Data Sampling / Voting; Start Bit Detection; Parity Error Detection; Figure 25-7 Receive State Machine Diagram - Motorola DragonBall MC9328MX1 Reference Manual

Integrated portable system processor
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25.3.3.2 Data Sampling / Voting

The receive state machine runs at the receiver clock rate (RCV_CK). This clock is oversampling the
received data at a sample rate of either 16X or 12X. For each input bit, the receive state machine captures
three samples. A majority voting algorithm is then applied to determine the value of the bit received. The
value common to two or more of the samples is selected as the bit value in the receive shift register.

25.3.3.3 Start Bit Detection

The SIM receive input is defined as high when not active. The data transmission is defined as beginning
with a low pulse for a bit duration. This is called the start bit. The receive state machine is responsible for
detecting and validating the start bit. The receive state machine samples the start bit three times and uses a
majority voting scheme to determine if the start bit is valid. This effectively filters out any low receive
inputs shorter than one RCV_CK period. Figure 25-8 shows a typical SIM data transaction with the start
bit identified.
LSB
Start

25.3.3.4 Parity Error Detection

The receive state machine is responsible for detecting parity errors in the received data. Data is always
transmitted with even parity, except when in inverse convention mode. In inverse convention mode, all
data bits and the parity bit are complemented making the data parity appear to be odd. The parity bit is
defined as the 10th bit of the received data. The parity of the 2nd through 10th received bits is calculated
by the receiver parity logic. This logic determines if the parity of the 9 received bits is correct. Figure 25-9
shows a typical SIM data transaction with the parity bit identified.
MOTOROLA
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Figure 25-7. Receive State Machine Diagram
MSB
Byte
i
Figure 25-8. Start Bit Diagram
SmartCard Interface Module (SIM)
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Start Bit
P
Stop Bits
Start
Functional Description
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25-11

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