5

MCECC

Difference from MEMC040: bit = DMCTL (data-mux- control) for MEMC040; bit = RWB (general purpose read write bit) for MCECC (data-mux-control not required for MCECC pair).

BAD22, BAD23

These are the lower two bits of the DRAM base address described in the previous register.

Difference from MEMC040: none.

BCLK Frequency Register

The Bus Clock (BCLK) Frequency Register should be programmed with the hexadecimal value of the operating clock frequency in MHz (i.e., $19 for 25 MHz and $21 for 33 MHz). The MCECC pair uses the value programmed in this register to control the Prescaler Counter. The Prescaler Counter increments to $FF and then it is loaded with the two’s compliment of the value in the BCLK Frequency Register. This produces a 1 MHz clock that is used by the refresh timer and the scrubber. When the BCLK Frequency Register is correctly programmed with the BCLK frequency, the DRAMs are refreshed approximately once every 15.6 microseconds. After power-up, this register is initialized to $19 (for 25 MHz).

Difference from MEMC040: none.

Note This register is configured only during power-up-reset and is unchanged by software or local reset.

ADR/SIZ

 

 

1st $FFF4301C/2nd $FFF4311C (8-bits)

 

 

 

 

 

 

 

 

 

 

 

BIT

31

30

 

29

28

27

26

25

24

 

 

 

 

 

 

 

 

 

 

NAME

BCK7

BCK6

 

BCK5

BCK4

BCK3

BCK2

BCK1

BCK0

 

 

 

 

 

 

 

 

 

 

OPER

R/W

R/W

 

R/W

R/W

R/W

R/W

R/W

R/W

 

 

 

 

 

 

 

 

 

 

RESET

0 P

0 P

 

0 P

1 P

1 P

0 P

0 P

1P

 

 

 

 

 

 

 

 

 

 

5-20

Computer Group Literature Center Web Site

Page 310
Image 310
Motorola MVME172 manual Bclk Frequency Register