USB interface controller chip

[0001] The present invention relates generally to an apparatus for supporting USB interfaces, and more particularly to a USB interface controller chip with a built-in pull-up resistor.

[0002] USB is a standard interface for connections or communications of computer systems with its peripherals, which is divided into low-speed devices and full-speed devices. FIG. 1 illustrates a connection manner of USB low-speed device, of which a host computer 10 and a USB low-speed device controller chip 12 are connected together by a cable 14 with connections of the respective D+ ports 16 and 20, and D− ports 18 and 22. There are two pull-down resistors Rpd of 15 KΩ connected from the D+ and D− ports 16 and 18 to ground, respectively, at the host computer 10 side as well as a pull-up resistor R_pu of 1.5 KΩ connected between the D− signal port 22 and a supply voltage of 3.3 V at the USB low-speed device controller chip 12 side. On the other hand, there is a difference between the USB full-speed device and the low-speed device in that the pull-up resistor R_pu for the USB full-speed device is connected to the D+ signal port, instead of the D− signal port as in the manner for the USB low-speed device shown in FIG. 1.

[0003] The USB specification defines the pull-up resistor R_pu of 1.5 KΩ with a tolerance not larger than 5%. The widely used ¼ Watts carbon film resistor has a resistance tolerance thereof just meet 5%. However, the USB low-speed device controller chip 12 together with an external pull-up resistor R_pu results in a higher cost, and the controller chip 12 needs to be provided with one additional pin for connection of the external pull-up resistor R_pu and thus not only increasing the cost, but also increasing the possibility of being damaged by electro-static discharge (ESD). In addition to the cost down of the device manufacturer, the pin count on the chip package will be decreased by one and thereby is decreased the possibility of ESD damages if the pull-up resistor R_pu could be integrated within a single chip. Nevertheless, the polysilicon resistor formed by the normal semiconductor processes in combination with the temperature drift may have a tolerance approximately ranged from 10% to 20%, it is therefore difficult for the pull-up resistor R_pu to be built in a single chip to satisfy the USB specification requirement, unless special and thus expensive process is used.

[0004] In consideration of the definition for the pull-up resistor R_pu in the USB specification, it is realized that the main function of that resistor R_pu is to identify a connected device is a USB full-speed one or a low-speed one depending upon which signal between D+ and D− signals is pulled high (logic “1”) as the USB device is plugged in a system. In terms of this aspect, it is noted that it will not destroy the normal operation of a system as long as the voltage divided by the pull-up resistor R_pu of 1.5 KΩ and the pull-down resistor R_pd of 15 KΩ will not result in a misjudgment of high level by digital logic, even though the pull-up resistor R_pu has a larger tolerance range than the USB definition.

[0005] It is therefore desired a chip integrated with a USB pull-up resistor.

[0006] One object of the present invention is to provide a USB interface controller chip integrated with a pull-up resistor without violating the consistence to USB transmission scheme, in which the pull-up resistor built in the controller chip could have a tolerance range larger than that defined by USB specification.

[0007] According to the present invention, a USB interface controller chip comprises a pair of D+ and D− signal ports for being connected to the USB ports of a host computer, a power node for connecting with a supply voltage, and a pull-up resistor built in the controller chip and connected between one of the pair of signal ports and the power node. The built-in pull-up resistor has a resistance tolerance typically up to or more than 20%.

[0011] FIG. 2 shows one embodiment arrangement of the present invention, in which the D+ ports 16 and 20, and D− ports 18 and 22 of the host computer 10 and a USB low-speed device controller chip 24 are connected together by a cable 14 as in the prior art connection, and the USB low-speed device controller chip 24 has a built-in pull-up resistor R_pu-bi of 1.5 KΩ connected between the D− port 22 and a power node 26 connected with a supply voltage of 3.3V, in addition to the pull-down resistors R_pd of 15 KΩ connected from the USB ports 16 and 18, respectively, to ground at the host computer 10 side. However, the tolerance range of the pull-up resistor R_pu-bi could be up to or more than 20%.

