Cypress CY7C67300 manual External Memory Interface

CY7C67300

External Memory Interface

EZ-Host provides a robust interface to a wide variety of external memory arrays. All available external memory array locations can contain either code or data. The CY16 RISC processor directly addresses a flat memory space from 0x0000 to 0xFFFF.

External Memory Interface Features

■Supports 8-bit or 16-bit SRAM or ROM

■SRAM or ROM can be used for code or data space

■Direct addressing of SRAM or ROM

■Two external memory mapped page registers

External Memory Access Strobes

Access to external memory is sampled asynchronously on the rising edge of strobes with a minimum of one wait state cycle. Up to seven wait state cycles may be inserted for external memory access. Each additional wait state cycle stretches the external memory access time by 21 ns (you must be running in internal memory when changing wait states). An external memory device with 12 ns access time is necessary to support 48 MHz code execution.

Page Registers

EZ-Host allows extended data or program code to be stored in external SRAM, or ROM. The total size of extended memory can be up to 512K bytes. The CY16 processor can access extended memory via two address regions of 0x8000-0x9FFF and 0xA000-0xBFFF. The page register 0xC018 can be used to control the address region 0x8000-0x9FFF and the page register 0xC01A controls the address region of 0xA000-0xBFFF.

Figure 1 illustrates that when the nXMEMSEL pin is asserted the upper CPU address pins are driven by the contents of the Page x registers.

Figure 1. Page n Registers External Address Pins Logic

	nXMEMSEL Pin
0000 + PC[14:0]		1	A[18:0]
PAGEx Register[5:0] + PC[12:0]		0

Where:

x = 1 or 2

PC = Program Counter

A = CPU Address Bus

Note:

PAGE 1 Register Active Range = 8000h to 9FFFh PAGE 2 Register Active Range = A000h to BFFFh nXMEMSEL Pin Active Range = 8000h to BFFFh

Merge Mode

Merge modes enabled through the External Memory Control register [0xC03A] allow combining of external memory regions in accordance with the following:

■nXMEMSEL is active from 0x8000 to 0xBFFF

■nXRAMSEL is active from 0x4000 to 0x7FFF when RAM Merge is disabled; nXRAMSEL is active from 0x4000 to 0xBFFF when RAM Merge is enabled

■nXROMSEL is active from 0xC100 to 0xDFFF when ROM Merge is disabled; nXROMSEL is active from 0x8000 to 0xDFFF (excluding the 0xC000 to 0xC0FF area) when ROM Merge is enabled

Program Memory Hole Description

Code residing in the 0xC000-0xC0FF address space is not accessible by the CPU.

DMA to External Memory Prohibited

EZ-Host supports an internal DMA engine to rapidly move data between different functional blocks within the chip. This DMA engine is used for SIE1, SIE2, HPI, SPI, HSS, and IDE but it can only transfer data between the specified block and internal RAM or ROM. Setting up the DMA engine to transfer to or from an external memory space might result in internal RAM data corruption because the hardware (for example, HSS/HPI/SIE1/SIE2/IDE) does not explicitly check the address range. For example, setting up a DMA transfer to external address 0x8000 might result in a DMA transfer into address 0x0000.

External Memory Related Resource Considerations:

■By default A[18:15] are not available for general addressing and are driven high on power up. The Upper Address Enable register must be written appropriately to enable A[18:15] for general addressing purposes.

■47K ohm external pull up on pin A15 for 12 MHz crystal operation.

■During the 3 ms BIOS boot procedure the CPU external memory bus is active.

■ROM boot load value 0xC3B6 located at 0xC100.

■HPI, HSS, SPI, SIE1, SIE2, and IDE cannot DMA to external memory arrays.

■Page 1 banking is always enabled and is in effect from 0x8000 to 0x9FFF.

■Page 2 banking is always enabled and is in effect from 0xA000 to 0xBFFF.

■CPU memory bus strobes may wiggle when chip selects are inactive.

Document #: 38-08015 Rev. *J Page 5 of 99

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