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A8251

Programmable Communications Interface

Altera Corporation 

a8251 Programmable Communications Interface Data Sheet

For example, after detecting a logic low in divide-by-1 mode, the a8251

assumes data is available on the next rising edge. However, after

detecting a logic low in divide-by-16 mode, the a8251 counts 8 nrxc

edges and samples again. The data must still be a logic low. At this point,

the a8251 assumes the data and clock are synchronized, and samples

data every 16 clock edges thereafter. Divide-by-64 mode is similar to

divide-by-16, with the start bit sampled at the first rising edge and the

32nd rising edge of nrxc. Data is then sampled every 64 rising edges.

Data Bit Sampling

After detecting a start bit, the a8251 samples and shifts the data into the

shift register. Data bit sampling occurs on every rising edge in divide-by-

1 mode, every 16 rising edges in divide-by-16 mode, and every 64 rising

edges in divide-by-64 mode. Each time a bit is sampled, parity is

calculated for future error detection. See Figure 3.

Figure 3. Receiver Clock Signals

nrxc (Divide-by-1)

nrxc (Divide-by-16)

Sampling Pulse

(Divide-by-16)

Parity/Stop Bit Detection

The a8251 counts the number of data bits as it shifts. When the number

of data bits received matches the number specified in the control register,

the a8251 expects either a parity bit or a stop bit.

If parity is enabled, the a8251 samples for the parity bit, which is

processed for parity but is not shifted into the shift register. After the

parity bit, or after the last data bit if parity is not enabled, the a8251

expects a stop bit (i.e., logic high). If a logic low is sampled, the fe bit is

set in the status register.

The a8251 receives data with one or two stop bits. If one stop bit is

specified in the control register, the a8251 will expect one stop bit before

starting the synchronization process. Similarly, if two stop bits are

specified, the synchronization process begins after detecting two logic

highs.

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Altera Corporation

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