Abstract: A bi-directional bus-interface chip has no direction-control input. A forward buffer and a reverse buffer are both normally disabled in the high-impedance state. When a transition occurs on one input bus, a driver transistor in the forward or reverse buffer is activated to pass the transition through the bus-interface chip. After a delay, the driver transistor is disabled. An optional bus-hold circuit maintains voltage levels on buses when driver transistors are disabled. The delay can be selectable by shorting delay resistors in the delay circuit. The high-level voltages on the two busses may differ. The bus-interface chip converts one voltage domain to another and can re-generate weak signals. A pre-buffer may be added to gradually step up the voltage level when differences in voltage domains are large.

Abstract: A clock generator uses two PLL loops and a digital-to-analog converter (DAC) to generate a variable output frequency from a single fixed-frequency reference clock. Each PLL loop receives the reference clock and phase-compares it with a feedback clock. The feedback clock in one loop is slightly faster in frequency than the feedback clock in the second loop. The input voltages to voltage-controlled oscillators (VCOs) in the two loops thus vary slightly. A DAC is connected between the two VCO inputs. The DAC's two reference-voltage inputs are connected to these VCO inputs. The DAC's output voltage is selected from within the voltage range between the two VCO voltages by a digital code-word input to the DAC. The DAC's output voltage is input to a final VCO that generates the variable output frequency. The output frequency is varied by selecting the digital code-word input to the DAC.

Abstract: A voltage booster drives the gate of a bus-switch n-channel transistor to a theoretical maximum of triple the power-supply voltage Vcc. The gate node is first driven to Vcc. Then the back-side of a first capacitor is driven from ground to Vcc, coupling a first voltage boost to the gate node. After a Schmidt-trigger detects the back-side of the first capacitor near Vcc, the back-side of a second capacitor is driven from ground to Vcc. The front-side of the second capacitor is connected to the back-side of the first capacitor. A second voltage boost is coupled across the first and second capacitors to increase the voltage boost of the gate node to near triple Vcc rather than just double Vcc.

Abstract: A multi-port Peripheral Component Interconnect (PCI) bus bridge allows for cascading of PCI buses and reduction of bus loading and traffic. The multi-port PCI bridge has three or more ports that connect to PCI buses. At each destination port, a pair of data FIFOs is provided for each source port, for read and write data. Each destination port has three address FIFOs, one for posted-memory-write (PMW) addresses, another for delayed-transaction-request (DTR) addresses and data, and a third for delayed-transaction-completion (DTC) addresses. An address mux receives addresses from all source ports and combines them into the three address FIFOs. When addresses arrive concurrently, the address mux delays one address until the first address has been written into the address FIFO, and then writes the delayed address. Since separate data FIFOs are used for each source port, data is not delayed. Concurrent transactions from different source ports to the same destination port can occur.

Abstract: Edge rates for output driver transistors are increased for slower conditions such as caused by supply-voltage, temperature, and process variations. The edge rates are increased by increasing charging and discharging currents to the gates of the driver transistors. Process-sensing transistors have gates tied to power or ground. Current through the process-sensing transistors changes with supply-voltage, temperature, and process variations. The currents through process-sensing transistors are used to generate process-compensated voltages that bias current sources and sinks to adjust process-dependent currents. Process-independent or fixed current sources and sinks use process-independent reference voltages ultimately generated from reference currents that are not sensitive to process variations. The process-dependent-currents are subtracted from the fixed currents to produce the charging and discharging currents.

Abstract: A power-down signal is encoded into a differential pair of lines between two chips. When the differential transmitter powers down, it enters a high-impedance state and floats the differential lines. A shunt resistor between a pair of differential lines equalize the voltages on the differential lines so they float to a same voltage when a differential transmitter is disabled and enters a high-impedance state. The condition of equal voltages on the differential lines is detected by an equal-voltage detector that generates a power-down signal when the differential lines are at equal voltages for a period of time. The period of time can be greater than the cross-over time during normal switching to prevent false power-downs during normal switching. Standard differential drivers can signal power-down using the high-impedance state, which is detected by equal voltages on the differential lines. A sensitive dual-differential amplifier and a simpler detector are disclosed.

Abstract: A voltage-variable capacitor uses the channel-to-substrate junction from a gated diode formed from a metal-oxide-semiconductor transistor. The transistor gate has at least two contacts that are biased to different voltages. The gate acts as a resistor with current flowing from an upper gate contact to a lower gate contact. The gate-to-source voltage varies as a function of the position. A critical voltage is where the gate-to-source voltage is equal to the transistor threshold. A portion of the gate that has gate voltages above the critical voltage has an inversion layer or conducting channel under the gate. Another portion of the gate has gate voltages below the critical voltage, and thus no channel forms. By varying either the gate voltages or the source voltage, the area of the gate that has a channel under it is varied, varying the channel-to-substrate capacitance. Separate gate arms reduce bias current.

