
IBM2520L8767
IBM Processor for ATM Resources
atmrm.chapt04.01
08/27/99
The DRAM Controllers (COMET/PAKIT)
Page 141 of 553
Memory Controlling Entities
Entity 5: The DRAM Controllers (COMET/PAKIT)
This section describes the function of the COMET/PAKIT entities. COMET is the memory controller for con-
trol memory and PAKIT is the memory controller for packet memory.
Each controller can support the following types of memory:
Extended Data Out (EDO) DRAM with a 60ns row access time and a 30ns page cycle. Supports memory
sizes of 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, 128MB.
Synchronous DRAM’s running at 66MHz (15ns cycle time) with a CAS latency of 2 or 3, a burst length of
1 or 2. Supports memory sizes of 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, 128MB. Note that the
cycle time of the SDRAM clock is a constant on the IBM2520L8767. Any SDRAM part selected must be
capable of running at 66MHz or faster at the desired CAS latency.
Synchronous SRAM running at 66MHz (15ns cycle time) with a read latency of 1 or 2 and a write latency
of 0 or 1 (late write). Supports memory sizes of 1MB, 2MB, 4MB, or 8MB.
For any memory configuration, modules must be selected such that the loading on any memory net
(including card wiring) does not exceed 120pF.
The number of column address lines is programmable, allowing both DRAMs with symmetric address (same
number of row and column address lines) and asymmetric (typically having more row than column address
lines).
The memory may be operated with one RAS line or two. If the memory is configured to have two RAS lines
(arrays), the memory address range is split equally between the two RAS lines (arrays).
Memory checking can be enabled/disabled and the method of checking selected can be either ECC or parity.
IF ECC is selected, seven data bits are used for ECC over the 32 data bits. If parity is selected, four data bits
are used to provide parity over the 32 data bits.
COMET/PAKIT are designed so that memory contents will be preserved over a reset. If the IBM2520L8767 is
reset while a memory write cycle is in progress, the cycle will be completed in an orderly fashion to ensure
that valid ECC/parity is written. Memory timings are not violated when reset goes active. Refresh is main-
tained during the reset.