Motorola MVME172 - Manual (Page 227)

Programming Model
http://www.mcg.mot.com/literature 3-39
3
BCK5-BCK0 The refresh rate is defined by the following equation:
Refresh Rate = BCK/BUS CLOCK * 16
where BCK is the value programmed in the Bus Clock
Register, and BUS CLOCK is the MC68xx060 bus clock
frequency.
PROM Access Time Control Register
The MVME172 is populated with a 150ns PROM memory device. Due to
the wide range of PROM speeds, the contents can be changed by software
to adjust for a specific speed.
ET2-ET0 PROM access time is controlled by the state of ET2-ET0.
The following table defines the ET2-ET0 encoding (note
that for the MVE172, whose bus frequency is
1 /
2
the
processor frequency, only the 33MHz column applies).
ADR/SIZ $FFF42040 (8 bits)
BIT 23 22 21 20 19 18 17 16
NAME ROM0 ET2 ET1 ET0
OPER R R R R/W R R/W R/W R/W
RESET 0 0 0 1 PL 0 1 PL 1 PL 1 PL
ET2-
ET0
PROM Access <= N
at 25 MHz where N =
PROM Access <= N
at 33 MHz where N =
$0 60 ns 40 ns
$1 100 ns 70 ns
$2 140 ns 100 ns
$3 180 ns 130 ns
$4 220 ns 160 ns
$5 260 ns 190 ns
$6 300 ns 210 ns
$7 340 ns 240 ns