Tuesday, June 26, 2007

The High Speed Signals in Hypertransport Technology

The High Speed Signals (One Set In Each Direction)

Each high-speed signal is actually a differential signal pair. CAD (Command/Address/Data) information consists of the two basic types of HyperTransport packets: control and data. When a link transmitter sends packets on the CAD bus, the receive side of the interface uses the CLK and CTL signals, also supplied by the transmitter, to latch in packet information during each bit time. CTL distinguishes control packets from data packets.

The CAD Signal Group

The CAD bus is always driven by the transmitter side of a link, and is comprised of signal pairs that carry HyperTransport requests, responses, and data. Each CAD bus may consist of between 2 bits (two differential signal pairs) and 32 bits (thirty-two differential signal pairs). The HyperTransport specification permits the CAD bus width to be different (asymmetrical) for the two directions. To enable the corresponding receiver to make a distinction as to the type of information currently being sent over the CAD bus, the transmitter also drives the CTL signal (see the following description).

Control Signal (CTL)

This signal pair is driven by the transmitter to qualify the information being sent concurrently over the CAD signals. If this signal is asserted (high), the transmitter is indicating that it is sending a control packet; if deasserted, the transmitter is sending a data packet. The receiver uses this information when routing incoming CAD information to appropriate request queues, data buffers, etc. There is one (and only one) CTL signal for each link direction, regardless of the width of the CAD bus.

Clock Signal(s) (CLK)

As a source-synchronous connection, each HyperTransport transmitter sends a differential clock signal along with CAD and CTL signals to the receiver at the other end of the link. There is one CLK signal pair for each byte of CAD width. While the timing on each clock pair is the same, replicating clocks help in routing of CAD signal pairs with respect to their clock signals. The current HyperTransport specification allows clock speeds from 200MHz (default) to 800MHz.

Scaling Hazards: Burden Is On The Transmitter

It is a requirement in HyperTransport that the transmitter side of each link must be aware of the capabilities of its corresponding receiver and avoid the double hazard of a scalable bus: running at a faster clock rate than the receiver can handle or using a wider data path than the receiver supports. Because the link is not a shared bus, the transmitter side of each device is concerned with the capabilities of only one target.

The Low Speed Signals

Power OK (PWROK) AndReset (RESET#)

PWROK used with RESET# indicates to HyperTransport devices whether a Cold or Warm Reset is in progress. Which system logic component is responsible for managing the PWROK and RESET# signals is beyond the scope of the HyperTransport specification, but timing and use of the signals are defined. The basic use of the signals includes:

  • At power up, PWROK is asserted by system logic when it can be guaranteed that system power and clocks related to HyperTransport are within proper limits.

  • RESET# is asserted by system logic to indicate that a reset is required. The state of PWROK when RESET# is seen asserted indicates the type of reset to be performed. PWROK and RESET# both asserted is a warm reset; PWROK deasserted and RESET# asserted indicates cold reset.

  • After initial system power up, reset, and initialization, a cold or warm reset may also be generated under software control writing configuration registers in the host bridge.

The HyperTransport specification describes the actions to be taken by devices during either type of reset event.


(Note: the signal names LDTSTOP# and LDTREQ# were carried forward from the earlier name AMD assigned to HyperTransport technologyLightning Data Transfer).

LDTSTOP# is an input to HyperTransport devices which is asserted by system logic to enable and disable link activity during power management state transitions. Support for this signal is optional for HyperTransport devices.

A transmitter which detects LDTSTOP# asserted finishes sending any control packet in progress, then commences a disconnect NOP sequence followed by disabling its output drivers (if so enabled in the transmitter's Configuration Space Tri-State Enable Bit). Upon receipt of the disconnect NOP sequence, the target also turns off its input receivers (if similarly enabled in it's Configuration Space Tri-State Enable Bit).

Later, when the transmitter detects LDTSTOP# deasserted, it re-enables its drivers and begins the initialization sequence. A receiver that responds to LDTSTOP# deasserted turns its input receivers on.


LDTREQ# is a wire-or'd output from HyperTransport devices that is used to request system logic to re-enable links previously disabled using the LDTSTOP# mechanism. Upon receipt of the LDTREQ# signal from one or more HyperTransport devices, system logic (typically the South Bridge) deasserts LDTSTOP# which triggers the sequence described previously. Specifically, the LDTREQ# signal indicates that a HyperTransport transaction is required somewhere in a system that is currently in the ACPI C3 state; the system is required to transition to the C0 state. Support for this signal is optional for HyperTransport devices.

Where Are The Interrupt, Error, And Wait State Signals?

The HyperTransport specification eliminates a number of control signals that are commonly found on other buses. While devices are not prohibited from implementing signals beyond those defined in the specification, HyperTransport is a generic, simple interface and handles interrupts, errors, and data wait states in the following general way:

Interrupt Signaling

Interrupts are conveyed in HyperTransport as messages sent over the link in the posted request channel. This eliminates the need for dedicated interrupt signal traces. Depending on the architecture, it may also eliminate the need for a separate interrupt controller (e.g. IOAPIC).

Error Signaling

HyperTransport error handling employs CRC checking of bit traffic across each link interface. In the event of an error, there are several possible handling schemes. All of this is done without any dedicated error signals.

Wait State Signaling

Wait states during transmission of data are a problem on any bus because they represent wasted time on the part of the devices performing the transfer and for other devices waiting to perform subsequent transfers. In HyperTransport, wait state, disconnect, and retry mechanisms used on other buses are eliminated. This is made possible through a coupon-based flow control scheme that guarantees that no transfer will be started by a transmitter which cannot be immediately accepted by the corresponding receiver on the other side of the link. Dynamic flow control information concerning buffer availability is embedded in NOP packets sent by each device — removing the need for dedicated transmitter and receiver ready signals.

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