MCP2122-E/SNG Microchip Technology, MCP2122-E/SNG Datasheet - Page 9

• Manufacturer Part Number: MCP2122-E/SNG
• Description: Infrared Encoder/Decoder
• Manufacturer: Microchip Technology

2.5.3
Table 2-4 shows the TXIR pin high-time at different
common baud rates. The internal TXIR pulse width
high-time limiter is a feature that minimizes the system
current consumption at lower baud rates. The IrDA
standard specification requires that optical receiver cir-
cuitry detect pulses as narrow as 1.41 µs (1.63 µs is
the typical time at 115200 baud). Therefore, the time
that the TXIR pin is high after this valid detection is
additional current that is driven by the emitter LED. The
MCP2122 will force the TXIR pin low after the pulse
width limiter has timed out. Figure 2-9 shows the
MCP2122 16XCLK, TX and TXIR waveforms at
115200 baud for a single TX low bit. In this case the
TXIR is high for three 16XCLK pulses. In Figure 2-10
the MCP2122 is at 9600 baud for a single TX low bit. In
this case the TXIR is high for 3.55 µs.
FIGURE 2-9:
 2004 Microchip Technology Inc.
ENCODING AND SCREEN
CAPTURES
TXIR
MCP2122 AT 115200 BAUD WAVEFORM
A B
Jitter of the TX input relative to the 16XCLK and TXIR
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Preliminary
TABLE 2-4:
9600
19200
38400
46875
57600
115200
Baud
Rate
Note 1: The pulse width limiter on the TXIR pin
2: This TXIR pulse width time is a design
3xT
19.53 µs
9.77 µs
4.88 µs
4.00 µs
3.26 µs
1.63 µs
saves system current for this baud rate.
target and is not tested. Actual times may
be greater than or less than this value.
TXIR Pulse Width
16XCLK
TXIR HIGH PULSE WIDTH
Limiter
16XCLK Pulse
4.00 µs
4.00 µs
4.00 µs
4.00 µs
4.00 µs
4.00 µs
Pulse
Width
MCP2122
(2)
Note 1
Note 1
Note 1
Target Crossover
Point
DS21894A-page 9
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