[0012] According to the definition in USB specification, a threshold voltage V_IH=2.0 V is defined for the USB port, i.e., it is ensured that logic “1” will be determined for the level of the on-line signal by a logical circuit as long as it is larger than 2.0 V. The tolerance range defined by the USB standard for the pull-down resistor R_pd of 15KΩ is also 5%, thereby the lowest value of the pull-down resistor R_pd is
R_pd(min)=15 KΩ×(1−5%)=14.25 KΩ.
While setting the tolerance of the pull-up resistor R_pu-bi to be 20%, that is,
R_pu-bi(max)=1.5 KΩ×(1+20%)=1.8 KΩ,
the pull-up voltage according to the definition in the USB specification is 3.0-3.6 V. When the pull-up resistor R_{pu-bi (max)} is connected to the lowest pull-up voltage in the worst case of 3.0V, it is known by calculation that the lowest voltage of the D− signal divided by the pull-up resistor R_{pu-bi (max)} and the pull-down resistor R_{pd (min)} in the USB idle mode is 2.66 V, which is still greater than the threshold definition V_IH=2.0 V significantly. As a result, the application of the built-in resistor R_{pu-bi (max)} with the tolerance of 20% to the USB pull-up resistor R_pu of 1.5KΩ may be practical. With further calculation, it could be known that the pull-up resistor R_pu must be more than 7 KΩ, then the determination of USB for the signal level could be affected.

[0013] Further, as mentioned in the USB specification, the signal of the low-speed device D−/full-speed device D+ must be pulled up to V_IH, i.e., 2.0 V, by the pull-up resistor R_pu within 2.5 μs after power-on and reset. To satisfy such requirement, for instance, the pull-up resistor R_pu must not be greater than 4.6 KΩ in case of a load capacitor C_L=200-600 pf on the D+/D− signal port. The detailed calculation is given as followed.
The RC step response can be described as $V\left(t\right)=3.3\times \left(1-e^{\frac{-t}{\tau }}\right)$
where τ is RC time constant. Thus, at time t=2.5 μs, $V\left(2.5u\right)=V_{\mathrm{IH}}=2.0=3.3\times \left(1-e^{\frac{-2.5u}{\mathrm{RC}}}\right)$
and
2.5 μs≈0.9RC=0.9×R_pu×600 p,
so that it is obtained
R_pu≈4.6 KΩ.

[0014] Moreover, as the resistance of the pull-up resistor R_pu becomes lower, the rising time of the rising-edge of the low-speed device D− signal/full-speed device D+ signal will become faster, resulting in that the crossover voltage will exceed the range of 1.3-2.0 V defined by the USB specification. As such, the lower limit thereof should depend upon the compensation capability of the output buffers in the USB transceiver.

[0015] It is known from the above embodiment illustration the normal operation of the chip 24 in the USB transmission scheme could be ensured as long as the largest resistance of the built-in pull-up resistor R_pu-bi of 1.5 KΩ for the USB low-speed device controller chip 24 does not exceed 4.6 KΩ (approximately 1.5 KΩ×306%) and the smallest one will not result in a rising time of the crossover voltage or rising-edge of signal exceeding the range defined by the USB specification.

[0016] As appreciated by those skilled in the art, the power source supplied to the pull-up resistor R_pu-bi could be served by a voltage source of electrical circuit embedded in the controller chip 24 that transforms an external power source to an appropriate voltage, for instance, a supply voltage of 5 V transformed by the chip to a supply voltage of 3.3 V in advance and, then connected to the power node 26 to which the pull-up resistor R_pu-bi is connected. In other words, the power node to which the pull-up resistor R_pu-bi is connected is an arrangement to supply the voltage to the pull-up resistor R_pu-bi, not necessarily connected to an external power source directly.

[0017] While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.