PIC18F4450-I/PT【PDF 159/241 页】IC PIC MCU FLASH 8KX16 44TQFP

收藏本站

您好，
买卖IC网欢迎您。
请登录
免费注册

买卖IC网

Sheet目录3854

155页 156页 157页 158页 [159页]160页 161页 162页 163页

241

XMEGA A [MANUAL]

8077I–AVR–11/2012

The recommendations for the maximum receiver baud rate error assume that the receiver and transmitter equally divide

the maximum total error.

21.9

Fractional Baud Rate Generation

Fractional baud rate generation is possible for asynchronous operation due to the relatively high number of clock cycles

for each frame. Each bit is sampled sixteen times, but only the three middle samples are of importance. The total number

of samples for one frame is also relatively high. Given a 1-start, 8-data, no-parity, and 1-stop-bit frame format, and

assuming that normal speed mode is used, the total number of samples for a frame is (1+8+1)×16 or 160. As stated

earlier, the UART can tolerate some variation in clock cycles for each sample. The critical factor is the time from the

falling edge of the start bit (i.e., the clock synchronization) until the last bit's (i.e., the first stop bit’s) value is recovered.

Standard baud rate generators have the unwanted property of having large frequency steps between high baud rate

settings. The worst case is found between the BSEL values 0x000 and 0x001. Going from a BSEL value of 0x000, which

has a 10-bit frame of 160 clock cycles, to a BSEL value of 0x001, with 320 clock cycles, gives a 50% change in

frequency. Ideally, the step size should be small even between the fastest baud rates. This is where the advantage of the

fractional baud rate generator emerges.

In principle, the fractional baud rate generator works by doing uneven counting and then distributing the error evenly over

the entire frame. A typical count sequence for an ordinary baud rate generator is:

2, 1, 0, 2, 1, 0, 2, 1, 0, 2, …

which has an even period time. A baud rate clock ticks each time the counter reaches zero, and a sample of the signal

received on RxD is taken for every 16th baud rate clock tick.

For the fractional baud rate generator, the count sequence can have an uneven period:

2, 1, 0, 2, 1-1, 0, 2, 1, 0, 2, 1-1, 0, ...

In this example, an extra cycle is added to every second baud clock. This gives a baud rate clock tick jitter, but the

average period has been increased by a fraction of 0.5 clock cycles.

Figure 21-9 on page 242 shows an example of how BSEL and BSCALE can be used to achieve baud rates in between

what is possible by just changing BSEL.

The impact of fractional baud rate generation is that the step size between baud rate settings has been reduced. Given a

scale factor of -1, the worst-case step then becomes from 160 to 240 clock cycles per 10-bit frame, compared to the

previous step of from 160 to 320. A higher negative scale factor gives even finer granularity. There is a limit however, to

how high the scale factor can be. The value 2|BSCALE| must be at most half the minimum number of clock cycles of a

frame. For instance, for 10-bit frames, the minimum number of clock cycles is 160. This means that the highest

applicable scale factor is -6 (2I-6I = 64 < (160/2) = 80).

For higher BSEL settings, the scale factor can be increased.

Table 21-5 on page 242 shows BSEL and BSCALE settings when using the internal oscillators to generate the most

commonly used baud rates for asynchronous operation and how reducing the BSCALE can be used to reduce the baud

rate error even further.

发布紧急采购，3分钟左右您将得到回复。

采购需求

(若只采购一条型号，填写一行即可)

*型号	*数量	厂商	批号	封装

添加更多采购

采购需求

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

我的联系方式