Energy-efficient MAC for Wireless Sensor Networks

Abstract

A compact blind oversampling data-recovery circuit was implemented by using a coarse data-recovery block and an add-drop first-in first out, and it was successfully applied to the universal serial bus (USB)2.0 high-speed packet data transmission. The proposed circuit recovered the serial input data by selecting the sampled data among the 5X oversampled data of a single data bit. It reduced the number of transistors by more than half and the lock time to zero, compared to the conventional blind oversampling data-recovery circuit using the de-multiplexing scheme for multibit-data. The proposed circuit was implemented with a 0.18- m CMOS process. It worked at the data rates ranging from 180 to 720 Mbps. The bit-error rate was measured to be less than 10 -12 with the 215- 1 PRBS data transmitted though a 5-m USB cable. The chip area and the measured power consumption for 480-Mbps operation were 0.185mm2 and 8.2mW, respectively.

INTRODUCTION

In the serial data transmission, the data recovery circuit is required because only the data signal is transmitted without the clock signal to reduce the number of signal wires. The blind oversampling data-recovery circuit is preferred in the case of universal serial bus (USB), which requires a fast lock time. Since this blind oversampling scheme is a full digital scheme, it is easier to port into a different process technology than analog scheme and its output jitter is relatively insensitive to the input data jitter. Also, its lock time is short (less than ten data periods) because of its feedforward structure. However, it requires a relatively large size first-in first-out (FIFO) and a large peripheral circuitry, because it needs to store a large size oversampled data in FIFO to locate the accurate sampling time point and process data in parallel in byte units.[1]–[3] Recently, a hybrid type clock and data recovery (CDR) circuit was proposed[4] to reduce the FIFO size by combining the analog tracking circuits into the blind oversampling CDR.

In this paper, a single-data-bit 5X blind oversampling data-recovery circuit is proposed. The proposed circuit consists of a 5X oversampler, a coarse data-recovery block, and an add-drop FIFO (AD-FIFO) block. A coarse data-recovery block recovers the serial input data by selecting the sampled data among the oversampled data of a single-data-bit. This coarse recovered data may have a missing or duplicate data, which is caused by the input data jitter or the frequency offset between transmitter and receiver. An AD-FIFO block corrects a missing or duplicate data by adding or removing a data to and from the FIFO data queue. Compared to the conventional blind oversampling data-recovery circuit using a de-multiplexing scheme, the proposed circuit reduces the size of FIFO and the peripheral circuitry, and also the lock time. This proposed circuit was successfully applied to the packet transmission of the USB2.0 high-speed interface (480 Mbps).

In Section II, the architecture of the proposed data-recovery circuit is compared to that of the conventional blind oversampling data-recovery circuit. In Section III, the operation of coarse data-recovery block is shown. The structure and the operation of AD-FIFO are presented in Section IV. In section V, the measurement results are presented, followed by the conclusion in Section VI.

ARCHITECTURE

A. Conventional Blind Over sampling Data-Recovery Circuit

The blind oversampling technique was used as a timing recovery technique in the universal asynchronous receiver/ - transmitter (UART). Recently, this technique was adopted to the serial link.[2] In this technique, the serial input data are oversampled using the multiphase clocks, and the recovered data are generated by using the de-multiplexing scheme.

Fig. 1(a) shows an application of this technique to the USB receiver. The serial input data are 3X oversampled by the 24 multiphase clocks. The 24-bit oversampled data are aligned to the receiver clock and transferred to both FIFO1 and the edge detector at the same time. The edge detector finds the transition of input data by XOR-ing the two sampled data adjacent in time. The voter accumulates the number of data transitions during each time interval between the multiphase clocks and determines one of the multiphase clocks to be the sampling phase by using the majority voting scheme. It takes several data periods for the voter to determine the sampling phase and the sampled data. During this time interval, all the oversampled data must be stored in FIFO1.

The frequency offset between transmitter and receiver may induce errors in the recovered data. To compensate for errors, the bit/byte synchronizer sends out 7, 8, or 9 bits in parallel to FIFO2 and FIFO2 packs the incident data into 8- bit units and sends out the packed 8-bit data as the recovered data. The conventional blind oversampling scheme requires a relatively large circuit because of the de-multiplexing architecture of converting the 1-bit serial input data into the 8-bit parallel recovered data. The data-recovery circuit of Fig. 1(a) has a lock time of several data periods because of the insufficient accumulation of data transition information at the beginning stage of data-recovery process.

B. Proposed Blind Oversampling Data-Recovery Circuit

Fig. 1(b) shows the proposed data-recovery circuit, which uses the five multiphase clocks with the same nominal frequency (f s ) as the serial input data rate. The coarse data-recovery block recovers a coarse data bit from the 5-bit over sampled data which corresponds to a single data bit. Because of this arrangement, FIFO1 of Figure 1 is removed and the logic circuitry to determine the sampling phase is simplified in the proposed circuit. With the frequency offset between transmitter and receiver or the input jitter, the coarse data-recovery circuit may generate some missing or duplicated data, which are corrected by the following AD-FIFO. AD-FIFO is a modified FIFO, where a data bit can be dropped from or inserted into the FIFO queue while it follows the FIFO sequence.

