3.6 Dynamic raMs

As we wish to maximize the number of cells in a memory, every effort must be made to implement them as simply as possible. They normally consist of just a few transistors [3.1]; in the case of static CMOS RAMs, a 6-transistor cell is normally used. In the simplest case, even the flip-flop is omitted and replaced by a MOSFET whose gate-source capacitance is used to store 1 bit as a charge.

Fig. 3.34 - Address decoding in a dynamic 1 Mbit memory.

RAS: Row Address Strobe (simultaneously Chip Enable)

CAS: Column Address Strobe

This makes a single-transistor cell possible. However, as the charge is only Stained for a short time, the capacitor must be recharged at regular intervals (every 2 to 8 ms approx). This operation is known as refresh, the memories are ailed dynamic RAMs.

This disadvantage is offset by several advantages. Dynamic memories can provide about four times more storage capacity on the same printed-circuit ward area, with the same current drain and at the same cost.

To save on pins, with dynamic memories the address is entered in two stages and buffered in the IC.

The block diagram of a 1 Mbit RAM is shown in Fig. 3.34. In the first step, address bits a₀ to a₈ are stored in the row-address latch with the RAS signal, and simultaneously bit a₉ in the column-address latch. In the second step, address bits a₁₀ to a₁₉ are loaded into the column-address latch with the CAS signal. This makes it possible to accommodate a 1 Mbit memory in an 18-pin package. Figure 3.35 lists commonly used IC types.

Capacity	Organization	Type	Manufacturer	Operating power, typical	Access time, max.	Pins
CMOS: (VDD = 5V, f = fmax)
256 kbit	256k x 1	51256	Hitachi	200 mW	100ns	16
	64k x 4	51464	Hitachi	250 mW	100ns	18
1 Mbit	1M x 1	511000	Toshiba	300 mW	80ns	18
	256k x 4	514256	Toshiba	300 mW	80ns	20
	128k x 8	6581282	Hitachi	200 mW	80ns	32
4 Mbit	4M x 1	514100	Toshiba	400 mW	80ns	20
	1M x 4	514400	Toshiba	400 mW	80ns	20
	512k x 8	514800	Hitachi	400 mW	80ns	28
	512k x 8	6585122	Hitachi	350 mW	80ns	32
9 Mbit	1M x 9	THM 910003	Toshiba	2500 mW	80ns	30
36 Mbit	4M x 9	THM 940003	Toshiba	3500 mW	80ns	30
	1M x 36	HB56 D136B3	Hitachi	3500 mW	80ns	72
72 Mbit	2M x 36	HB56 D236B3	Hitachi	4000 mW	80ns	72

Fig. 3.35 - Examples of dynamic RAMs.

¹ Other manufacturers: Fujitsu, Hitachi, NEC, Oki, Texas Instr.

² Pseudo-static since it has an integrated refresh controller

³ Hybrid circuit (module)