AT89LP428 datasheet(84/149 Pages) ATMEL

84 / 149 page

3654A–MICRO–8/09

AT89LP428/828

Figure 16-2. Mode 0 Waveforms

Mode 0 may be used as a hardware accelerator for software emulation of serial interfaces such

as a half-duplex Serial Peripheral Interface (SPI) in mode (0,0) or (1,1) or a Two-wire Interface

(TWI) in master mode. An example of Mode 0 emulating a TWI master device is shown in Figure

16-3. In this example, the start, stop, and acknowledge are handled in software while the byte

transmission is done in hardware. Falling/rising edges on TXD are created by setting/clearing

SM2. Rising/falling edges on RXD are forced by setting/clearing the P3.0 register bit. SM2 and

P3.0 must be 1 while the byte is being transferred.

Figure 16-3. UART Mode 0 TWI Emulation (SMOD1 = 1)

Mode 0 transfers data LSB first whereas SPI or TWI are generally MSB first. Emulation of these

interfaces may require bit reversal of the transferred data bytes. The following code example

reverses the bits in the accumulator:

EX:

MOV

R7, #8

REVRS: RLC

; C << msb(ACC)

XCH

A, R6

RRC

; msb(ACC) >> B

XCH

A, R6

DJNZ R7, REVRS

RXD (TX)

TXD

RXD (RX)

01234567

RXD (TX)

TXD

RXD (RX)

01234567

RXD (TX)

TXD

RXD (RX)

01234567

RXD (TX)

TXD

RXD (RX)

01234567

SMOD1 = 0

SM2 = 0

SMOD1 = 1

SM2 = 0

SMOD1 = 0

SM2 = 1

SMOD1 = 1

SM2 = 1

(SDA) RXD

(SCL) TXD

ACK

SM2

P3.0

Write to SBUF

Sample ACK

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