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Ultra Low Power RC Oscillator for System Wake-Up Using Highly Ultra Low Power RC Oscillator for System wake-up using highly precise Auto-Calibration Technique Joonhyung, Lim#1, Kwangmook, Lee#2, Koonsik, Cho#3 # Ubiquitous Conversion Team, Samsung Electro-Mechanics, Suwon, Gyunggi-Do, Korea, 443-743 [email protected], [email protected], [email protected] Abstract— An ultra low power RC oscillator for system wake-up A. The design of RC oscillator is implemented using 0.18um CMOS process. The modern mobile systems need system clock which consumes low power and thus saves limited battery power in order to wake up from sleep-mode. A RC oscillator operates in the subthreshold region to reduce current consumption. The output frequency of RC oscillator is very weakly dependent on process and temperature variation using auto-calibration. This RC oscillator is featured as follows: the current consumption is 0.2 ㎂; the supply voltage is 1.8V; the output frequency is 31.25 KHz with 1.52(Relative 3σ)% accuracy after calibration; it has only 0.4%/℃ temperature coefficient; its size is 190 um x 80 um exclude bonding pad. I. INTRODUCTION Many applications, such as PDA, mobile communication devices need to operate with low power consumption and low supply voltage to fulfil the requirement of long-term operation. For such System-on-Chips (SoCs), embedded a system wake up circuit is necessary [1]. Several analogue and digital blocks such as clock generator, timer and SRAM for retention must be activated to wake system up from a sleep-mode. Especially, a clock generator, which is the heart of digital circuits, will output unknown waveform of clock and the system will enter Fig. 1 The proposed RC oscillator into an unknown condition. Largely, there are two types of clock generator. One is crystal oscillator, the other is RC oscillator for clock generation. Advantages of the first type The proposed ultra low power RC oscillator is shown in Fig. oscillator are lower jitter noise, more accurate frequency and 1. This circuit includes four blocks: start up, current reference, frequency operation than the second one. Disadvantage is charge/discharge sensing and clock generation part. For low finite start up time, higher cost and more current consumption power operation, it may be desirable to operate a current than the second, the second type of oscillator has fast start up source/sink in the weak inversion, or subthreshold region. The time, lower cost and less current consumption including easy current flows when VGS<VTHN is called the subthreshold. on-chip integration, but its disadvantage is high jitter noise, When the MOSFET is operating in the region, it is said to be low accurate frequency and low frequency operation [2]. in the weak inversion region. This current is due mainly to In this paper, we propose an extremely low power RC diffusion current between the drain and the source, similar to oscillator for waking up SoC from a sleep-mode that includes the bipolar junction transistor (BJT). The total current IDS is highly precise auto-calibration function to overcome low the sum of the weak inversion component and the strong accurate frequency against process, temperature and supply inversion component. The start-up circuit prevents all self- voltage variation. This circuit has features of low current biased circuit to operate zero bias point. The possibility that consumption, small silicon area, and frequency accuracy. the current, I, may be zero exist in all self-biased circuits. Fig. Thus, it is suitable to be embedded in a battery operate low 2 illustrates this graphically. Point B in this figure is the power SoC in the sleep mode. desired operating point, while point A corresponds to I=0. The circuit description and simulation results are shown in Once the self-bias circuit is operating at point B, the start-up section. Section III and IV presents experimental results and circuit does not affect the reference operation. conclusions, respectively. II. THE PROPOSED RC OSCILLATOR 978-1-4244-6664-1/10/$26.00 ©2010 IEEE 274 β IVV=−1 ()2 DGSTHN112 Δ = × VC (5) TD 2RC VR V I = GS 2 D2 R we can get output frequency of RC oscillator, that is: 1 V Freq. = = R (6) Δ TD 2RC VC Fig. 2 Two possible operating points of self-biased circuit The cascade connection of current mirrors is used to increase the output resistance of a current source or sink. This generated current is mirrored by the current mirror and fed to capacitor, hysteresis controller M1 and current-starved invertors for clock generation. The drain current I5 charges capacitor and when Vc is equal to Vth of M1, the hysteresis controller M1 will turn on. This mechanism is described in the Fig. 3. For the clock generator part, with the assumption of Fig.3 Waveform above Capacitor