Accessing the OnCE Module

Figure 12-21. Executing a OnCE Command by Reading the OCR

In the first shift sequence, the 8-bit shifter is selected. Whenever the 8-bit shifter is selected, the OCMDR is written (on Update-DR) with the value shifted into TDI. In this case, $82 is shifted in. This is the OnCE opcode for Read OCR. Similarly, whenever the 8-bit shifter is selected, it captures the value of the OSR when passing through Capture-DR. This value is then shifted out of TDO in the ensuing shift. In this case, $18 was shifted out, indicating that the DSP is in debug mode.

When Update-DR is passed through in an OCMDR write, the OnCE module begins decoding the opcode in the OCMDR. During command decoding, the OnCE module determines whether a 16-bit shift is to occur (in this case, yes) and, if so, whether it is a read or write. If it is a read, the register selected by the RS field in the OCMDR is captured in the 16-bit shifter on Capture-DR. If it is a legal write, the selected register is written on Update-DR following the 16-bit shift.

12.10.4.3 Executing a OnCE Command by Writing the OCNTR

In this sequence, the 8-bit OCNTR is written. First the Write OCR opcode is entered, followed by a 16-bit shift sequence, even though the OCNTR is only 8 bits long. If a selected register is less than 16 bits, it always reads to or writes from the LSB of the 16-bit shifter. The initial setup is identical to the previous example. See Figure 12-22.