LSI for Radio Clocks

LSI for Radio Clocks Takayuki Kondo Katsuya Maruyama Tsunao Oike Radio clocks sustain accurate time at all times, as The broadcasting of time information is conducted in they are equipped with functions for receiving low Japan by the National Institute of Information and frequency signals (the standard-time and frequency- Communications Technology, an incorprated signal emission) that carry time information and administrative agency, with the standard frequency automatically correct time and calendars. Products fitted transmitting offices established at two locations including with a radio clock function are on the increase, since the Ohtakado-yama standard frequency station in Fukushima broadcast range of the standard frequency was extended Prefecture (1997) and Hagane-yama standard frequency to cover all Japan in 2001, which resulted in radio clocks station in Saga Prefecture (2001). Each transmitting getting into the limelight. station covers a radius of 1,200km and the two Conventional radio clocks were used as ordinary transmitting stations combined cover almost the entire clocks (table clocks, watches, wall clocks, etc.) but with area of Japan (Figure 2). Further, mutual interference is the recent improvement of LSIs for radio receiver and avoided with different frequencies assigned, 40KHz from antennas combined with a reduction in power Ohtakado-yama station (Fukushima-Pref.) and 60KHz consumption, raised sensitivity and miniaturization, from Hagane-yama station (Saga-Pref.). application of the device in various products is anticipated, such as consumer appliances, mobile devices and home electric appliances. This paper will provide a general summary of the “ML6191”, a real-time clock LSI with an automatic time correction function that is a combination of the RF for the Approx. 1,200km standard frequency and a real-time clock in a single chip, > 45dBuV/m developed by Oki Electric Industry, as well as examples of system applications and future efforts. Approx. 1,200km >45dBuV/m About standard-time and frequency-signal emission The standard frequency is a low frequency signal that carries time information (time code). The time error of this signal is approximately one second in 100,000 years, Fig. 2 Low frequency standard frequency broadcasting which is extremely high accuracy. The standard frequency stations and their range of coverage. transmitting stations are located at major regions on a global scale, such as Japan, Germany, United Kingdom, The time code carried by the standard frequency is North America and China (Figure 1). composed of 60-bit (60 seconds) data. The time indicated by this time code and its data format varies from country to country. Figure 3 shows the Japanese time code format, Seconds 10 20 30 Minutes O’ clock Total days from 30 40 50 0 Jan. 1 of A.D Day of the Intercalary seconds week : Marker (M) and position markers (P0 to P5) Pulse width 0.2s ±5ms : Binary “0” Pulse width 0.8s 5ms : Binary “1” Pulse width 0.5s ±5ms Example: Thursday, Day 92 (April 1) of 2004 at 17:25 hours with no intercalary seconds within one month. Fig. 1 Low frequency standard frequency broadcasting 1) stations and worldwide coverage. Fig. 3 Japanese time code format 88 Oki Technical Review April 2005/Issue 202 Vol.72 No.2 Special Issue on Technologies of Oki Electric Group of Companies which indicates the “0”, “1” and marker data with 1-bit data in the TCO signal is inhibited. In other words the signals. Further, the marker expresses the time improvement of reception sensitivity through the synchronization position every ten seconds and the implementation of noise countermeasures is the largest separation of digit data. issue on hand for the purpose of system configuration. Mechanisms of radio clocks and issues Relationship between noise and reception relating to system configurations sensitivity A radio clock is composed of a radio receiver, When the strength of a radio wave signal is defined decoder, timer clock and display which are generally as the signal (S) and the strength of noise is defined as created through the combination of two LSIs, one of the noise (N), the reception sensitivity of a radio clock for which is used for RF and the other for a microcomputer practical purposes is the extent to which the S/N ratio can (Figure 4). be reduced while the clock continues to receive signals. A correlation chart showing the variance of the S/N ratio Radio reception Decoding (demodulation) (code deciphering) of the TCO signal waveform in four steps is shown in Figure 5. Low frequency Time Time standard Time code Time display frequency information correction There is no deformity with the TCO signal waveform using waveform A, which displays the largest S/N ratio. Radio clock When the S/N ratio is lowered to waveform B the spiked R F Microcomputer deformity arising from noise starts to show, while with Radio receiver waveform