| ??? 10/10/05 16:52 Modified: 10/10/05 16:57 Read: times |
#102175 - How to count seconds Responding to: ???'s previous message |
Rohit said:
This means we can\\\'t simply count the number of overflows from it counting from 0 to 65536. We\\\'ll be introducing even more inaccuracy. In the case of an 11.0592Mhz crystal, this inaccuracy will be about 0.44%, but if we were to use this same program with a 12.000Mhz crystal, the inaccuracy would be 1.70% which is much worse. Other crystal frequencies could result in even less accuracy. For a software based RTC you need to count seconds. So, what happens if you have a 11.0592MHz crystal and use the internal timer/counters? When each overflow from counting from 0 to 65536 gives you a tick, then you get a time base of 1 / (11.0592MHz / 12 / 65536) = 0.07111111sec. If you now count 14 of such ticks, you will get a period of 14 x 0.07111111sec = 0.99555554sec. If you count 15 ticks, you will get 15 x 0.07111111sec = 1.06666665sec. So, you will have a problem, because neither 14 nor 15 ticks will give you excatly 1sec. Best deviation is (1 - 0.99555554) x 100 / 1 = 0.44%. Kai |
| Topic | Author | Date |
| Software Based RTC | 01/01/70 00:00 | |
| Context | 01/01/70 00:00 | |
| Link to my RTC - sftware crystal freq. | 01/01/70 00:00 | |
| The precision is depending on the fracti | 01/01/70 00:00 | |
| How to count seconds | 01/01/70 00:00 | |
| How to decide count for a 3.5795Mhz, | 01/01/70 00:00 | |
| the frequency range does not matter | 01/01/70 00:00 | |
| the "tricks" | 01/01/70 00:00 | |
| 3.93216MHz is better | 01/01/70 00:00 | |
| 4.915200 | 01/01/70 00:00 | |
| 3.93216 ?? | 01/01/70 00:00 | |
| 3.93216 is standard frequency | 01/01/70 00:00 | |
| Can i have some clarifications on XTALS | 01/01/70 00:00 | |
| manufacturing tolerances | 01/01/70 00:00 | |
| low frequencies Crystals are bigger ? | 01/01/70 00:00 | |
| 32.768kHz means low current consumption | 01/01/70 00:00 | |
| so higher Frequeny crystals are accurate | 01/01/70 00:00 | |
Yes! | 01/01/70 00:00 | |
| The Right Frequency | 01/01/70 00:00 |