Patent ID: 7368951
Filing Date: 2008-05-06
Classification: H04L

Abstract:
1. A data transmission circuit having a first transmission mode and a second transmission mode transmitting data at a lower speed than the first transmission mode, the data transmission circuit outputting output data to a data output line in accordance with input data, comprising: a constant current driver having a constant current source and circuitry connected to the data output line; a buffer circuitry connected to the data output line, having a plurality of switching elements and controlling impedance by the switching elements; a dummy buffer having a plurality of corresponding switching elements, each of the corresponding switching elements corresponding to each of the switching elements of the buffer and having substantially the same capacitance as each of the switching elements of the buffer; and a selection circuit selecting between the switching elements of the buffer and between the corresponding switching elements of the dummy buffer in accordance with an impedance control signal, the selection circuit not selecting corresponding switching elements of the dummy buffer corresponding to selected switching elements of the buffer, while selecting corresponding switching elements of the dummy buffer corresponding to non-selected switching elements of the buffer; wherein, in the first transmission mode, the selected switching elements of the buffer selected by the selection circuit are set to on or off state, and the constant current driver operates in accordance with the input data to output data, and in the second transmission mode, the selected switching elements of the buffer selected by the selection circuit are on/off controlled in accordance with the input data to output data.