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Case Studies
Fred Beckhusen-Z80 Power-DRAM Test

How does DRAM refresh itself?

I wrote this DRAM test to test this program in 1979 and  ran it on a 2.5 Mhz Z80. Threw the CPU and card cage in a temperature chamber and let it run all morning.  It had some interesting and practical results.  It turns off the memory refresh by banging on the R ( Refresh ) register  as fast as it can for 1 millisecond, then tests memory for any memory cells that have discharged. If any bit fails it prints it.

August 1979 Mostek Microcomputer Newsletter

At last, a program that used the R register!

When Z80 Systems use the processor generated REFRESH signal to refresh dynamic RAMs, the Z80 must be able to execute op codes. Several factors can cause the execution of opcodes to be inhibited and will destroy the contents of dynamic memory.

These are:

(1) Prolonged reset greater than 1ms

(2) Prolonged wait state greater than 1ms

(3) Prolonged bus acknowledge greater than 1ms.

(4) Clock period less than 2.432 Mhz (some 64k RAMS)

less than 1.216 Mhz (most 16k RAMS)

less than .608 Mhz (all 4k RAMS)

The situation gets worse if combinations of wait states and bus acknowledges (DMA) are added. Add in the effects of higher temperature on refresh characteristics, and the system designer can no longer be confident that the Z80 will perform reliably in worst case conditions. Since possible combinations of #2, #3, and #4 are difficult to calculate, an alternative method of determining system performance is desirable.

The first of two programs presented are designed to quantitatively measure the refresh interval of dynamic RAM that uses the Z80 generated REFRESH signal.

The program inhibits refresh for progressively longer periods of time by repeatedly loading the R register with 00 until memory fails. Because of this, the program must run in PROM or static RAM while testing. Any errors due to bits being dropped force the CPU to store the first failing address, data, and refresh period in memory locations ADDR (0002H), DATA (0004H), and TIME (000H).

To test the program, a Mostek MDX system was heated in a test chamber and the resulting output of the program (X-AXIS) was plotted against the temperature (Y-AXIS) as shown in Figure 1. The system remained operational at 100 degrees C, but as the graph clearly shows, operation would not be reliable as the slope of the curve gets very steep at the higher temperatures.

Other system parameters affect Z80 system performance. With high temperature, bus drivers and receivers slow down, and RAM access times rise. All these degrade system performance. Compounding this problem on the Z80, opcode fetches, (M1 cycles) occur in 1/2 less clock cycles than normal memory read operations. This means that conventional store/read diagnostics cannot test a Z80 memory, either dynamic or static, at the memorys full access time requirement. For example, a 4 MHz Z80 will need memory data in 240 nanoseconds (worst case) during opcode fetches, but conventional Z80 memory diagnostic tests only require a 345 nanosecond access time (worst case. access time for a Z80 memory read cycle). This is similar to the 8080 problem where stack operations (push, pop) occur faster than normal memory references. Clearly, a good Z80 memory diagnostic must execute each memory location, fetching both 0’s and 1’s, and insure that the correct opcode was fetched (executed).

The second of the two programs executes every available memory location using four routines. The program tests 1/2 the memory for correct operation fetching 0’s, then the other half for 0’s, then two tests that check correct operation of each half fetching all 1’s. THe program sequentially generates the instructions. CB 00 and CB FF, RLC B and SET 7, A. As both bytes of these opcodes are fetched with an M1 cycle at the fast rate, half the memory will be tested for O’s or 1’s being correctly fetched. If opcodes other than RLC B and SET 7, A are fetched, then the B register will not be shifted or the A register will not have bit 7 set, and the program will halt, leaving the error address and error data in memory locations ADDR (0080H) and EXP (0092H).

Mostek MK 4096P

Dram Refreshes Itself

Computers use random access memory, or RAM, as their workspace when running programs, and RAM chips today come in two flavors—static and dynamic RAM, or SRAM and DRAM. As long as the computer is powered up, SRAM will retain its contents, while DRAM must be constantly refreshed. The advantage of DRAM over SRAM is that the simplicity of each memory cell means a lot more data can be packed into a given space. Today most computers use DRAM for their main memory.

Future Chip

The first DRAM chip was put out by Intel. But Mostek’s 4-kilobit DRAM chip brought about a key innovation, a circuitry trick called address multiplexing, concocted by Mostek cofounder Bob Proebsting. Basically, the chip used the same pins to access the memory’s rows and columns. It did so by sending row and column addressing signals in turn, rather than all at once as in earlier designs. As a result, the chip didn’t require more pins as memory density increased and could be made for less money. There was just a little compatibility problem. The 4096 used 16 pins, whereas the memories made by Texas Instruments, Intel, and Motorola had 22 pins. What followed was one of the most epic face-offs in DRAM history. With Mostek betting its future on the chip, its executives set out to proselytize customers, partners, the press, and even its staff. Fred K. Beckhusen, who as a recent hire was drafted to test the 4096 devices, recalls when Proebsting and chief executive L.J. Sevin came to his night shift to give a seminar—at 2 a.m. “They boldly predicted that in six months no one would hear or care about 22-pin DRAM,” Beckhusen says. They were right. The 4096 and its successors became the dominant DRAM for years, and address multiplexing became the standard way to handle ever larger memories.The 4096 used 16 pins, whereas the memories made by Texas Instruments, Intel, and Motorola had 22 pins. What followed was one of the most epic face-offs in DRAM history.