Operating Mode Register (Omr) - Motorola DSP56156 Manual

as a 6-bit counter which addresses (selects) a fifteen location stack with its four least sig-
nificant bits.
1.6.3.7

Operating Mode Register (OMR)

The OMR is a 16-bit register which defines the current chip operating mode of the proces-
sor. The OMR bits are only affected by processor reset and by instructions which directly
reference the OMR.
UF SE P3 P2 P1 P0
1 1 1 1
1 1 1 1
0 0 0 0
0 0 0 0
...
...
0 0 1 1
0 0 1 1
0 1 0 0
0 1 0 0
During processor reset, the chip operating mode bits will be loaded from the external
Mode Select pins (see Table 1-7 and Table 1-9). The operating mode register format is
shown in Figure 1-12.
The Mode C bit (MC) selects the initial bus operating mode. When set, the DSP is pro-
grammed for the master mode (BR output and BG input) and when cleared, the DSP is
programmed for the slave mode (BR input and BG output).
The Saturation bit (SA), when set, selects automatic saturation on 32 bits for the results
from the MAC array back to the accumulator. This saturation is done by a special satura-
tion circuit inside the MAC unit. The purpose of this bit is to provide a saturation mode for
16-bit algorithms which do not recognize or cannot take advantage of the extension accu-
mulator. The saturation logic operates by checking three bits of the 40-bit result: two bits
of the extension byte (exp[7] and exp[0]) and one bit on the MSP (msp[15]). The result
obtained in the accumulator when SA =1 is shown in Table 1-10.
The Rounding bit (R)selects between convergent rounding and two's complement round-
ing. When set, two's complement rounding (always round up) is used.
The Stop Delay bit (SD) is used to select the delay that the DSP needs to exit the STOP
mode.
MOTOROLA
PROGRAMMING MODEL
Table 1-7 Stack Pointer Values
CAUSE
← Stack Underflow condition after double pull.
1 0
← Stack Underflow condition.
1 1
← Stack Empty (reset). Pull causes underflow.
0 0
← stack location 1.
0 1
← Stack location 14.
1 0
← Stack location 15 (stack full). Push causes overflow.
1 1
← Stack overflow condition.
0 0
← Stack Overflow condition after double push.
0 1
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