SMC Networks ARM720T_LH79520 manual Reading from a Slave Wishbone Memory Device, Wishbone Timing

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ARM720T_LH79520 – Sharp LH79520 SoC with ARM720T 32-bit RISC Processor

Reading from a Slave Wishbone Memory Device

Data is read by the host processor (Wishbone Master) from a Wishbone-compliant memory device or memory controller (Wishbone Slave) in accordance with the standard Wishbone data transfer handshaking protocol. This data transfer cycle can be summarized as follows:

•The host presents an address on its ME_ADR_O output for the address in memory that it wishes to read. It then negates its ME_WE_O output to specify a Read cycle

•The host defines where it expects the data to appear on its ME_DAT_I line using its ME_SEL_O signal

•The slave device receives the address at its ADR_I input and prepares to transmit the data from the selected memory location

•The host asserts its ME_STB_O and ME_CYC_O outputs, indicating that the transfer is to begin. The slave device, monitoring its STB_I and CYC_I inputs, reacts to this assertion by presenting the valid data from the requested memory location at its DAT_O output and asserting its ACK_O signal – to indicate to the host that valid data is present

•The host, monitoring its ME_ACK_I input, responds by latching the data appearing at its ME_DAT_I input and negating the ME_STB_O and ME_CYC_O signals. At the same time, the slave device negates the ACK_O signal and the data transfer cycle is naturally terminated.

Wishbone Timing

Figure 9 shows the signal timing for a standard single Wishbone Write Cycle (left) and Read Cycle (right), respectively. The timing diagrams are presented assuming point-to-point connection of the Master and Slave interfaces, with only signals on the Master side of the interface shown. Note that cycle speed can be throttled by the Slave device inserting wait states (represented as WSS on the diagrams) before asserting its acknowledgement line (ACK_I input at the Master side).

Figure 9. Timing diagrams for single Wishbone Write (left) and Read (right) cycles

CR0162 (v2.0) March 10, 2008

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Contents
Summary FeaturesAvailable Devices Soft Fpga Processors Why use Soft Processors?Risc Processor Background HistoryWishbone Bus Interfaces ARM720TLH79520Processor Abstraction System Wishbone OpenBUS Processor WrappersDesign Migration Architectural Overview SymbolPin Description Name Type Polarity/Bus size Description Control SignalsPhysical LH79520 Interface Signals Name Type Polarity/Bus size Description Configuring the ProcessorCurrent configuration settings for the processor Defining the Memory Map Memory & I/O ManagementBuilding the Bridge between the Hardware and Software CR0162 v2.0 March 10 Division of Memory Space Configuring the ProcessorDedicated System Interconnect Components Internal Memory Peripheral I/O External MemoryWords, Half-Words and Bytes Data OrganizationPhysical Interface to Memory and Peripherals #define Port32 *volatile unsigned int* Port32Address Interrupts ResetHardware Description ClockingWriting to a Slave Wishbone Memory Device Wishbone CommunicationsWriting to a Slave Wishbone Peripheral Device Reading from a Slave Wishbone Peripheral DeviceWishbone Timing Reading from a Slave Wishbone Memory DevicePlacing an ARM720TLH79520 in an Fpga design Design using a Schematic onlyDesign Featuring an OpenBus System Facilitating Communications Downloading Your Design Additional Soft Devices in Your DesignEnabling the Soft Devices Jtag Chain On-Chip Debugging Accessing the Debug EnvironmentStarting an embedded code debug session CR0162 v2.0 March 10 CR0162 v2.0 March 10 Revision History Instruction Set