Intel 80C188XL Refresh BUS Cycles, Guidelines for Designing Dram Controllers, Data Bus Width

REFRESH CONTROL UNIT

7.5REFRESH BUS CYCLES

Refresh bus cycles look exactly like ordinary memory read bus cycles except for the control sig- nals listed in Table 7-1. These signals can be ANDed in a DRAM controller to detect a refresh bus cycle. The 16-bit bus processor drives both the BHE and A0 pins high during refresh cycles. The 8-bit bus version replaces the BHE pin with RFSH, which has the same timings. The 8-bit bus processor drives RFSH low and A0 high during refresh cycles.

Table 7-1. Identification of Refresh Bus Cycles

Data Bus Width	BHE/RFSH	A0
16-Bit Device	1	1
8-Bit Device	0	1

7.6GUIDELINES FOR DESIGNING DRAM CONTROLLERS

The basic DRAM access method consists of four phases:

1.The DRAM controller supplies a row address to the DRAMs.

2.The DRAM controller asserts a Row Address Strobe (RAS), which latches the row address inside the DRAMs.

3.The DRAM controller supplies a column address to the DRAMs.

4.The DRAM controller asserts a Column Address Strobe (CAS), which latches the column address inside the DRAMs.

Most 80C186 Modular Core family DRAM interfaces use only this method. Others are not dis- cussed here.

The DRAM controller’s purpose is to use the processor’s address, status and control lines to gen- erate the multiplexed addresses and strobes. These signals must be appropriate for three bus cycle types: read, write and refresh. They must also meet specific pulse width, setup and hold timing requirements. DRAM interface designs need special attention to transmission line effects, since DRAMs represent significant loads on the bus.

DRAM controllers may be either clocked or unclocked. An unclocked DRAM controller requires a tapped digital delay line to derive the proper timings.

Clocked DRAM controllers may use either discrete or programmable logic devices. A state ma- chine design is appropriate, especially if the circuit must provide wait state control (beyond that possible with the processor’s Chip-Select Unit). Because of the microprocessor’s four-clock bus, clocking some logic elements on each CLKOUT phase is advantageous (see Figure 7-4).

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