoder (the MCP2120 or the
MCP2122) would work for the application. When both
ends of the application are owned by the hardware
developer, a simple protocol could be implemented in
each embedded systems firmware to address data
integrity and the communication link.
If the application is expected to interface to a standard
device (PCs or PDAs with IR ports), the following
question is asked:
“Can the IrDA standard protocol be
implemented in the Host Controller?”
Some embedded systems have a powerful Host
Controller that has the performance and available
throughput to implement both the desired functionality
and the IrDA standard protocol stack. In this case, the
embedded system only requires an encoder/decoder
device (the MCP2120 or MCP2122).
If the Host Controller is already fully utilized, does not
have the processing capabilities to implement the IrDA
standard protocol in firmware or if there is no expertise
to implement the firmware IrDA standard protocol, a
dedicated device to perform that functionality is
required, such as the MCP215X or MCP2140.
To determine which of these devices to choose, the
following question is asked:
“Is the design new or existing?”
If the IrDA interface is added to a new design, any of
the three devices may be appropriate. However, the
data throughput requirement of the application needs
to be evaluated. For applications where the communication rate may need to be greater than 9600 baud, the
MCP215X devices should be used since the Host
UART and IR interfaces can operate up to
115.2 Kbaud. The MCP2140 is fixed at 9.6 Kbaud for
both the Host UART and IR interface.
DS91073B-page 2
When deciding between the MCP2150 and the
MCP2155, the MCP2150 would typically be selected
since it has a signal to indicate when the device has
completed initialization. This status information is
useful in the design debug stage.
If the IrDA interface is added to an existing design, the
following question is asked:
“Does the existing design use the UART
like a PC (DTE) or a modem (DCE)?”
With an existing application, the selection of the device
will be determined by how the existing application uses
its UART control signals. The MCP215X and MCP2140
devices require flow control, so the existing application
must use the CTS signal to determine when data can
be transmitted from the Host Controller to the
MCP215X or MCP2140 device.
If the existing application uses its UART in the same
configuration as a PC (DTE), the MCP2150 is the
recommended choice.
If the existing application uses its UART in the same
configuration as a modem (DCE), the MCP2155 or
MCP2140 may be used, depending on the data
throughput requirements of the application.
For DCE applications where communication rate may
need to be greater than 9600 baud, the MCP2155
device should be used since the Host UART and IR
interfaces can operate up to 115.2 Kbaud. The
MCP2140 is fixed at 9.6 Kbaud for both the Host UART
and IR interface.
“Does the IrCOMM protocol application
require very low power consumption?”
In some IrCOMM protocol applications, such as
battery-powered, low current consumption is one of the
top requirements (even higher than the communication
(baud) rate). In such applications, the MCP2140 should
be the perferred device due to it’s automatic entry into
Low-power mode and it’s wake-up on IR detect feature.
This allows it to have the lowest average current
consumption, typically.
While the MCP215X devices’ Low-power mode has a
lower current, the Host Controller must use an I/O pin
to control the MCP215X operating mode (Normal or
Low-power mode). In a typical application, the Host
Controller needs to ensure that the MCP215X is in the
normal operating mode at regular intervals to look for
an IR Primary device.
The MCP2140 can remain in the low-power state until
the Primary device polls for devices. The MCP2140 will
automatically enter the Low-power mode approximately 10 seconds after IR activity has stopped. It will
remain in the Low-power mode until either IR activity is
detected or the device exits the reset state.
 2004 Microchip Technology Inc.
TB073
Determining which encoder/decoder
device to use
Once it has been determined that the system will use
one of the encoder/decoder devices, the decision tree
in Figure 2 will aid in selecting the specific device. The
first question that needs to be asked is:
“Does the baud rate need to be
controlled by software or can it be fixed
by hardware?”
Some systems like the ability to change the baud rate
of the encoder/decoder by sending commands during
operation. This type of system requires the MCP2120.
If the application is expected to have additional system
clocks, the following question is asked:
“Does the design have an available 16x
clock for the UART signals?”
Depending on the implementation of the application
design, a 16x clock may be available in the system.
The frequency of this clock is 16 times the frequency of
the desired baud rate. The MCP2122 requires a 16x
clock, while the MCP2120 requires that the clock (or
crystal) have a frequency of at least 64 times the
desired baud rate.
The Host Controller may have resources available to
generate this 16x clock. The PICmicro® MCU’s CPP
and Timer2 modules can be used to create a PWM
output of the desired frequency with a 50% duty cycle.
Another question to ask is:
“Will this device be replacing the
HSDL-7000?”
