AMD Am79C930 manual Physical Header Accommodation, DC Bias Control, Baud Determination Logic

AMD P R E L I M I N A R Y

register of TCR4. ADIN2 becomes active after ADIN1 by the amount of delay specified in the RSSI Sample Start time of TCR24. ADIN2 remains active for the time pro- grammed in the A2DT register (TCR25). The converter output should be connected to the SAR pins, which act as inputs in this mode.

External D/A mode allows the user to connect an exter- nal D/A converter to the Am79C930 device. The SAR pins function as outputs and values written to the SAR register (TIR27) will be driven onto these pins for con- version by the external D/A device.

The following table indicates the programming required in order to effect each mode of the A/D section of the Am79C930 device:

ADDA	ENEXT	ENSAR	UXA2DST	A/D
TIR26[2]	TCR25[6]	TCR25[5]	TCR25[7]	mode
0	0	0	0	internal_A
0	0	0	1	reserved
0	0	1	0	internal_B
0	0	1	1	internal_C
0	1	0	X	external
0	1	1	X	reserved
1	X	0	X	reserved
1	X	1	X	D/A mode

Physical Header Accommodation

The Am79C930 device can accommodate physical header information by delaying the start of CRC8 and CRC32 calculations on outgoing and incoming frames, until a specified number of bytes beyond the Start of Frame Detection has become asserted. The length of the physical header may be anywhere from 0 to 15 bytes as indicated by the value in the PFL bits of TCR3.

DC Bias Control

An optional DC bias control circuit exists within the Am79C930 device. This circuit may be disabled through software control. The circuit uses 16-bit block inversion and bit stuffing to insure a proper DC balance to the out- going signal on transmit. Receive signals will automati- cally have the DC Bias Control removed before further operations inside of the Am79C930 device. Bit stuffing may begin with the first bit transmitted after SFD, or at the beginning of a programmable number of byte times following the SFD. Receive frames may be “de-stuffed” in a similar manner. DC Bias Control may be disabled for transmit through a control bit located in TCR1. DC Bias Control may be disabled for receive through a control bit located in TCR3. Bit stuffing start control is located in TCR2 [7].

Baud Determination Logic

The TAI contains Baud Determination logic that sam- ples the incoming bit stream to determine the data rate. The result of the Baud Determination is used in making decisions regarding Clear Channel Assessment and in selecting an antenna. The Baud Determination logic functions as follows:

Baud Determination testing is performed on a periodic basis, where the period is determined by the Antenna Diversity time of TCR4. Baud Determination is intended to be alternately performed on up to two separate anten- nas. The antenna diversity decision logic is coupled to the Baud Determination logic in such a manner that each successive set of Baud Determination tests is per- formed on alternating antenna selections. Baud Deter- mination continues for CCA when an antenna is chosen, but baud detect results will not affect antenna selection once an antenna has been locked. Baud detect tests continue with the periodicity of the dwell timer. Antenna diversity switching ceases when a satisfactory antenna has been found. See the section on Automatic Antenna Diversity logic for antenna selection criteria and testing. Antenna selection testing resumes following the asser- tion of either the RXRES bit (RX RESET) of TIR16 or the RXS bit (RX Start) of TIR16. This action causes the dwell timer to reset to the value found in TCR4 [5:0] and then to resume.

Because antenna switching can cause transient noise to appear at the RXD input of the Am79C930 device, the start of Baud Determination testing is delayed for a pe- riod of time immediately following the antenna switching process. In order to accommodate different transceiver/ antenna settling times, the amount of test start delay is programmable through the Baud Detect Start Timer of TCR16. Therefore, the duty cycle of the Baud Determi- nation test period (i.e., the portion of the period during which Baud test measurements are performed) is equal to the Antenna Diversity time of TCR4 minus the value of the Baud Detect Start time of TCR16, minus an addi- tional three CLKIN periods (6 CLKIN periods if CLKGT20=1). The three CLKIN periods are used for fi- nal calculations of Baud Determination, Clear Channel Assessment, and Antenna selection once a set of meas- urements has been taken and before a new cycle is al- lowed to begin.

The Baud Determination measurement process is con- ducted as follows:

Two counters track the separation between adjacent falling edges and adjacent rising edges of incoming receive data. One counter measures the separation between adjacent falling edges of incoming receive data, and the other counter measures the separation between adjacent rising edges of incoming receive data. Measurement resolution is equal to the CLKIN pe- riod with the CLKGT20 bit of MIR9 set to 0, and

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