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). Refer to Chapter 8, entitled "HT Interrupts," on page 199 for a discussion of HyperTransport interrupt management.

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. Refer to Chapter 10, entitled "Error Detection And Handling," on page 229 for a discussion of HyperTransport error detection and handling.

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. Refer to Chapter 5, entitled "Flow Control," on page 99 for a discussion of HyperTransport flow control.