Keyboard

Library:	Input/Output
Introduced:	2.2.0
Appearance:

Behavior

This component allows the circuit to read keys typed from the keyboard - as long as the keys are representable in the 7-bit ASCII code. After clicking the component using the poke tool, the user can type characters, which accumulate in a buffer. At all times, the ASCII value for the leftmost character in the buffer is sent out the rightmost output. When the clock input is triggered, the leftmost character disappears from the buffer and the new leftmost character is sent on the rightmost output.

The supported characters for the buffer include all the printable ASCII characters, as well as space, newline, backspace, and control-L. In addition, the left-arrow and right-arrow keys move the cursor within the buffer, and the delete key deletes the character to the right of the cursor (if any).

The component is asynchronous in the sense that when the buffer is empty and the user types a character, that character is sent immediately as an output, without any wait for a clock pulse.

Pins

West edge, marked by a triangle (input, bit width 1)

Clock - when triggered while the read-enable pin isn't 0, the leftmost character from the buffer is deleted, and the outputs are updated to reflect the buffer's new status.

South edge, leftmost pin (input, bit width 1)

Read Enable - when 1 (or floating or error), a clock edge will consume the leftmost character from the buffer. The clock input is ignored when Read Enable is 0.

South edge, second pin from left (input, bit width 1)

Clear - when 1, the buffer is emptied and does not accept further characters.

South edge, second pin from right (output, bit width 1)

Available - this is 1 when the buffer contains at least one character and 0 when the buffer is empty.

South edge, rightmost pin (output, bit width 7)

Data - the 7-bit ASCII code for the leftmost character in the buffer, or 0 if the buffer is empty.

Properties

Buffer Length

The number of characters that the buffer can hold at once.

ASCII Size

The number of bits to use for the output port.

Trigger

If the value is Rising Edge, then when the clock input changes from 0 to 1, the leftmost character is consumed (when enabled by the Read Enable input). If it is Falling Edge,, then this happens when the clock input changes from 1 to 0.

Label

The text within the label associated with the component.

Label Location

The location of the label relative to the component.

Label Font

The font with which to render the label.

Poke Tool Behavior

Pressing the mouse button into the component gives keyboard focus to the component, and a vertical-bar cursor will be displayed.

Each character typed will then be inserted into the buffer, as long as the buffer hasn't reached its capacity and the character is one of those that the component supports: the printable characters within the 7-bit ASCII code, as well as space, backspace, newline, and control-L. Additionally, the user may type the left-arrow and right-arrow keys to change the location of the cursor within the buffer, and the user may type the delete key to delete the buffer character (if any) just to the right of the cursor.

FPGA Synthesis

Supports VHDL synthesis. The Clear signal is not yet supported for synthesis.

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