The present invention relates to a data transmitter in a semiconductor device used to transmit data to the outside of a chip, especially, a data transmitter employing a pulse amplitude modulation (PAM) method, and more particularly, to a data transmitter capable of outputting data in a normal mode and a low-power mode.
FIG. 1 illustrates voltage levels at output nodes depending on logic levels of data output through a conventional data transmitter employing a four level PAM (four level Pulse Amplitude Modulation, henceforth 4PAM). In this and other waveform diagrams, D0D1 represents the logic levels of bits D0 and D1 sequentially, e.g. 10 means that D0 is high and D1 is low.
The conventional data transmitter of FIG. 1 outputs two bits of data, D1 and D0 through two output nodes TX_OUT and TX_OUTB in a differential manner. Therefore, the data D0 and D1 output through the two output nodes TX_OUT and TX_OUTB have four logic levels of ‘Fully High’, ‘High’, ‘Low’ and ‘Fully Low’, not two logic levels of ‘High’ and ‘Low’.
As illustrated in FIG. 1, when (D0, D1) is (0, 0), the output nodes TX_OUT and TX_OUTB are at ‘Fully Low’ and ‘Fully High’ levels, respectively. When (D0, D1) is (1, 0), the output nodes TX_OUT and TX_OUTB go to ‘Low’ and ‘High’ levels, respectively; when (D0, D1) is (0, 1), the output nodes TX_OUT and TX_OUTB go to ‘High’ and ‘Low’ levels, respectively; and when (D0, D1) is (1, 1), the output nodes TX_OUT and TX_OUTB go to ‘Fully High’ and ‘Fully Low’ levels, respectively.
FIG. 2 is a circuit diagram illustrating a conventional data transmitter outputting data having waveforms shown in FIG. 1.
The conventional data transmitter outputting data by the 4PAM includes a first driver 210 and a second driver 220. The first and second drivers 210 and 220 pull down the first and second output nodes TX_OUT and TX_OUTB, which are terminated with pull-up resistors.
The first driver 210 pulls down the first output node TX_OUT or the second output node TX_OUTB in response to the data D0. Specifically, when the data D0 changes to ‘High’ level (i.e., the data D0B changes to ‘Low’ level), the first driver 210 pulls down the second output node TX_OUTB. On the contrary, when the data D0 changes to ‘Low’ level (i.e., the data D0B changes to ‘High’ level), the first driver 210 pulls down the first output node TX_OUT.
In detail, when the data D0 changes to ‘High’ level, a transistor M21 is turned on but a transistor M22 is turned off. Accordingly, the driving control signal NET1 goes to ‘Low’ level and the driving control signal NET2 goes to ‘High’ level, thus turning off a transistor M24 and turning on a transistor M25. Therefore, the first driver 210 pulls down only the second output node TX_OUTB. On the contrary, when the data D0 changes to ‘Low’ level, the transistor M24 is turned on so that the first driver 210 pulls down the first output node TX_OUT. Transistor M23, biased by voltage VBIAS1, provides a current sink for transistors M21 and M22, and transistor M26, biased by voltage VBIAS2, provides a current sink for transistors M24 and M25.
The second driver 220 pulls down the first output node TX_OUT or the second output node TX_OUTB in response to the data D1. When the data D1 changes to ‘High’ level (i.e., the data D1B changes to ‘Low’ level), the second driver 220 pulls down the second output node TX_OUTB. On the contrary, when the data D1 changes to ‘Low’ level (i.e., the data D1B changes to ‘High’ level), the second driver 220 pulls down the first output node TX_OUT. A detailed operating principle of the second driver 220 is the same as the first driver 210, and thus further description will be omitted herein. Transistor M29 provides a current sink for transistors M27 and M28, and transistor M32 provides a current sink for transistors M30 and M31.
The second driver 220 is designed to have twice the driving force of the first driver 210. That is, transistor M32 sinks twice the current of transistor M26.
Since the first and second drivers 210 and 220 have different driving forces, the output nodes TX_OUT and TX_OUTB may have logic levels shown in FIG. 1.
Hereinafter, description will be concentrated on the first output node TX_OUT for convenience in description. The first output node TX_OUT is at ‘Fully High’ level when both the first and second drivers 210 and 220 do not pull down the first output node TX_OUT. When only the first driver 210 pulls down the first output node TX_OUT, the first output node TX_OUT has ‘High’ level. When only the second driver 220 pulls down the first output node TX_OUT, the first output node TX_OUT has ‘Low’ level because the second driver 220 has twice the driving force of the first driver 210. When both the first and second drivers 210 and 220 pull down the first output node TX_OUT, the first output node TX_OUT changes to ‘Fully Low’ level.
Basically, the first and second drivers 210 and 220 make current sink from the first and second output nodes TX_OUT and TX_OUTB, that is, pull down the first and second output nodes TX_OUT and TX_OUTB, thereby changing logic levels of the first and second output nodes TX_OUT and TX_OUTB. Such a method, however, causes the first and second output nodes TX_OUT and TX_OUTB to always consume current except for the case of ‘Fully High’ level. Accordingly, the data transmitter consumes a large amount of current unnecessarily.
In FIG. 2, the symbol PWDN denotes a power down signal, which maintain its ‘Low’ level when a circuit operates normally.
FIG. 3 illustrates voltage levels at output nodes TX_OUT and TX_OUTB of another conventional data transmitter for low power consumption.
The output nodes TX_OUT and TX_OUTB of the conventional data transmitter have logic levels shown in FIG. 3, which differs from FIG. 1. Referring to FIG. 3, at least one of the output nodes TX_OUT and TX_OUTB always maintains ‘Fully High’ level. The meaning the logic level of at least one of the output nodes TX_OUT and TX_OUTB is ‘Fully High’ is that there is no current sink to a ground voltage (VSS) terminal from the output nodes TX_OUT and TX_OUTB. Therefore, when the output nodes TX_OUT and TX_OUTB maintain the logic levels shown in FIG. 3, current consumption is reduced in comparison with the conventional data transmitter outputting data with the waveforms shown in FIG. 1.
As described above, there have been employed the conventional data transmitter outputting data with the waveform of FIG. 1 and another conventional data transmitter outputting data with the waveform of FIG. 3. Hence, a variety of data transmitters are required to meet various data transmission standards.