Port Number: 09h
Function: Link Assist Status/CPU Speed 0 Signaling Rate
This port gives information on the status of the hardware link assist.
- Bit 0: Set if an interrupt was generated by receiving a byte.
- Bit 1: Set if an interrupt was generated when a byte can be sent.
- Bit 2: Set if an interrupt was generated due to an error in transmission.
- Bit 3: Set if the assist is currently receiving data
- Bit 4: Set if the assist has read a complete byte (which can be read from port 0A)
- Bit 5: Set if the assist is ready to write data (via port 0D)
- Bit 6: Set if there was an error during transmission
- Bit 7: Set if the link assist is currently sending a byte.
Writing to this port sets the signaling rate of the link assist in CPU speed mode 0 (6 MHz); the link assist uses the same clock as the CPU. Bits 5 through 7 control a divisor to the clock before it reaches the link assist, affecting how fast the link assist can detect changes in the signal states. It will divide by 2^n, allowing you to divide by 1, 2, 4 ... 64. Value 111b (7) will halt the link assist. Bits 0 through 4 control how much time to wait between bits. TI desires a minimum time between bits of 2 microseconds.
This port only exists on the 83+ SE and the 84+.
Credits and Contributions
- Michael Vincent: Original documentation of the link assist