Lecture 9

Lecture 9: The OPB Bus and IPIF Interface Cores

ECE 412: Microcomputer Laboratory

Lecture 9

Outline• The OPB Bus• Bus arbitration• Using the IPIF core generators to build logic to

interface your hardware to the PLB/OPB busses.

Lecture 9

Review Questions• What is time-multiplexed data transfer?

– Share a single set of wires for multiple pieces of data

– Saves wires at expense of time

• What are the two basic bus control protocols? What is the difference between them?– Strobe: Servant puts data on bus within time taccess

– Handshake: Servant puts data on bus and asserts ack

• How do the REQUEST, ADDRACK, RDACK, and WRDACK signals interact to control transfers over the PLB bus?– REQUEST starts a transaction, ADRACK acknowledges that the

slave device has accepted the transaction, and RDDACK/WRDACK signal when data are received.

Lecture 9

Multilevel Bus Architectures Revisit

• Processor-local bus– High speed, wide, most frequent

communication– Connects microprocessor, cache,

memory controllers, etc.

• Peripheral bus– Lower speed, narrower, less

frequent communication– Typically industry standard bus

(ISA, PCI) for portability

Processor-local bus

Micro-processor

Cache Memorycontroller

DMAcontroller

BridgePeripheralPeripheralPeripheral

Peripheral bus

• Don’t want one bus for all communication– Peripherals would need high-speed, processor-specific bus interface

• excess gates, power consumption, and cost; less portable

– Too many peripherals slow down bus

Lecture 9

The OPB Bus• “On-Chip Peripheral Bus”• Standard developed by IBM for connecting logic

cores to microprocessors/other cores• Not implemented in hardware by either the

PowerPCs or the FPGA portion of the Virtex II Pro– Use Xilinx cores to build PLB-OPB “bridge” and interfaces– Use OPB because it’s enough of a standard that IP cores

are designed to be compatible with it• Ex: the UART, SystemACE cores

Lecture 9

OPB Bus

• Two components: Arbiter and Interconnect– Interconnect implemented as OR of output bits from each

master

Lecture 9

OPB Bus• Specification supports up to 16 bus masters, arbitrary

number of slaves– Number of slaves affects how many resources the OPB bus

core uses– Xilinx recommends you use at most 16 slaves

• 32-bit bus protocol• Signaling conventions similar to PLB

– Xilinx core supports either combinational or registered control signals

• Combinational allows responses in same cycle as requests

• Registered generally allows higher clock rate on bus

Lecture 9

Arbitration: Priority Arbiter

• Consider the situation where multiple peripherals request service from single resource (e.g., the OPB bus) simultaneously - which gets serviced first?

• Priority arbiter– Peripherals make requests to arbiter, arbiter makes requests to

resource

Micro-processor

Priority arbiter

Peripheral1

System bus

Int3

57

IntaPeripheral2

Ireq1

Iack2

Iack1

Ireq2

2 2

6

Lecture 9

Arbitration Using a Priority Arbiter

1. Microprocessor is executing its program.2. Peripheral1 needs servicing so asserts Ireq1. Peripheral2 also needs servicing so asserts Ireq2. 3. Priority arbiter sees at least one Ireq input asserted, so asserts Int.4. Microprocessor stops executing its program and stores its state.5. Microprocessor asserts Inta.6. Priority arbiter asserts Iack1 to acknowledge Peripheral1.7. Peripheral1 puts its interrupt address vector on the system bus8. Microprocessor jumps to the address of ISR read from data bus, ISR executes and returns

(and completes handshake with arbiter).9. Microprocessor resumes executing its program.

Micro-processor

Priority arbiter

Peripheral1

System bus

Int3

57

IntaPeripheral2

Ireq1

Iack2

Iack1

Ireq2

2 2

6

Lecture 9

Arbitration: Priority Arbiter

• Types of priority– Fixed priority

• each peripheral has unique rank

• highest rank chosen first with simultaneous requests

• preferred when clear difference in rank between peripherals

– Rotating priority (round-robin)• priority changed based on history of servicing

• better distribution of servicing especially among peripherals with similar priority demands

Lecture 9

OPB Bus Arbitration• Two modes

– “Dynamic” gives priority to the LRU requester– “Fixed” defines a priority order for bus masters, gives bus to

the highest-priority requester

• Set of registers define priority order for fixed mode– Can be programmed before or during operation

• Xilinx implementation of this is somewhat different than IBM spec in order to allow variable # of masters

– User responsibility to make sure that every master has a slot in the table

Lecture 9

OPB Bus Modes of Operation• Standard: Masters do individual requests for each

transaction– Simple, but requires request-reply delay on each transaction– OPB bus does allow requests to overlap with transactions

