Motorola DSP56301 - Manual (Page 99)

Bus Interface Unit (BIU) Registers
Core Configuration 4 -25
Table 4-10. DRAM Control Register (DCR) Bit Definitions
Bit
Number
Bit Name
Reset
Value
Description
23 BRP 0 Bus Refresh Prescaler
Controls a prescaler in series with the refresh clock divider. If BPR is set, a
divide-by-64 prescaler is connected in series with the refresh clock divider. If BPR is
cleared, the prescaler is bypassed. The refresh request rate (in clock cycles) is the
value written to BRF[7–0] bits + 1, multiplied by 64 (if BRP is set) or by one (if BRP is
cleared). When programming the periodic refresh rate, you must consider the RAS
time-out period. Hardware support for the RAS time-out restriction does not exist
.
Note: Refresh requests are not accumulated and, therefore, in a fast refresh request
rate not all the refresh requests are served (for example, the combination
BRF[7–0] = $00 and BRP = 0 generates a refresh request every clock cycle,
but a refresh access takes at least five clock cycles).
22–15 BRF[7–0] 0 Bus Refresh Rate
Controls the refresh request rate. The BRF[7–0] bits specify a divide rate of 1–256
(BRF[7–0] = $00–$FF). A refresh request is generated each time the refresh counter
reaches zero if the refresh counter is enabled (BRE = 1).
14 BSTR 0 Bus Software Triggered Reset
Generates a software-triggered refresh request. When BSTR is set, a refresh request
is generated and a refresh access is executed to all DRAM banks (the exact timing of
the refresh access depends on the pending external accesses and the status of the
BME bit). After the refresh access (CAS before RAS) is executed, the DRAM controller
hardware clears the BSTR bit. The refresh cycle length depends on the BRW[1–0] bits
(a refresh access is as long as the out-of-page access).
13 BREN 0 Bus Refresh Enable
Enables/disables the internal refresh counter. When BREN is set, the refresh counter is
enabled and a refresh request (CAS before RAS) is generated each time the refresh
counter reaches zero. A refresh cycle occurs for all DRAM banks together (that is, all
pins that are defined as RAS are asserted together). When this bit is cleared, the
refresh counter is disabled and a refresh request may be software triggered by using
the BSTR bit. In a system in which DSPs share the same DRAM, the DRAM controller
of more than one DSP may be active, but it is recommended that only one DSP have
its BREN bit set and that bus mastership is requested for a refresh access. If BREN is
set and a WAIT instruction is executed, periodic refresh is still generated each time the
refresh counter reaches zero. If BREN is set and a STOP instruction is executed,
periodic refresh is not generated and the refresh counter is disabled. The contents of
the DRAM are lost.
12 BME 0 Bus Mastership Enable
Enables/disables interface to a local DRAM for the DSP. When BME is cleared, the
RAS and CAS pins are tri-stated when mastership is lost. Therefore, you must connect
an external pull-up resistor to these pins. In this case (BME = 0), the DSP DRAM
controller assumes a page fault each time the mastership is lost. A DRAM refresh
requires a bus mastership. If the BME bit is set, the RAS and CAS pins are always
driven from the DSP. Therefore, DRAM refresh can be performed, even if the DSP is
not the bus master.