TX rise/fall time control for multi-rate serial link

For a multi-rate serial link, the proposed TX rise/fall time (tTR) control scheme enables tTR to be changed according to data rate. The main driver is split into multiple legs and each leg is controlled by its own pre-driver. All pre-drivers are biased on for the smallest tTR while some pre-drivers are biased off for large tTR. Due to RC delay among pre-drivers, the transition of a biased-off pre-driver's output occurs later but becomes overlapped with those of adjacent pre-drivers' outputs so that there is no stair-step waveform shown. The measurement results show that the driver has a sufficient tTR control range of around three times.