參數(shù)資料
型號(hào)： MQ80C52EXXX-12P883
廠商： ATMEL CORP
元件分類： 微控制器/微處理器
英文描述： 8-BIT, MROM, 12 MHz, MICROCONTROLLER, CQFP44
封裝： CERAMIC, QFP-44
文件頁(yè)數(shù)： 41/80頁(yè)
文件大?。?/td> 5152K
代理商： MQ80C52EXXX-12P883
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.
相關(guān)PDF資料
PDF描述
MD80C52CXXX-16SCD 8-BIT, MROM, 16 MHz, MICROCONTROLLER, CDIP40
MQ80C52TXXX-25SCR 8-BIT, MROM, 25 MHz, MICROCONTROLLER, CQFP44
MQ80C52CXXX-30/883D 8-BIT, MROM, 30 MHz, MICROCONTROLLER, CQFP44
MR83C154XXX-25P883 8-BIT, MROM, 25 MHz, MICROCONTROLLER, CQCC44
MS80C51-12D 8-BIT, MROM, 12 MHz, MICROCONTROLLER, PQCC44
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
MQ82370-20 制造商:Rochester Electronics LLC 功能描述:- Bulk
MQ8238020 制造商:Intel 功能描述:CONTROLLER: OTHER
MQ82380-20 制造商:Rochester Electronics LLC 功能描述:- Bulk
MQ82380-20/R 制造商:Rochester Electronics LLC 功能描述:
MQ82592 制造商:Rochester Electronics LLC 功能描述:- Bulk
發(fā)布緊急采購(gòu)，3分鐘左右您將得到回復(fù)。

采購(gòu)需求

(若只采購(gòu)一條型號(hào)，填寫一行即可)

發(fā)布成功！您可以繼續(xù)發(fā)布采購(gòu)。也可以進(jìn)入我的后臺(tái)，查看報(bào)價(jià)

發(fā)布成功！您可以繼續(xù)發(fā)布采購(gòu)。也可以進(jìn)入我的后臺(tái)，查看報(bào)價(jià)

*型號(hào) *數(shù)量 廠商 批號(hào) 封裝
添加更多采購(gòu)

我的聯(lián)系方式

*
*
*