Programming Model

BCK5-BCK0The 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.

3

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.

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-ET0PROM 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).

ET2-

PROM Access <= N

PROM Access <= N

ET0

at 25 MHz where 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

 

 

 

http://www.mcg.mot.com/literature

3-39

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Image 227
Motorola MVME172 manual Prom Access Time Control Register, ET2 Prom Access = N, At 25 MHz where N = At 33 MHz where N =