MCP2122 Microchip Technology, MCP2122 Datasheet - Page 9

• Manufacturer Part Number: MCP2122
• Description: Converts serial data from UART bitstreams to IrDA standard bitstreams (encodes) and from IrDA standard bitstreams to UART bitstreams ...
• 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 once 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 (determined by
pulse-width limiter circuit).
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
57600
115200
Baud
Rate
Note 1: The pulse-width limiter on the TXIR pin
2: This TXIR pulse width time is a design
3: This time (determined by the pulse-width
19.53 µs
3xT
9.77 µs
4.88 µs
3.26 µs
1.63 µs
Circuit
saves system current for this baud rate.
target and is not tested. Actual times may
be greater than, or less than, this value.
limiter circuit) is device dependent.
16
XCLK
TXIR HIGH PULSE WIDTH
(1)
(1)
(1)
TXIR Pulse Width
Pulse-width
16XCLK Pulse
Limiter
4.00 µs
4.00 µs
4.00 µs
4.00 µs
4.00 µs
Circuit
MCP2122
(2)
DS21894B-page 9
Actual Pulse
4.00 µs
4.00 µs
4.00 µs
3.26 µs
1.63 µs
Width
(3)
(3)
(3)