1 00:00:00,210 --> 00:00:00,530 Hello. 2 00:00:00,540 --> 00:00:01,240 Welcome back. 3 00:00:01,240 --> 00:00:04,800 In this lesson we shall give a brief overview of time. 4 00:00:05,250 --> 00:00:09,080 Let's begin by talking about assisted time or the system. 5 00:00:09,080 --> 00:00:16,590 Take timer the system tick timer is present in all cortex microcontrollers irrespective of whether it's 6 00:00:16,770 --> 00:00:26,820 SDM thirty two LPC TLC series etc. This system tick timer allows the system to initiate an action on 7 00:00:26,820 --> 00:00:28,890 a periodic basis. 8 00:00:28,890 --> 00:00:34,290 This action is performed internally at a fixed rate without external signal. 9 00:00:34,470 --> 00:00:42,630 For example in a given application we can use cystic to read a sense of value let's say every 200 milliseconds 10 00:00:43,410 --> 00:00:52,050 cystic is widely used when in real time operating systems so that this system software may interrupt 11 00:00:52,080 --> 00:00:59,300 the application software periodically to monitor and control the system operations and do so few take 12 00:00:59,300 --> 00:01:03,660 in my other course on good and your own real time operating system. 13 00:01:03,660 --> 00:01:11,020 You saw how we use cystic as our time base to control everything and to sort of control our scheduler. 14 00:01:11,100 --> 00:01:19,840 The cystic is a 24 bit down counter driven by either the system clock or an internal oscillator it's 15 00:01:19,850 --> 00:01:25,920 counts down from an initial value to zero when it reaches zero in the next clock. 16 00:01:25,920 --> 00:01:34,140 It's under floors and it raises a flag known as Count and reloads the initial value and starts all over 17 00:01:34,140 --> 00:01:35,220 again. 18 00:01:35,220 --> 00:01:41,580 Now let's take a look at the cystic registers when program in the cystic we need to configure just three 19 00:01:41,580 --> 00:01:49,380 registers we've got the the system control and status register the system reload value register in the 20 00:01:49,380 --> 00:01:56,680 system current value register and which we'll see how to configure these registers programmatically. 21 00:01:56,690 --> 00:02:02,100 Now let's see how to calculate the value to load into the cystic LeWitt register. 22 00:02:02,120 --> 00:02:05,510 Let's say we want an action to occur every second. 23 00:02:06,410 --> 00:02:14,160 If our clock frequency is 16 megahertz we simply need to load 16 million minus one into the cystic load 24 00:02:14,210 --> 00:02:18,250 register and we shall see how this works in code. 25 00:02:18,320 --> 00:02:27,740 Well 60 million minus one because if our process is running at 16 megahertz This means there are 16 26 00:02:27,740 --> 00:02:30,920 million cycles in one second. 27 00:02:30,950 --> 00:02:38,750 So in one second we are able to run 60 million processor cycles and because we count from zero we do 28 00:02:38,750 --> 00:02:42,140 16 million minus one over here. 29 00:02:42,230 --> 00:02:44,030 How about one millisecond. 30 00:02:44,060 --> 00:02:47,570 Let's say we want an auction to occur every millisecond. 31 00:02:47,570 --> 00:02:49,390 How do we compute this. 32 00:02:49,400 --> 00:02:57,310 Well we know there are 60 million cycles in a single second and we also know that one second equals 33 00:02:57,470 --> 00:02:59,980 a thousand milliseconds. 34 00:03:00,420 --> 00:03:06,360 Then this means death 16 million cycles in a thousand milliseconds. 35 00:03:06,360 --> 00:03:15,360 So if 60 million cycles in a thousand milliseconds then how many cycles do we get in a single millisecond 36 00:03:15,630 --> 00:03:23,400 and we compute that by doing a thousand milliseconds divided by thousand equals sixteen thousand cycles 37 00:03:23,460 --> 00:03:25,200 divided by thousand. 38 00:03:25,290 --> 00:03:28,770 And this gives us sixteen thousand. 39 00:03:29,160 --> 00:03:35,640 So therefore in a single millisecond we have sixteen thousand cycles. 40 00:03:35,640 --> 00:03:43,860 So if we want an action to occur every millisecond we have to load sixteen thousand minus one into the 41 00:03:43,860 --> 00:03:46,290 cystic node register. 42 00:03:46,350 --> 00:03:49,110 Now let's take a look at the general purpose timer. 43 00:03:49,110 --> 00:03:57,060 Apart from having the cystic timer in every cortex microcontroller the silicon manufacturers like SD 44 00:03:57,090 --> 00:03:59,150 and XP Texas Instruments. 45 00:03:59,310 --> 00:04:04,980 They also provide additional time is known as general purpose time is and we can use these timers to 46 00:04:04,980 --> 00:04:05,940 create delays and.