Again, this matches the count-down timer behavior we originally discussed. If the high bit is set upon any write, shown as r in Fig 4 above, then the timer will enter into interval mode. In all other cases, the timer will be started as a one-shot countdown timer. This is our first break from the original countdown timers functionality, allowing us to run in an interval timer mode. Further, if set to interval mode, then the value written to the timer will become the interval definition. Hence, when the timer finishes counting down to zero, well just automatically restart it again with the same new counter value just written. On writing a zero to the counter, all ongoing counts will be ended and the counter will return to idle.
Do you see any of the problems with this implementation? For example, what happens if you want to switch from a 4-second intervals to 10ms intervals? Just how essay many counts will that first 10ms interval contain? Up to 4 seconds? If thats not the response you want, then how should this timer respond? While we consider this, lets also consider merging the countdown timer together with the interval timer in a way that both respond to bus requests. Heres the capability or requirement well build to then: On any reset, the counter will set itself to zero and wait to be configured This matches the count-down timer behavior we discussed above. The zipTimer Register On any write, the counter will assume the value written to it, as shown in Fig 4 as the new counter bits, and will then start counting down. If the number written was a zero, then the counter will stay at zero and stop.
Simply put, an interval timer is one that counts down to zero, and then resets itself to count down again. Initial r_value 0; always posedge i_clk) if (r_value! 0) r_value r_value - 1'b1; else r_value interval_count; As before, well generate an interrupt anytime this timer hits zero, initial o_int 1'b0; always posedge i_clk) o_int (r_value 1 but what if we wanted to allow this reload value to be externally set? To create this capability, well attach this interval timer to wishbone bus. Perhaps we want something like, initial r_value 0; always posedge i_clk) if r_interval_count i_wb_data; always posedge i_clk) if (r_value! 0) r_value r_value - 1'b1; else r_value r_interval_count; This is almost identical to our original counter above, save that every time it resets it goes back to r_interval_count instead of the original timeout parameter. Since r_interval_count is programmable from the bus, we now have a programmable interrupt timer. How hard can this be? In this case, the devil is in the details.
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0) r_value r_value - 1'b1; Thats quite the configurable counter, no? Lets now return to our bus interface and properly set the rest of the required bus control values. Since we can respond on every clock cycle, theres no reason to ever stall the bus. Assign o_wb_stall 1'b0; Well also need to create a response to the wishbone bus. Since this operation takes only a single cycle, well acknowledge the bus any time we are selected. Assign o_wb_ack (i_wb_stb we can do this if the current counter state data is always valid on the bus s o_wb_data lines. Assign o_wb_data r_value; voila!
A simple, wishbone bus controlled count-down timer! The former code homework works great for a one-shot timer. However, if you want to create a timer that interrupts the cpu every 10ms (as an example only to be reset by the cpu in an interrupt service routine, then you will find that the interval pseudorandomly walks in phase. The intervals will all stationery be longer then 10ms. How can we fix this? An Interval Timer One solution is to use an interval timer.
The problem with the above implementation of a counter is that it isnt very reusable. If you are going to generate a counter that will be programmable, then youll want to add a bus interface. If we use the wishbone bus, then anytime (i_wb_stb) (i_wb_we) is true, and the address reflects our timers address, then we can reload our timer from i_wb_data. This would give us a more adaptable, configurable timer. Such a bus controlled timer could easily become a cpu peripheral. Initial r_value 0; always posedge i_clk) if r_value i_wb_data; else if (r_value!
0) r_value r_value - 1'b1; This works fine for fpga implementations, but what if you want this counter to run in a context where initial statements are ignored? In that case, you need an i_reset input. On a reset, that is when i_reset is high, the counter should return to idle, r_value. Initial r_value 0; always posedge i_clk) if (i_reset) r_value 0; else if r_value i_wb_data; else if (r_value! 0) r_value r_value - 1'b1; In a similar fashion, with only a tiny adjustment, we can use this module to count events. Well use an incoming i_ce signal to denote when an event has taken place. Examples of such events include not only clock cycles (i_ce1 but also incoming or outgoing samples in a dsp system, or lines or frames in a video system. All of these options can be created by appropriately setting an i_ce input to one any time the timer is to step. Put together, our original counter now becomes, initial r_value 0; always posedge i_clk) if (i_reset) r_value 0; else if r_value i_wb_data; else if (i_ce) (r_value!
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Initial o_int 1'b0; always posedge i_clk) o_int (r_value 1 Thats not all that dissertation hard, right? Did you notice the subtlety associated with checking r_value1 here? One of my readers pointed this out. If i_start happens to be true on the same cycle that r_value1, then o_int might be true on a clock cycle when r_value! Yes, this is a bug. Itll come back in the next section as well. However, Im going to leave this bug in place because this was how i originally designed the zipTimer with this bug within. (Oops!) It wasnt until years later when i attempted to formally verify the timer code presented below that I discovered this subtlety. For now, lets evernote just peel this onion back a bit further.
The beginners Exercise hopefully everyone reading this blog has at one time built a countdown timer in Verilog. Indeed, i use a basic countdown timer as one of the first examples in the formal Verification course i now teach. Below is the simple example timer that well start with today. A countdown Timer initial r_value 0; always posedge i_clk) if (i_start) r_value timeout; else if (r_value! 0) r_value r_value - 1'b1; This counter starts at zero. Any time an i_start signal takes place, the counter is set to timeout xiv and then counts down to zero, as illustrated in Fig. Note that setting this counter to timeout doesnt guarantee that it will take timeout clock ticks until it returns to zeroit is possible the i_start signal resets this counter back to timeout before it hits zero. Well also create an interrupt signal that we will set anytime the counter becomes zero.
(x3). I call these timers ZipTimers. Each of these zipTimers supports generating either a regular interrupt or a one-shot delay based interrupt. These zipTimers have been a part of the zipSystem since i started. Their simplicity makes them perfect candidates for beginner exercises, and even better candidates for learning formal verification. The zipTimer has two capabilities beyond the traditional beginners counter exercise. These are first the ability to be programmed over a wishbone bus, and second the ability to interrupt the cpu when the specified delay runs out. Therefore, lets examine this timer peripheral as an exercise in learning Verilog, formal verification, and connecting a simple item to a bus using Autofpga. Along the way, ill do my best to avoid calling this a counter example.
O/s, is an interval timer. An interval timer is little more than a reconfigurable counter. All it does is issue an interrupt to the, cpu at a periodic interval. Embedded systems, such as dissertation those found within. Fpga s, have an additional timing need. These systems often need to insert known delays between different operations. Instead of an interval timer, these systems need what are known as one-shot timers.
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If you are a beginning, fpga designer, the first example you will be given to learn is that of a counter. Its sort of a tradition. Class room exercises all illustrate concepts with simple counters. If you ask a question, the instructor will go to the board and start his explanation with a counter. At least, thats what I would do if I trying to teach an Verilog concept. But just how useful is a counter in the end anyway? Lets try examining a counter all the way from an irrelevant classroom discussion to a vital system component. How can this be? Well, one peripheral necessary to any multitasking operating system, whether, unix, linux, windows, or some other.