Input / Output Expansion Without Control Lines

Expansion of input and output lines in an embedded system is sometimes complicated by the unavailability of any additional control or select lines. This paper discusses a technique to expand the input and/or output of an embedded system without requiring any control signals or requiring that there be any unused states on existing signals. The key idea is to utilize transitions between states of existing output lines to generate new control signals and then reprogram the embedded system such that transitions on those output lines are controlled in a specific way. Introduction Embedded systems typically incorporate a host microprocessor or microcontroller coupled to peripherals devices. Typically, signals coupled between the microcontroller and the peripheral are used for the input of data from such peripherals and for the output of data to such peripherals. These data signals can be directly wired to the microcontroller or there can be intervening buffers or registers. Typically certain of these signals are control signals, such as enable signals or strobe signals, which indicate to the peripheral when to perform data input or output respectively. Alternatively, control signals can be implemented in the form of select lines, which are used to indicate to the peripheral how to interpret other signals. A problem that arises with embedded systems is the need to expand the I/O beyond the number for which it was originally designed. For example, it may be necessary to add an additional eight output signals to an embedded system that was only designed with the consideration of handling 16 output signals. Unless there are unused control signals that can be utilized for such an expansion, significant changes may be required, including substantial redesign and additional wires. In the case of select lines, expansion is sometimes simplified if there are unused states, but often all states have been defined and are utilized by existing peripherals. Again this means that a significant redesign effort may be required. This paper discusses a mechanism to expand the I/O capabilities of an embedded system without requiring any additional control signals and without requiring that there are any unused states in existing control signals. In particular, output expansion incorporates a logic circuit coupled to two existing signals and an output register. The logic circuit generates a strobe signal in response to a direct transition from one state to another state of the two signals. The host controller is then programmed such that no such direct transition takes place when input/output is being performed to existing peripherals, and such that a direct transition will be detected by the logic circuit when expansion output is being performed. This means that when a transition between the two states is needed to satisfy the existing peripheral, the host controller ensures that the signals sequence through other states and do not go directly between the two states that are detected by the logic circuit. In the case of input expansion, a logic circuit is coupled to two existing signals and an input buffer. The logic circuit activates an enable signal in response to a direct transition from one state to another state of the two signals, and deactivates the enable signal in response to a transition to a third state of the two signals. As with output expansion, the host controller is programmed such that the first detected direct transition will not take place when input/output is being performed to existing peripherals. Additionally, the host controller is programmed to generate the two transitions detected by the logic circuit when expansion input is being performed. Host Microcontroller Existing Peripheral