Timekeeping. ECE Rick

Transcription

1 Timekeeping ECE Rick

2 Reading "Assignment" You don t really have to read these. Only if you re curious about the real-time clock: Textbook, Chapter 18, "Real-time Clock (RTC)", pages FRM, Chapter 24, Real-time clock (RTC)

3 So you want to build a clock... Every semester, several teams want to build an alarm clock. This is a fine thing to do, and we encourage it. But it can t be the same clock we gave you in lab 8. It also has to be accurate. Your STM32F0Discovery board, alone, is not enough. Why not?

4 Clock Accuracy If you look at the user manual for your dev board, you ll find a schematic. There are spots for two external crystal oscillators. And notes that say "Not Fitted." This means they re not there.

5 What clock is used? All timing is done by an internal 8MHz RC circuit. This is not precisely 8 MHz. +/- 0.2% +/- 3 minutes per day. And it drifts faster and slower. Due to temperature & voltage. So even if you determined how much "off" the clock is, it would still change, and you would be wrong again.

6 How about the RTC? There is a real-time clock on the STM32. But it has to use either the built-in RC clocks or an external oscillator (which you don t have). To make an accurate clock, you need an accurate timebase. Let s look at various sources of accurate time.

7 Crystal Oscillators A crystal oscillator is very accurate. Accuracy is measured in terms of frequency tolerance and stability. Each can be rated as good as +/- 10 ppm. i.e., +/- 1 / There are seconds in a day, so this is a gain or loss of less than 1 second per day.

8 Power Lines Power in the Americas is regulated to oscillate at 60 Hz. Sometimes the frequency has to slow down or speed up for grid synchronization, and other reasons. Deviations are subject to Time Error Correction (TEC). In North America, whenever the cumulative error exceeds 10 seconds in the eastern states, 3 seconds in Texas, 2 seconds in other western states, a correction of +/ Hz is applied. In the US, TEC is mandated by the Federal Energy Regulatory Commission (FERC). Or, at least, it used to be.

9 Maybe don t count on it. From Wikipedia article on "Electric Clock": In 2011, the North American Reliability Corporation (NERC), a consensus-based industry organization, petitioned the Federal Energy Regulatory Commission (FERC) to eliminate the TEC. While this would have freed the power companies from the threat of fines and also provided an extremely modest increase in frequency stability, it was also noted that synchronous clocks, which include wall clocks, alarm clocks, and other clocks computing the time on the basis of their electrical power, would accumulate several minutes of error between the biannual resets for Daylight Saving Time. This consequence was reported in the American news media, and the initiative was dropped. However, in late 2016 a similar proposal was again filed by the NERC to the FERC, which was approved two months later. It is contingent upon the removal of the standard WEQ-006, and the NERC also petitioned the North American Energy Standard Board (NAESB), a non-governmental organization that is business-oriented, for removing that standard. If the FERC adopts the NAESB petition, TECs will no longer be utilized in the United States and Canada, and clocks timed by them will likely wander uncontrolled until manually reset; it was noted in a technical paper by employees of the National Institute of Standards and Technology and the U.S. Naval Observatory that, had TECs not been inserted in 2016, there would have been over seven minutes lost by electrically timed clocks over much of the United States and Canada, as shown in Figure 8 of their paper.

10 How about "atomic clocks" Consider one of these clocks. Sometimes they re called "atomic clocks" There s not really an atomic clock in there. How does it work? It listens to a radio station. Image also from Wikipedia.

11 WWV Broadcast by NIST, Fort Collins, CO. Where they do have an atomic clock. Ticks within 100 ns of UTC and 20 ns of the national time standard. They ve been broadcasting, continuously, since Frequencies: 2.5, 5, 10, 15, 20 MHz Also WWVB which transmits no-voice on 60 khz. The carrier waves are accurate to 1 part in and can be used as a frequency standard.

12 WWV Receivers 60khz-atomic-clock-receiver-module-wwvb-msf-jjy60/ Receives 60 khz waves. Digital serial output of the WWVB time code format. Some decoding needed.

13 You can t always hear WWV Having several frequencies to choose from helps. Sometimes, the signal is too weak due to: Interference Mountains Building materials

14 Global Positioning System GPS requires an accurate timebase to work. So it broadcasts one. Any GPS receiver, in addition to telling you the time, will also tell you where you are. With a reasonable set of rules, your clock could automatically determine if it should honor daylight saving time. For instance:

15 Sometimes GPS doesn t work. Poor reception indoors when too much intervening building material are present. Doesn t work underground.

16 FM Radio RBDS subcarrier FM radio has subcarriers e.g., stereo difference is between 19 khz to 38 khz On a 57 khz subcarrier (in the US) is the Radio Broadcast Data System (RBDS) Transmits data at baud RBDS content: Alternative frequencies, program information, station call letters, program type, radio text (station slogan, song title, and artist), clock time and date. Time is only within 100 ms of UTC. Not all radio stations broadcast time on RBDS. Not all radio stations broadcast RBDS.

17 Cellular Receivers Cellular phone networks transmit time. Cellular receivers are rather expensive.

18 Other sources of time Internet. Network Time Protocol. Phone lines. Caller ID reports hour & minute. Others?

19 Build a clock If you re going to build a clock, build it right. It s not too hard to understand, and you ll learn something useful. I d actually like a team to build a clock to put in EE 69. I want it built it to last decades. How do you build something to last? That will be the subject of another lecture.