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resolution is equal to twice the CLKIN period when the CLKGT20 bit of MIR9 is set to 1. (For a 1 MB data rate with CLKIN = 20 MHz and CLKGT20 = 0, resolution is 50 ns.) After each pair of rising edges is detected, the value of the rising edge separation counter is compared against the Baud Detect upper limit register value (TCR17) and also against the Baud Detect lower limit register value (TCR18). If the rising edge counter value is between these limits, then a GOOD counter is incre- mented. If the rising edge counter value is outside of these limits, then a FAIL counter is incremented.
A similar comparison is made whenever the falling edge detection circuit locates a pair of falling edges. The GOOD counter and the FAIL counter are shared by both the rising edge and falling edge measurement circuits. Both rising edge measurements and falling edge meas- urements will contribute to the total GOOD and FAIL counts during any Baud Determination cycle period. Note that the falling and rising edge separation counters begin counting at 0 and count up to 30 decimal and then wrap back to 10 decimal before continuing. This means that all multiples of 20 counts are aliased to a final counter indication of 20. Neither of the rising or falling edge separation counters is accessible to the user.
At the end of any Baud Determination cycle, the value in the GOOD counter is compared against the Baud De- tect Accept Count for Carrier Sense (TCR19). If the GOOD count is less than the value of TCR19, then the Baud Detect determination of Carrier Sense is uncondi- tionally FALSE. If the GOOD count exceeds the value of TCR19, then the GOOD count is compared against the value of the Baud Detect Ratio register (TCR21) multi- plied by the FAIL count. If the GOOD count exceeds this value, then the Baud Detect determination of Carrier Sense is TRUE. The Baud Detect determination of Car- rier Sense may, in turn, be used in the final determina- tion of Carrier Sense (Clear Channel Assessment), depending upon the setting of the UBDCS bit of TCR28.
At the end of any Baud Determination cycle, the value in the GOOD counter is compared against the Baud De- tect Accept Count for Stop Diversity (TCR20). If the GOOD count is less than the value of TCR20, then the Baud Detect determination for Stop Diversity Switching is unconditionally FALSE. If the GOOD count exceeds the value of TCR20, then the GOOD count is compared against the value of the Baud Detect Ratio register (TCR21) multiplied by the FAIL count. If the GOOD
count exceeds this value, then the Baud Detect determi- nation for Stop Diversity is TRUE. The Baud Detect de- termination for Stop Diversity may in turn, be used in the final determination of antenna selection, depending upon the setting of the UBDSD bit of TCR28.
Clear Channel Assessment Logic
The Am79C930 device gathers CCA information from one of two possible sources. One source is the Am79C930 device's internal CCA logic, which is de- scribed in the following paragraphs. The other possible CCA source is externally computed CCA information, which is then passed into the Am79C930 device through the USER5/IRQ4/EXTCHBSY pin. Regardless of the source of CCA information, a path through the Am79C930 TAI section is provided allowing the embed- ded 80188 controller to either poll the status of the CCA result or to be interrupted by any change to the CCA status, or to be interrupted whenever the CCA status changes to the “Busy” state. Selection of CCA source is through the ENXCHBSY bit of TCR15.
The TAI contains CCA logic that relies on two inputs to determine whether or not a carrier is present on the me- dium. One, both, or none of the two inputs may be se- lected to determine whether or not a carrier is present. One input that may be used to determine carrier sense is the result of the Baud Determination of Carrier Sense as described in the Baud Determination logic section. The other input used by the CCA logic is whether or not the value of the converted RSSI input exceeds a pro- grammed lower limit (RSSI Lower Limit of TIR28).
Note that Baud Determination of Carrier Sense meas- urements are made on a periodic basis where the period and duty cycle of the measurements depends upon the settings of the Antenna Diversity Timer (TCR4) and the Baud Detect Start timer (TCR16).
Either input or both inputs may be used to make the CCA decision. Each input to the CCA logic is enabled by a specific bit of TCR28. The UBDCS bit of TCR28 is used to select/deselect the Baud Determination of Carrier Sense for use in CCA decisions, and the URSSI bit of TCR28 is used to select/deselect RSSI information in CCA decisions. Note that URSSI bit of TCR28 is also used to select/deselect RSSI information for use in Stop Diversity decisions.
The possible CCA results are as follows.
Am79C930 | 53 |
