ST20-GP1 datasheet(74/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Figure 13.2 8-bit data frames

9-bit data frames

9-bit data frames consist of:

•

nine data bits D0-8;

•

eight data bits D0-7 plus an automatically generated parity bit;

•

eight data bits D0-7 plus a wake-up bit.

Parity may be odd or even, depending on the ParityOdd bit in the ASCControl register. An even

parity bit will be set, if the modulo-2-sum of the eight data bits is 1. An odd parity bit will be cleared

in this case. The parity error flag (ParityError) will be set if a wrong parity bit is received. The parity

bit itself will be stored in bit 8 of the ASCRxBuffer register, see Table 13.4.

In wake-up mode, received frames are only transferred to the receive buffer register if the ninth bit

(the wake-up bit) is 1. If this bit is 0, no receive interrupt request will be activated and no data will

be transferred. This feature can be used to control communication in multi-processor systems.

When the master processor wants to transmit a block of data to one of several slaves, it first sends

out an address byte which identifies the target slave. An address byte differs from a data byte in

that the additional ninth bit is a 1 for an address byte and a 0 for a data byte, so no slave will be

interrupted by a data byte. An address byte will interrupt all slaves (operating in 8-bit data + wake-

up bit mode), so each slave can examine the 8 least significant bits (LSBs) of the received

character (the address). The addressed slave will switch to 9-bit data mode, which enables it to

receive the data bytes that will be coming (with the wake-up bit cleared). The slaves that are not

being addressed remain in 8-bit data + wake-up bit mode, ignoring the following data bytes.

2nd

stop

bit

1st

stop

bit

start

bit

(LSB)

8th

bit

• Data bit (D7)

• Parity bit

ST20-GP1 Datasheet(PDF) 74 Page - STMicroelectronics