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S1D13505 Hardware Functional Specification
X23A-A-001-14 Issue Date: 01/02/02
Note
Changing this register to non-zero value, or to a different non-zero value, should be done only
when there are no read/write DRAM cycles. This condition occurs when all of the following are
true: the Display FIFO is disabled (REG[23h] bit 7 = 1), and the Half Frame Buffer is disabled
(REG[1Bh] bit 0 = 1), and the Ink/Cursor is inactive (Reg[27h] bits 7-6 = 00). This conditi on also
occurs when the CRT and LCD enable bits (Reg[0Dh] bits 1-0) have remained 0 since chip reset.
For further programming information, see S1D13505 Programming Notes and Examples, docu-
ment number X23A-G-003-xx.
bit 7 Reserved
bits 6-5 RC Timing Value (NRC) Bits [1:0]
These bits select the DRAM random-cycle timing parameter, tRC. These bits specify the number
(NRC) of MCLK periods (TM) used to create tRC. NRC should be chosen to meet tRC as well as
tRAS, the RAS pulse width. Use the following two formulae to calculate NRC then choose the larger
value. Note, these formulae assume an MCLK duty cycle of 50 +/- 5%.
NRC = Round-Up (tRC/TM)
NRC = Round-Up (tRAS/TM + NRP)if N
RP = 1 or 2
= Round-Up (tRAS/TM + 1.55) if NRP = 1.5
The resulting tRC is related to NRC as follows:
tRC = (NRC) TM
Performance Enhancement Register 0
REG[22h] RW
Reserved RC Timing
Value Bit 1 RC Timing
Value Bit 0
RAS#-to-
CAS# Delay
Value
RAS#
Precharge
Timing Value
Bit 1
RAS#
Precharge
Timing Value
Bit 0
Reserved Reserved
Table 8-12: Minimum Memory Timing Selection
REG[22h] bits [6:5] NRC Minimum Random Cycle
Width (tRC)
00 5 5
01 4 4
10 3 3
11 Reserved Reserved