VHDL: Modeling Open Collector Outputs for FPGAs
FPGAs offer a versatile platform for implementing digital circuits, and understanding how to model various output types is crucial for effective design. Open collector outputs are particularly interesting, as they provide a way to achieve wired-AND logic without dedicated gates. However, accurately modeling these outputs in VHDL requires a careful approach to ensure correct behavior and prevent potential design errors. This article will delve into the intricacies of modeling open collector outputs in VHDL for FPGAs, exploring the nuances and best practices for achieving reliable and efficient implementations.
Understanding Open Collector Outputs
Before diving into VHDL modeling, it's essential to understand the core concept of open collector outputs. In traditional logic gates, the output is driven by a transistor that can be switched on or off, effectively connecting the output to either the high (Vcc) or low (GND) voltage level. In an open collector output, the transistor is only connected to GND. When the output is "high," the transistor remains off, leaving the output floating. Conversely, when the output is "low," the transistor turns on, pulling the output to GND.
Advantages of Open Collector Outputs
- Wired-AND Logic: Multiple open collector outputs can be connected to a common pull-up resistor. The logic level at the output is determined by the "AND" of the outputs, with the output only being high when all outputs are high (transistors off). This allows for efficient implementation of AND logic without dedicated gates.
- Current Limiting: The open collector output can be used to limit current, as the transistor can handle only a limited current flow. This is particularly useful in applications like driving LEDs where current control is crucial.
Challenges of Open Collector Outputs
- Output State Uncertainty: The output state can be ambiguous when multiple open collector outputs are connected without a pull-up resistor. This is because there is no defined voltage level when all transistors are off.
- Pull-Up Resistor Requirement: An external pull-up resistor is typically required to define the output state when no outputs are actively pulling it low. The value of the pull-up resistor influences the output voltage and current levels.
Modeling Open Collector Outputs in VHDL
VHDL provides various ways to model open collector outputs. The most common approach is to use the "OUT" signal type with an additional "OPEN" state to represent the floating condition.
Example: Simple Open Collector Output
library ieee;
use ieee.std_logic_1164.all;
entity open_collector_example is
port (
input_signal : in std_logic;
output_signal : out std_logic
);
end entity;
architecture behavioral of open_collector_example is
begin
process (input_signal)
begin
if input_signal = '1' then
output_signal <= '1'; -- Transistor off, output floating
else
output_signal <= '0'; -- Transistor on, output low
end if;
end process;
end architecture;
In this example, the output_signal is declared as an out std_logic. The OPEN state is not explicitly used here, as the default state of an output signal is OPEN. When input_signal is '1', the output signal is set to '1' (floating), and when input_signal is '0', the output signal is set to '0' (transistor on, output low).
Example: Wired-AND Logic with Open Collector Outputs
library ieee;
use ieee.std_logic_1164.all;
entity wired_and_example is
port (
input1 : in std_logic;
input2 : in std_logic;
output_signal : out std_logic
);
end entity;
architecture behavioral of wired_and_example is
begin
process (input1, input2)
begin
if input1 = '1' and input2 = '1' then
output_signal <= '1'; -- Both transistors off, output floating
else
output_signal <= '0'; -- At least one transistor on, output low
end if;
end process;
end architecture;
This example demonstrates how to model a wired-AND logic using open collector outputs. The output_signal is '1' only when both input1 and input2 are '1', otherwise it is '0'. This behavior simulates the wired-AND logic where the output is high only when all inputs are high (transistors off).
Modeling Pull-Up Resistors
To ensure proper operation of the open collector output, a pull-up resistor is usually required. This can be modeled using a simple resistor element in the FPGA's configuration tool or by explicitly modeling the pull-up behavior in VHDL. However, directly modeling the pull-up resistor behavior in VHDL is typically not recommended, as it can introduce unnecessary complexity and might not be the most efficient approach.
Advanced Modeling Considerations
- Tri-State Buffer Simulation: While not a true open collector output, a tri-state buffer can be used to simulate the behavior of an open collector output. This can be achieved by setting the enable input of the tri-state buffer to '0' to simulate the output being open and setting it to '1' to enable the output.
- Output Strength: FPGAs often allow you to configure the output strength of the signal. For open collector outputs, using a lower output strength can prevent potential damage to the pull-up resistor or other connected circuits.
- Timing Considerations: The timing of open collector outputs can be influenced by the pull-up resistor value and the characteristics of the connected circuits. Carefully consider these aspects during design to ensure correct operation.
Conclusion
Modeling open collector outputs in VHDL for FPGAs requires understanding their unique characteristics and carefully choosing appropriate techniques. The OPEN state and the "OUT" signal type in VHDL provide a powerful mechanism to model open collector outputs, and careful consideration of pull-up resistors, output strength, and timing is crucial for achieving reliable and efficient implementations. By leveraging these techniques, you can harness the benefits of open collector outputs for implementing wired-AND logic, current limiting, and other applications in your FPGA designs.