Abstract: A bus switch has a p-channel and an n-channel transistor in parallel between two buses. When power is disconnected to the bus switch, and one bus is hot and has a voltage above ground, this higher voltage is conducted to the gate and substrate of the p-channel transistor. This biasing keeps the p-channel transistor turned off. A gate connecting p-channel transistor connects the hot bus to the p-channel gate node, while a substrate connecting p-channel transistor connects the hot bus to the substrate under the p-channel transistor. A third connecting p-channel transistor connects the hot bus to a power-down node. The power-down node is normally driven low through a delay line when power is applied. The power-down node is applied to the gate of a source transistor that connects power to the substrate and to an inverter that normally drives the p-channel gate node.

Abstract: A fail-safe circuit for a differential receiver can tolerate noise. A latch is enabled when both differential inputs V+, V− rise above a reference voltage that is close to Vcc. The latch, once enabled, is set by an offset amplifier, signaling the fail-safe condition. The offset amplifier sets the latch when V+ is above or equal to V−. The differential amplifier has a small offset voltage to allow the latch to remain set when V+ and V− are equal in voltage. An output from a differential amplifier receiving V+ and V− can be blocked by a gate when the fail-safe condition is latched. Pullup resistors pull V+, V− to Vcc when an open failure occurs. The latch remains set when common-mode noise occurs on V+, V−, preventing noise from prematurely disabling the fail-safe condition. Such noise coupled into a broken cable is usually common-mode.

Abstract: Electro-static-discharge (ESD) protection of an integrated circuit chip is enhanced by an EOS protection circuit using external components. An external MOSFET is placed in series with the ground pin of the integrated circuit chip. The external MOSFET has a gate coupled to a power bus through a gate resistor, and is bypassed by an ESD capacitor. The external MOSFET turns on after a delay when power is applied during hot insertion. The delay is determined by a power-to-ground bypass capacitor. The time delay of the on stage of the MOSFET inhibits ground current generated by EOS voltage leaked from the power supply through parasitic resistances, capacitances, and inductances, preventing ESD-protection diodes inside the chip from burning out from this EOS pulses that occur during hot insertion. The ESD bypass capacitor shunts the initial ESD pulse to ground before the external MOSFET turns on.

Abstract: A voltage-controlled crystal oscillator (VCXO) has variable load capacitors on the crystal nodes. The variable load capacitors are p-channel or n-channel transistors with their source and drain nodes connected to a crystal node. The gates are driven by an input voltage that is generated from a full-swing control voltage by a voltage conversion circuit. The input voltage has a half-swing of only half of the power-supply voltage, or VDD/2. The input voltage driving n-channel capacitors swings from VDD to VDD/2, which is just above the source voltage of VDD/2 on the crystal node and ensures that the n-channel capacitors remain on for most of the range. A series of resistors can divide the input voltage into a series of differing voltages that drive gates of multiple n-channel capacitors that have their source/drains connected in parallel to the crystal node. Capacitance increases as an n-channel capacitor channel turns on.

Abstract: A bus switch has an n-channel bus-switch transistor between two buses and a p-channel pullup transistor. When power is disconnected from the bus switch, and one bus is hot and has a voltage above ground, this higher voltage is conducted to the gate and substrate of the p-channel pullup transistor. This biasing keeps the p-channel transistor turned off. When power is off, a connecting p-channel transistor connects the higher voltage on the hot bus to the p-channel gate node, while an inverting p-channel transistor connects the gate node to the substrate under the p-channel transistor. Inverting transistors receive an inverse enable signal and drive the gate node when power is applied, turning on the pullup transistor when the n-channel bus-switch transistor is off, and vice-versa. The gate node is fed back and applied to the gate of a source transistor that connects power to the substrate.

Abstract: A power-down signal is encoded into a differential pair of lines between two chips. When the differential transmitter powers down, it enters a high-impedance state and floats the differential lines A shunt resistor between a pair of differential lines equalize the voltages on the differential lines so they float to a same voltage when a differential transmitter is disabled and enters a high-impedance state. The condition of equal voltages on the differential lines is detected by an equal-voltage detector that generates a power-down signal when the differential lines are at equal voltages for a period of time. The period of time can be greater than the cross-over time during normal switching to prevent false power-downs during normal switching. Standard differential drivers can signal power-down using the high-impedance state, which is detected by equal voltages on the differential lines. A sensitive dual-differential amplifier and a simpler detector are disclosed.

