NEC PD78212, User Manual

Page: 397 / 487

368
µ
PD78214 Sub-Series
13.5.3 Operation
(1) Pulse refresh operation
To support the pulse refresh cycle of pseudo static RAM, the REFRQ pin outputs refresh pulses, synchronized
with the bus cycle.
Adjust the oscillator frequency and bits 1 and 0 (RFT1 and RFT0) of the refresh mode register (RFM) so that
at least 512 refresh pulses are output in 8 ms.
Table 13-2 System Clock Frequency and Refresh Pulse Output Cycle When Pseudo Static RAM Is Used
64/f
CLK
32/f
CLK
16/f
CLK
System clock frequency
(f
CLK
) MHz
4.096 < f
CLK
≤
6
(8.192 < f
XX
≤
12)
2.048 < f
CLK
≤
4.096
(4.096 < f
XX
≤
8.192)
2 < f
CLK
≤
2.048
(4
≤
f
XX
≤
4.096)
Refresh pulse output cycle
0
1
0
1
0
0
RFT1 RFT0
Pulse refresh is controlled so that it does not overlap external memory access. During the refresh cycle, the
external memory access cycle is held pending (ASTB, RD, and WR are inactive). During the external memory
access cycle, the refresh cycle is held pending.
When pulse refresh does not cause a contention with external memory access, the refresh cycle can be
executed without affecting instruction execution by the CPU.
(a) Accessing internal memory
Even when external pseudo static RAM is not accessed, and internal memory is accessed, the refresh bus
cycle is output at the intervals specified by the RFM register, ensuring that the contents of pseudo static
RAM are retained.
Fig. 13-19 Pulse Refresh When Internal Memory Is Accessed
1t
CYC
Refresh cycle
REFRQ
pin output
t
CYC
: system clock cycle time (1/f
CLK
) [ns] (f
CLK
= 6 MHz)
Refresh timing counter