• Locked: Master can assert its lock signal to retain control of the bus for multiple transactions– Improves bandwidth, has potential for starvation if master

doesn’t release bus

• Parked: Master retains control of the bus as long as no other master requests it– Good bandwidth, no starvation, but masters need to watch

for other requests

Lecture 9

Interfacing Logic to the OPB/PLB Busses

A number of challenges• PLB/OPB bus spec is somewhat complex• Each slave needs to detect whether the address of a

request falls in its assigned range• PLB/OPB bus width may not match your logic• Many user cores require functions that aren’t built

into the OPB/PLB bus spec– Bursts– DMA access

• PowerPC handles interrupts and PLB with separate hardware

Lecture 9

One Approach: Xilinx IPIF Cores• EDK includes a core generator that creates

VHDL/Verilog for modules that act as the interface between user logic and the PLB or OPB busses

• Core is parameterizable so that user can include just the functions that a given piece of logic needs

• EDK provides a graphical interface to generate cores with most options – Refer to back-up slides

Lecture 9

User Logic

IPIF

IP Interconnect (IPIC)

OPB or PLB Bus

IPIF Functional Overview

Lecture 9

IPIF Functionality• Handles address range checking • Implements user-defined registers• “Byte Steering” lets devices connect to PLB/OPB

busses that are wider than they are• Interrupt handling with collection/latching• Supports fixed length burst transfers• Read or Write FIFOs• DMA engine

Lecture 9

IPIF Block Diagram

Lecture 9

Things you can do with IPIF• Create a set of control registers for your logic so that

you don’t have to deal with the PLB bus– We use this trick in MP2.1

• Instantiate a DMA engine to move data between a buffer (like a BlockRAM) and another device on the PLB bus (like, say, the SDRAM controller)– Using DMA engine simplifies your logic– Lets you ignore timing of the SDRAM– DMA controller can be configured to do scatter/gather to

non-contiguous locations, making dealing with data structures much easier

Lecture 9

More Things to do with the IPIF• Create input or output FIFOs

– FIFOs are great for decoupling modules so that they don’t have to operate in lockstep

– Also good for cases where your logic operates on blocks of data -- lets you think about transferring blocks instead of individual words

• Issue reset operations to the processor– Doubt you’ll need that for this course, but useful in a number

of real applications

Lecture 9

Still more things to do with the IPIF• Manage interrupts

– IPIF controller generates interrupts for DMA, FIFO overflow, etc.

– Need to merge user interrupts with IPIF controller interrupts– IP cores may implement interrupts differently than PowerPC

• Different timing, hold requirements, etc.

• IFIP provides up to 32-bit bus of interrupt inputs– Some bits reserved for IFIP’s interrupts– If you need more interrupt signals than this, you need to

build a second device or an interrupt hierarchy.

Lecture 9

Interrupts in the IPIF• IPIF provides two sets of interrupt registers: Device and IP

– IP registers handle interrupts from user logic

– Device registers are the interrupt information presented to the processor (merged user + IPIF interrupts)

– Device registers always 32-bit, IP registers vary depending on number of interrupts defined

• Interrupt Status Register is a bit vector of which interrupts are asserted

• IPISR has three modes (defined per bit)– Pass-through (no latching)

– Sample and hold (samples on clock edge and holds until cleared)

– Registered edge detect (detect 0->1 transitions and hold)

– Can also invert interrupt signals

Lecture 9

Interrupts in the IPIF (cont’)• DISR merges IP and IPIF interrupt signals, is always

pass-through• Device Interrupt Enable Register lets you enable or

disable individual interrupts – Can also globally disable all interrupts– IP Interrupt Enable Register masks interrupts at the IP logic

side

• Device Interrupt Pending Register is bitwise and of DISR and DIER

• Device Interrupt ID register contains ID of interrupt currently being sent to processor– IPIF interrupt logic selects highest-priority interrupt out of the

ones asserted at any time

Lecture 9

Using IPIF in MP2

• Core is provided to you in CP1– V:\ece412\edk_user_repository\MyProcessorIPLib\pcores– Need to import existing core using “Create and Import

Peripheral Wizard”

• Need to create new core in CP2– Design a median filter attached to OPB or PLB bus

Lecture 9

Topology of Instantiated Modules for MP2 CP1

Mp2_checkpoint1(mp2_checkpoint1.vhd)

OPB_IPIF_I * User_logic(user_logic.vhd)

Student_logic(student_logic.vhd: whereyour code will be written)

My_logic(my_logic.vhd: simplified

version of the DIO2 code weused in MP1 CP3)

* Part of the EDK, not your project

Lecture 9

BACKUP Slides for Creating the IPIF Core

Lecture 9

Bus Interface

Lecture 9

IPIF Services

Lecture 9

Interrupt Service

Lecture 9

User S/W Register

Lecture 9

IPIC

Lecture 9

Peripheral Simulation Support

Lecture 9

Peripheral Implementation Support

Lecture 9

You are done!

Lecture 9

Next Time

• Device drivers in Linux