Abstract: A differential output buffer has a primary stage and a secondary stage that each directly drive differential outputs. Link transistors between the secondary stage and the differential outputs are eliminated. The primary stage continuously receives differential inputs applied to gates of n-channel sourcing and sinking transistors. The sources of the sourcing transistors and the drains of the sinking transistors are connected to the true and complement differential outputs. The secondary stage also has n-channel sourcing and sinking transistors directly connected to the differential outputs. Pulsed inputs applied to secondary-stage gates are normally low, disabling the sourcing and sinking transistors in the secondary stage to disable the secondary stage. However, during a switching transient, the pulsed inputs are pulsed on, allowing the secondary stage to drive a boost current to the differential outputs.

Abstract: A differential clock driver uses feedback to reduce timing skews between the true and complement differential outputs. Each of the differential outputs has a pull-up driver and a pull-down driver. Each pull-up or pull-down driver has an initial transistor and a final transistor in parallel to drive the output. A resistor separates gates of the initial and final transistors, causing a delay to enable the final transistor. A transmission gate provides feedback from the other output to the gate of the final transistor. When the other output is faster that the output being driven, the transmission gate transfers charge from the other output to the gate of the final transistor, causing it to speed up driving its output. This helps compensates for the timing skew between the outputs. Skews present on differential inputs can be compensated by the transmission gate feedback.

Abstract: A clock driver chip has several banks of clock outputs driven by a single clock reference. Each clock output is driven by large pull-up and pull-down transistors, which have gates driven by pre-driver lines generated by a pre-driver circuit. Individual clock outputs, or a bank of outputs, are enabled by enable signals. A shorting switch is activated when enables for a pair of clock outputs are in a same state. The shorting switch has two transmission gates. One transmission gate shorts the pre-driver lines to the large p-channel transistors of the pair of outputs, while the other transmission gate shorts the pre-driver lines to the large n-channel transistors of the pair of outputs. Pre-driver lines to the pull-up transistors within a bank driven by the same enable can be hardwired together, as can the pre-driver lines to the pull-down transistors. Shorting switches can short banks together to reduce output skew.

Abstract: A phase-locked loop (PLL) includes a final mixer on its output. The final mixer subtracts out a noise or error term from the PLL's output to reduce noise and jitter. A first mixer generates the error term by subtracting a feedback clock from the reference clock. This error term is near D.C. since the feedback and reference clocks are at the same frequency. When this error term is subtracted from the PLL output, a secondary maxima in the noise plot at the PLL's loop bandwidth is removed. A feedback counter receives the output of the voltage-controlled oscillator (VCO) before the final mixer. Outer-band noise created by the VCO is subtracted out by the final mixer, using the error term generated by the first mixer. The mixers reduce noise generated by the VCO or from other sources in the PLL.

Abstract: A bus switch is protected from undershoots on either of its terminals. The bus switch transistor is an n-channel metal-oxide-semiconductor (MOS) with its source connected to a first bus and its drain connected to a second bus. During isolation, the gate node of the bus switch transistor is discharged to ground by a pulsed transistor, and then kept at ground by a leaker transistor. Sense-pulse circuits are attached to the first and second bus. When a low-going transition is detected by a sense-pulse circuit, an n-channel connecting transistor is turned on, connecting the bus with the low-going transition to the gate node through a grounded-gate n-channel transistor. If an undershoot occurs, it is coupled to the gate node. Since both the gate and source of the bus switch transistor are coupled to the undershoot, the gate-to-source voltage never reaches the transistor threshold and the bus switch transistor remains off.

Abstract: When the clock is stopped during a power-down mode, a clock duty-cycle detector asserts a power-down signal. The clock input is filtered to produce an average clock voltage over several clock periods. The average clock voltage is compared to an upper reference voltage to determine when the clock's duty cycle (high pulse-width percent) is above an upper limit. The average clock voltage is also compared to a lower reference voltage to determine when the clock's duty cycle is below a lower limit. When the clock's duty cycle is above the upper limit or below the lower limit the power-down signal is activated by logic. The logic disables the power-down signal when the clock's duty cycle is between the upper and lower limits. High-frequency clock glitches do not falsely trigger a power-up, since glitches are usually narrow and not sufficiently wide to reach the lower limit.

Abstract: Large output driver transistors are used to shunt electro-static-discharge (ESD) pulses. ESD pulses are capacitivly coupled to the gates of the large driver transistors by R-C networks. The capacitive coupling causes a gate-to-source voltage to exceed the transistor threshold, turning on the large driver transistor to shunt the ESD current. Transistor switches are inserted into the R-C networks. These transistor switches disconnect the R-C networks during normal operation, and ensure that the R-C networks couple the I/O pad to the gates of the output driver transistors only when power is turned off. Since ESD events normally occur when power is disconnected, such as during handling by a person or machine, the ESD protection is only needed when power is off. Thus an active ESD-protection device can be disabled during normal powered operation of the IC. A feedback circuit detects power and biases the gates of the transistor switches.