ATmega325P datasheet(178/364 Pages) ATMEL

178 / 364 page

178

8023F–AVR–07/09

ATmega325P/3250P

larger time variation when using the Double Speed mode (U2Xn = 1) of operation. Samples

denoted zero are samples done when the RxD line is idle (i.e., no communication activity).

Figure 18-6. Start Bit Sampling

When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the

start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in

the figure. The clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and sam-

ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the

figure), to decide if a valid start bit is received. If two or more of these three samples have logical

high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts

looking for the next high to low-transition. If however, a valid start bit is detected, the clock recov-

ery logic is synchronized and the data recovery can begin. The synchronization process is

repeated for each start bit.

18.8.2

Asynchronous Data Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data

recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight

states for each bit in Double Speed mode. Figure 18-7 shows the sampling of the data bits and

the parity bit. Each of the samples is given a number that is equal to the state of the recovery

unit.

Figure 18-7. Sampling of Data and Parity Bit

The decision of the logic level of the received bit is taken by doing a majority voting of the logic

value to the three samples in the center of the received bit. The center samples are emphasized

on the figure by having the sample number inside boxes. The majority voting process is done as

follows: If two or all three samples have high levels, the received bit is registered to be a logic 1.

If two or all three samples have low levels, the received bit is registered to be a logic 0. This

majority voting process acts as a low pass filter for the incoming signal on the RxD pin. The

recovery process is then repeated until a complete frame is received. Including the first stop bit.

Note that the Receiver only uses the first stop bit of a frame.

Figure 18-8 shows the sampling of the stop bit and the earliest possible beginning of the start bit

of the next frame.

START

IDLE

BIT 0

123

678

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

BIT n

123

678

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

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