COARSEDATA-RECOVERY BLOCK

A. Block Diagram

The block diagram of the coarse data-recovery block is shown in Figure 2.(a)

It accepts the 5-bit data (D [4:0]) from the 5X sampler as input, and generates the 1-bit coarse recovered data (CRD) and the Add/Drop signals as output. The transition detector finds the time interval of input data transition out of the five time intervals between the adjacent multiphase clocks, by using the current 5-bit oversampled data (D [4:0] ) and the previous 1-bit oversampled data (D’[0]). It sends out the 5-bit flag data to both the phase selector and the error detector. The phase selector determines the sampling phase from the 5-bit flag data. It maintains the previous sampling phase if the 5-bit flag data is all zero, which indicates no transition in the input data. The 5-to-1 multiplexer (MUX) selects the 1-bit data corresponding to the sampling phase determined by the phase selector, among the 5 oversampled data input. It sends out the selected sampled data as the (CRD). The 1-bit delay block compensates for the time delay of the transition detector to synchronize the oversampled data to the selected sampling phase at the MUX. The error detector compares the current and previous values of the 5-bit flag data and generates Add/Drop the signals.

B.Center Picking Method Within a Single Sampling Window

Fig. 2(b) shows the center picking method within a single sampling window. This method is used by the coarse datarecovery block of this work to determine the sampling phase and the sampled data, depending on the transition position of input data (Case1-5). The sampling window (SW) is defined to be a time period starting from one rising edge to the following rising edge of the P[0] clock which is one of the 5 multiphase clocks. The coarse data-recovery block accepts the 5-bit oversampled data of a single sampling window, and finds the transition position of input data. Then, it determines the sampling phase and the sampled data within the same sampling window. For example, in the Case1, where the input data changes in between the rising edges of P[0] and P[4], the following P[2] phase in the same sampling window is selected as the sampling phase because the center point (max. data eye point) of input data is located around the rising edge of P[2] clock. Therefore, this scheme is called the center picking method within a single sampling window. Since this scheme requires the data transition information only within a single sampling window, it reduces the hardware by removing the FIFO1 and the voting logic of the conventional scheme.

Fig. 2(c) shows the timing diagram of the coarse data-recovery block. If the data transition position always belongs to one of the three cases (Case1-3) continuously in time, the third phase after the data transition (P[2],P[1],P[0]) is chosen as the sampling phase. If the data transition position always belongs to one of the two cases (Case4-5) continuously in time, the third phase before the data transition (P[4],P[3]) is chosen as the sampling phase. These two groups of cases ensure the normal data recovery without any errors.

C. Correction of Missing Or Duplicate Data

If there is a frequency offset between transmitter and receiver, the input data transition position moves in the forward or back ward direction monotonically with time in a sampling window. Eventually it will hit the sampling window boundary. In this case, the missing or duplicate data will occur in the CRD, and the coarse data-recovery block activates the Add/Drop signals, which are sent to the following AD-FIFO block, as shown in Fig.1(b)

When the transmitter frequency is higher than the receiver frequency, the duplicate data will appear in the CRD. In Fig.3(a), the input data transition belongs to the Case3 during the sampling window SW1 and the Case4 during the following sampling window SW2. During SW1, the third phase (P[0]) after the data transition is selected as the sampling phase and Data2 is recovered as CRD. During SW2, the third phase (P [4]) before the data transition is selected as the sampling phase, the same Data2 as in SW1 is recovered as CRD.When the receiver frequency is higher than the transmitter frequency, the missing data will occur in the CRD. Fig.3(b) shows one of these cases. The input data jitter may also induce the missing or duplicate data in CRD.

To resolve the problem of missing or duplicate data, the error detector within the coarse data-recovery block activates the Add or Drop signal, synchronized to CRD, as shown in Fig. 3(c). The Add/Drop signals are activated only during a single clock cycle after the particular change of the selected sampling phase. When there is no transition in the input data, the Add/Drop signals are not activated because the selected sampling phase retains the previous value. The AD-FIFO block receives the CRD and Add/Drop signals, and recovers the correct data by adding the missing bit or removing the duplicated bit in the data queue.

AD-FIFO BLOCK

A.Operation Of AD-FIFO Block

The AD-FIFO block performs the basic FIFO operation. However, it can add or remove a single bit to or from the data queue by the control of the Add or Drop signal. Fig.4(a) shows the block diagram of the AD-FIFO block with 2N + 1AD FIFO cells. The number of AD-FIFO cells is determined by the given maximum packet size and the frequency offset between transmitter and receiver. The maximum packet size is given by the interface standard, for example, it is 8255 bits in the USB standard. The CRD and Add/Drop signals of an AD-FIFO cell are shifted to the next right ADFIFO cell at every rising edge of clock. Fig.4(b) shows the block diagram of an AD-FIFO cell. Each cell has a tri-state buffer at output and all the output nodes (CO) of all the AD-FIFO cells are connected to a single node RD, which is the output node of the AD-FIFO block.