C starts to break down as the frequency and Time clock Decoder Display extent of the deformity increases. By the time the deterioration reaches waveform D the deformity is so extensive the original shape of the waveform is no longer identifiable. Further, an overall review indicates that there is no regularity as to the occurrence of deformity due to Fig. 4 Radio clock configuration. noise. TOC signal • Radio receiver: The low frequency signal sent by the standard frequency transmitting station is received Small and the time codes are output as digital signals (TCO D signal). • Decoder: TCO signals are read, time information is extracted based on the time data format, validation of C the time data is verified and the received time is S/N ratio determined. • Time clock: Determined time data is reflected on the B clock and sustains the clock function. • Display: Time being clocked internally is displayed on A a display such as an LCD panel. Large There are two major issues with the radio clock relating to the configuration of the system. One second The first is the fact that a few minutes are required to Fig. 5 Correlation chart of the S/N ratio and TCO signal start receiving the time code to determine the time, as the waveform. transmission of the utilized time code takes 60 seconds. This means that the microcomputer that controls the The ability of a radio clock to extract time information radio clock must maintain the RF turned to ON and scan from even a TCO signal waveform that has deteriorated the signal level of the TCO signal in a few tens of in quality, due to the low S/N ratio as shown in waveform milliseconds to determine the bit data and also verify the D, would lead to an improvement in the reception validation of the digit data, all during the few minutes it sensitivity of the clock for practical purposes. For this takes to receive the time code. For this reason the power reason it is necessary to incorporate a noise reduction consumption is lowered for the microcomputer and the filter when configuring the system in order to bring the RF, operating for the duration of a few minutes at a time. waveform closer to that of waveform A. The noise The other issue is the fact that the quality of time data reduction filter used for this purpose removed the noise of extracted is directly influenced by the receiver sensitivity. small spiked shapes by assigning low-pass The limit for radio clocks to receive a weak standard characteristics to the filter. frequency is determined by the performance of the In reality, however, partial abnormalities occur with antenna and the radio reception circuit used. However, waveforms due to reflected waves and such, which leave since radio receiving can be inhibited by noise it is remnants of noises that cannot be removed even with a difficult to determine with any certainty the information of noise reduction filter. This in turn results in erroneous the correct time even when reception of just a single bit of interpretation of data bits in decoded signals, such as Oki Technical Review 89 April 2005/Issue 202 Vol.72 No.2 data “0”, data “1” or a marker. It is necessary to follows: implement a regulative data bit in order to correct such • Automatic start and stop of reception. erroneous interpretation of data bits and turn it into • Reception start time and reception interval time can normal data. be set. There are a variety of methods used to correct data • Automatic switchover between the signals of two bits, such as those that utilize the regularity of data. This frequencies (40/60KHz). is when individual manufacturers draw on their unique know-how. • Selectable reception modes (Auto/Manual). This means that it would not be an exaggeration to say • Selectable time systems (12/24 hours). that the improvement of noise resistance and reception • Clock output (32.768KHz). sensitivity, made possible through noise reduction filters • Internally set time (Japan time). and data bit regulative technologies, is directly • Four-wire serial interface. proportional to the commercial value of radio clocks. • High sensitivity with typical reading of 1.0µVrms. • Low voltage operation of 1.1 to 3.6V. Summary of ML6191 • Low electric current consumption of 0.3µA in the The ML6191 is an LSI for radio clocks and a real-time HALT mode (operating at 32KHz in standby mode of clock LSI that incorporates a radio receiver, decoder and radio receiving). timer clock inside a single chip to make it possible to • Temperature range of -40 to +85ºC. control radio receiver, and the time setting of clocks through serial communications conducted from a main Application implementation examples external CPU (Photo 1). The ML6191 is optimally suited for applications that require a low voltage and low current consumption besides accurate time for devices that require time settings (Figure 6).

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