8.3
CPU Interface
The free-running counter (FRC), output compare registers (OCRA and OCRB), and input capture
registers (ICRA to ICRD) are 16-bit registers, but they are connected to an 8-bit data bus. When
the CPU accesses these registers, to ensure that both bytes are written or read simultaneously, the
access is performed using an 8-bit temporary register (TEMP).
These registers are written and read as follows:
•
Register Write
When the CPU writes to the upper byte, the byte of write data is placed in TEMP. Next, when
the CPU writes to the lower byte, this byte of data is combined with the byte in TEMP and all
16 bits are written in the register simultaneously.
•
Register Read
When the CPU reads the upper byte, the upper byte of data is sent to the CPU and the lower
byte is placed in TEMP. When the CPU reads the lower byte, it receives the value in TEMP.
Programs that access these registers should normally use word access. Equivalently, they may
access first the upper byte, then the lower byte by two consecutive byte accesses. Data will not be
transferred correctly if the bytes are accessed in reverse order, or if only one byte is accessed.
Figure 8.3 shows the data flow when FRC is accessed. The other registers are accessed in the same
way. As an exception, when the CPU reads OCRA or OCRB, it reads both the upper and lower
bytes directly, without using TEMP.
Coding Examples
To write the contents of general register R0 to OCRA:
To transfer the contents of ICRA to general register R0:
MOV.W R0, @OCRA
MOV.W @ICRA, R0
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