Only the tri-state buffer of a single AD-FIFO cell is enabled among those of the 2N + 1 cell by using a token signal (Token[i]). Only the token signal of a selected cell is set to 1 with those of all the other 2N cells set to 0. An AD-FIFO cell passes the token signal to the left or to the right by activating the Pass Token to the Left (PTL) or Pass Token to the Right (PTR) signals. When there is a missing bit, the token signal is passed to the right. When there is a duplicate bit, the token signal is passed to the left. Fig.4(c) shows the token control circuit of the ith AD-FIFO cell. The upper two D F/Fs form the shift registers to synchronize the Add and Drop signals to the CRD signal. The lower two D F/Fs form a one-stage pipe-line to maximize the operation clock frequency by minimizing the time delay of the Token[i] and PTR[i] signals from the clock rising edge.

The one-clock latency caused by the one-stage pipeline is compensated for by using the prediction logic with the preceding cell output as input.

Fig.5(a) shows the state transition diagram of the entire AD-FIFO block. The State (i) represents the state where the token signal of the ith cell (Token[i]) is set to 1 with those of all the other cells set to 0. If the AD-FIFO block is in the State (i) and there is a duplicated bit, the Drop [i-1] signal is activated to 1. Then the AD-FIFO block change to the State (i-1) at the next rising edge of clock, and RD (output of the AD-FIFO block) is set to CRD [i-1] output of the (i-1) th cell. In this way, the token moves to the left adjacent cell in one clock period, and the duplicated bit is eliminated in the recovered data (RD).

If there is a missing bit, the Add [i-1] signal is activated to 1. Then, the AD-FIFO block changes to the State (i+1/2) at the next rising edge of clock, and RD is set to! CRD[i] (the inverted value of the th cell output). At the following rising edge of clock, the AD-FIFO block changes to the State (i+1). In this way, the token moves to the right adjacent cell in two clock periods, and the missing bit (! CRD[i]) is added in RD at the State (i+1/2). After that, the normal data recovery is continued with RD connected to the output of the (i+1)th cell. The! CRD[i] is added to RD for the missing bit, because the missing bit CRD[i] (DATA2 of Figure 7) is the inverted value of the following data [DATA3 of Figure 3.4 This is because the missing bit occurs after the change of the selected sampling phase and the selected sampling phase changes with the input data transition, the missing bit always occurs just after the input data transition

In Fig.5(b), the operation of the AD-FIFO block is presented in the form of timing diagram. Assume that the ADFIFO block is initially in the State (i), where the ith cell is selected by the token signal to be the output cell which sends out CRD[i] as RD. When there is no Add or Drop signal activated, as shown for the first four data (D0 – D3) of Fig.5(b), the AD-FIFO block remains in the State (i) and the ith cell sends out CRD[i] as RD at the next rising edge of clock. When there is an Add signal activated (AD4), the AD-FIFO block changes to the State (i+1/2) at the next rising edge of clock and the ith cell sends out! D4 as RD. After that, the AD-FIFO block changes to the State (i+1) at the next rising edge of clock and sends out D4 as RD. This case corresponds to the transition of the selected sampling phase from Case4 to Case3, as shown in Fig.3(b).

When there is a Drop signal (DR7) activated, the AD-FIFO block changes to the State(i) at the next rising edge of clock and it skips D7, which is the data signal synchronized to the Drop signal (DR7), This case corresponds to the transition from Case3 to Case4, as shown in Fig.3(a). The successive occurrence of Add, Drop and Add signals (AD9, DR10, AD11) indicates the worst case of input data jitter, where the selected sampling phase changes from Case4, to Case3 and back to Case4. Even in this case, the correct data sequence of! D9, D9,! D11, D11 appears in RD, as desired.

CONCLUSION

A single-data-bit blind oversampling data-recovery circuit is proposed. It consists of a 5X oversampler, a coarse data-recovery block, and an AD-FIFO. The coarse data-recovery block operates on a single sampling window, which consists of 5 oversampled data corresponding to a single-data bit. It generates a coarse recovered data. The AD-FIFO corrects any missing or duplicated data in the coarse recovered data, and generates the final recovered data. The coarse data-recovery block reduced the number of transistors by more than half and the lock time to zero, compared to the conventional blind oversampling data recovery using the de-multiplexing scheme for 8-bit-data. The AD-FIFO performs the correct data recovery even with the frequency offset between transmitter and receiver or the input data jitter, by adding or removing a data bit to and from the data queue.

References

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