The MCP2122 is an 8-pin device that has the same
pinout as the HSDL-7000. The MCP2122 8-pin SOIC
package is smaller than the HSDL-7000 package and
can be easily placed in an existing HSDL-7000
application circuit board to validate functional
operation.
Table 3 shows a comparison of some key features
between the MCP2122 and the HSDL-7000. The
HSDL-7000 does not specify the supply current, but
does specify power dissipation. The MCP2122’s power
dissipation has been calculated based upon the supply
current specification.
TABLE 3:
MCP2122 VS. HSDL-7000
COMPARISON
Device
Feature
MCP2122
Supply Current 2 mA (max)
Power
Dissipation
Low-power
Mode Current
If there is not a 16x clock available, then the MCP2120
must be used. So the next question should be asked.
“What is the required baud rate for the
system?”
The IrDA standard SIR specification has a maximum
baud rate of 115.2 Kbaud. If both ends of the system
are controlled by the developer, this specification can
be ignored. The MCP2120 allows a maximum baud
rate of 312.5 Kbaud (although some work may need to
be done with the optical transceiver), while the
MCP2122 has a maximum of 115.2 Kbaud.
—
300 µA (typ)
—
11 mW (max) (1)
220 mW (max)
540 µW 1.65 mW (typ) (2)
4.9 mW (typ)
4 µA
(maximum, in
Reset) (3)
No
7.2 µW - 22 µW (4) —
Supply Voltage 1.8V to 5.5V
2.7V to 5.5V
Package Size
154 x 193 mils
220 x 260 mils
Temperature
Range
-40°C to +125°C
-40°C to +85°C
Note 1:
2:
3:
4:
 2004 Microchip Technology Inc.
HSDL-7000
Calculated from maximum supply current
times maximum voltage (2 mA * 5.5V)
Calculated from typical supply current
times minimum and maximum voltage
(300 µA * 1.8V to 300 µA * 5.5V)
Lowest power consumption mode when
device RESET pin is forced low.
Calculated from Low-power mode current
times minimum and maximum voltage
(4 µA * 1.8V to 4 µA * 5.5V)
DS91073B-page 3
TB073
If this is a new design, another question to ask is:
SUMMARY
“Is there a package size or supply current
requirement?”
The MCP21XX family of IR devices allows a customer
flexibility in partitioning a system to address application
requirements including interface, data throughput and
cost. This Technical Brief should assist you in selecting
the MCP21XX device for your application.
The MCP2122 is an 8-pin device that has a SOIC
package, while the MCP2120 is a 14-pin device also
available in a SOIC package. The MCP2122 also
requires less power (refer to Table 1). If board space or
supply current are at a premium, the MCP2122 may be
the better choice.
FIGURE 1:
MCP21XX SELECTION DECISION TREE
Start
Do You
Need an IrDA®
Standard
Protocol Stack?
Yes
Does Host Have
H/W and Firmware
Resources for
IrDA Standard
Protocol Stack?
No
Yes
Expertise to
Implement
IrDA Standard
Protocol Stack
in Host
Firmware?
Use MCP212X
Use MCP212X
No
Is Design
New or
Existing?
PC (DTE)
Use MCP2150
No
New
Does the
Yes
System Require
the Lowest
Power?
Existing
Does
Design
Use UART Like
a PC or
Modem?
Yes
Use MCP2140
Modem (DCE)
No
Does the
Yes
System Require
the Lowest
Power?
What Is
the Required
Baud Rate?
Use MCP2140
No
What Is
the Required
Baud Rate?
≤ 115200
Use MCP2150
9600
Use MCP2140
≤ 115200
Use MCP2155
9600
Use MCP2140
DS91073B-page 4
 2004 Microchip Technology Inc.
TB073
FIGURE 2:
MCP212X SELECTION DECISION TREE
Start
MCP212X
Baud Rate
Controlled by
Hardware (H/W)
or Software (S/W)?
S/W
Use MCP2120
H/W
Is 16x
Clock
Available?
Y
What Is
the Required
Baud Rate?
N
Can
Host Controller
Generate
Baud Clock?
N
> 115200
Use MCP2120
Y
≤ 115200
HSDL-7000
Replacement?
Use MCP2120
Y
Use MCP2122
N
Need
Smallest
Package,
Lowest
Current?
Y
Use MCP2122
N
Use either the MCP2120
or the MCP2122
 2004 Microchip Technology Inc.
DS91073B-page 5
TB073
APPENDIX A:
REVISION HISTORY
Revision B:
• Updated to include MCP2122 device
• Enhanced information for MCP2120, MCP2140,
MCP2150 and MCP2155 devices.
Revision A:
Initial Release
DS91073B-page 6
 2004 Microchip Technology Inc.
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DS91073B-page 7
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DS91073B-page 8
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