HDL Counter
Free-running or count-limited hardware counter
- Library:
HDL Coder / Sources
Description
The HDL Counter block models a free-running, count-limited, or modulo
hardware counter that supports signed and unsigned integer and fixed-point data types.
The counter emits its value for the current sample time. During simulation, this block
does not report warnings or errors due to wrap on overflow. To report these warnings,
see Simulink.restoreDiagnostic.
By default, the counter does not have input ports. The counter counts up from an initial value to a threshold value based on the Counter type, the Count to value, and the Word length. The output data type of the counter depends on the Counter output data, Word length, and Fraction length.
Ports
Input
Output
Parameters
Algorithms
Free Running Counter Behavior
By default, when you do not enable the control ports, the counter counts upwards
from zero in free-running mode. In this mode, the counter increments in steps of one
at each sample time unit until it reaches the threshold value. The count threshold
value is calculated as 2^(Word length) - 1. When it reaches the
count value, the counter resets to the initial value.
The counter behavior depends on control ports that you specify. This table shows the priority of the control signals and how the counter value is updated in relation to the control signals.
Local reset, rst | Load trigger, load | Count enable, enb | Count direction, dir | Next Counter Value |
|---|---|---|---|---|
| 1 | – | – | – | Initial value |
| 0 | 1 | – | – | load_val value |
| 0 | 0 | 0 | – | Current value |
| 0 | 0 | 1 | 1 | Current value + step value |
| 0 | 0 | 1 | 0 | Current value - step value |
The Step value parameter and optional count direction port,
dir, interact to determine the actual count
direction.
dir Signal Value | Step Value Sign | Actual Count Direction |
|---|---|---|
| 1 | + (positive) | Up |
| 1 | - (negative) | Down |
| 0 | + (positive) | Down |
| 0 | - (negative) | Up |
This figure illustrates the free running mode of operation. In this example, the
counter has a Word length of 4. The
Initial value is 2, load value is
7, and the Step value is
1. When the Enb signal is high, the
counter increments by steps of one. When Rst signal becomes
high, the counter resets to the initial value, 2. When the
Dir port goes low, the counter decrements from
4 to 3 at time step
11s. The count value is tied to the load value when the
Load port becomes high.
Count Limited and Modulo Operation Modes
You can use the Counter type parameter to specify the counter
behavior. The Count limited mode of the counter wraps the
count to the initial value when the counter exactly reaches the Count to
value. If the counter does not exactly reach the Count to
value, it can exceed this value. For an up counter, the count value
can reach a threshold value that is calculated as 2^(Word length) -
1. For a down counter, the count value can reach the Initial
value. When it reaches or exactly matches this threshold value, the
counter resets to a value that is determined by the wrapping step value.
The Modulo mode of the counter wraps the count when it
reaches or exactly matches the Count to value. Instead of
restarting at the initial value, the counter wraps back to a value that is
determined by a wrapping step value. For an up counter, the wrapping step value is
calculated as step value - (count to value + 1) + count from
value. For a down counter, the wrapping step value is calculated as
count from value - step value + (count to value +
1).
This figure illustrates the Count limited and
Modulo modes of operation. In this example, the
counter has a Word length of 4,
Initial value of 2, Step
value of 3, and Count to
value of 12.
In the Count limited mode, as the count value reaches
11, it exceeds the Count to value and
reaches 14. As the threshold value is 15
(2^4-1), the counter resets to a value that is determined by the
wrapping step value. When counting down, the counter exceeds the Initial
value and can reach zero. It then resets to a value that is
determined by the wrapping step value.
In the Modulo mode, as the count value reaches
11, the counter resets to a value that is determined by the
wrapping step value. The wrapping step value is 3 - (12 + 1) + 2 =
-8. Therefore, the counter resets to the value 3
(11 + (-8). When counting down, as the count value reaches
4, the counter resets to a value that is determined by the
wrapping step value. The wrapping step value is 2 - 3 + (12 + 1) =
12. Therefore, the counter resets to the value
12.