MCP2155 datasheet(9/38 Pages) MICROCHIP

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Preliminary

DS21894C-page 9

MCP2122

2.5.3

ENCODING AND SCREEN

CAPTURES

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).

TABLE 2-4:

TXIR HIGH PULSE WIDTH

FIGURE 2-9:

MCP2122 AT 115200 BAUD WAVEFORM

Baud

Rate

TXIR Pulse Width

3xT16XCLK

Circuit

Pulse-width

Limiter (2)

Circuit

Actual Pulse

Width

9600

19.53 µs (1)

4.00 µs

4.00 µs (3)

19200

9.77 µs (1)

4.00 µs

4.00 µs (3)

38400

4.88 µs (1)

4.00 µs

4.00 µs (3)

57600

3.26 µs

4.00 µs

3.26 µs

115200

1.63 µs

4.00 µs

1.63 µs

Note 1: The pulse-width limiter on the TXIR pin

saves system current for this baud rate.

2: This TXIR pulse width time is a design

target and is not tested. Actual times may

be greater than, or less than, this value.

3: This time (determined by the pulse-width

limiter circuit) is device dependent.

1 2 3 4 5 678 9 10 11 12 13 14 15 16

Jitter of the TX input relative to the 16XCLK and TXIR

16XCLK Pulse

TXIR

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