and RC oscillator frequency constant current into the capacitor, the voltage of the capacitor, Vc, will increase in proportional to current(I5). We designed trimming resistor and capacitor array to That is: generate precise output frequency using auto-calibration. The simulation results of RC oscillator are shown in Fig. 4. I ×Δt The output frequency is 31.25 KHz and current consumption Δ=V (1) C C is 0.19 ㎂. Meanwhile the voltage of the capacitor, ∆ Vc, is also controlled by the logical threshold voltage of the hysteresis control transistor M1. The charging and discharging time can be controlled by I5 and M1. At last a triangular voltage wave form as in Fig.2 will be generated for the capacitor and the period time for one cycle of charging and discharging will be: ×Δ CVC Tt− =Δ = (2) Ramp up, down I For current generator part, a constant current source is built using current mirror. A feedback circuit always stabilizes the Fig. 4 RC oscillator output plots at time and frequency domain voltage above the resistor denoted as VR. So the resistance will decide the current, that is: V B. Precise Auto-Calibration I = R (3) R Fig.5 shows the interface between RC oscillator and the other blocks: 1) Auto-calibration engine generates frequency Replacing the current expression into the equation (2), we control bits, 2) Sleep timer calculates the sleep time of main get(equ.4,5): system using RC oscillator’s clock. The RC oscillator operates with initial output frequency by CV×Δ Δ V (4) default R/C trim bits. The control bits of R/C are decided by Tt=Δ =CC = RC × Ramp− up, own V digital auto-calibration engine block which counts RC R VR R 275 oscillator clock by reference clock. It compares the reference Firstly, the coarse tuning is performed. If the value of count is value of count with value of count until two values are equal. smaller than 420, calibration engine increases resistor trim bits After calibration is done, R/C trim bits will be stored to by 2xLSB to quickly generate the desired frequency. If it is retention RAM and voltage regulator be turned off to save less than 499 and more than 421, it increases resistor trim bits power in sleep-mode. by LSB. Secondly, it carries out the first and second fine calibration by sweeping capacitor trim bits. The second fine calibration is fulfilled for generating precise frequency. Its procedure is as follows. After calibration engine divides the frequency of RC oscillator by N(4~8) in order to increase the FREQ_Fine_C_TRIM<5:0> FREQ_R_TRIM<3:0> FREQ_C_TRIM<2:0> resolution of counting frequency, the fine tuning is performed by varying capacitance until it is equal to the wanted value of count. The oscillator’s frequency is drifted about 1 KHz per LBS (resistor trim bit), 100 Hz per LBS (first capacitor trim bit) and 10 Hz per LBS (second capacitor trim bit). This auto- calibration can make precise frequency of RC oscillator. III. EXPERIMENTAL RESULTS A photograph of the RC oscillator is shown in the Fig. 7. Its Fig. 5 Auto-calibration interface of RC oscillator size is 190 um x 80 um exclude bonding pad. Fig. 6 exhibits the flow chart of auto-calibration in detail. Firstly, auto-calibration is started, and then RC and crystal oscillator is activated. RC oscillator operates with some output frequency. Reference clock counts the duty cycle of RC oscillator for generating the control bits. The reference value of count is 512 (16 MHz/31.25 KHz =512) Fig. 7 A photograph of RC oscillator’ The experimental results in this paper were obtained from an in plastic molded package measurement. Fig. 8 shows the measured output frequency of RC oscillator. Its frequency is 31.24 KHz after calibration. A RC oscillator consumes 0.2 ㎂ with supply voltage 1.8V. FIG. 6 AUTO-CALIBRATION FLOW CHART The auto-calibration engine for the precise output of Fig.8 The measured output frequency of RC oscillator frequency adopts the coarse and fine frequency tuning method. 276 A calibration time is less than 5ms. A temperature The authors would like to thank Kwangmook Lee, Boyle coefficient is 0.4%/℃ over a temperature range from -45℃ to Seo, for their composing test software programs. 80℃. A Supply voltage coefficient is 5%/V after auto- calibration. The measured results of 200 sample chip is a REFERENCES mean value of the oscillation frequency of 31.25 KHz and standard deviation of 0.16KHz, leading to a 3σ of about [1] Hung-wei Chen, “A Low Power and Fast Wake up Circuit”, IEE Conference, pp.223~228, Vol. 152, Issue3, June 2005 1.52%, which is excellent for many applications. [2] Shane A., “Quick Start Crystal Oscillator Circuit”, Micrieletronics The performance of the proposed oscillator is compared Symposium, pp.78~81, July 2003 with those of other designs already reported in the literature in [3] C.
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