SC68C752B datasheet(12/49 Pages) PHILIPS

12 / 49 page

SC68C752B_3

Product data sheet

Rev. 03 — 29 November 2005

12 of 49

Philips Semiconductors

SC68C752B

5 V, 3.3 V and 2.5 V dual UART, 5 Mbit/s (max.), with 64-byte FIFOs

6.3.3 Software ﬂow control example

6.3.3.1

Assumptions

UART1 is transmitting a large text ﬁle to UART2. Both UARTs are using software ﬂow

control with single character Xoff (0F) and Xon (0D) tokens. Both have Xoff threshold

(TCR[3:0] = F) set to 60, and Xon threshold (TCR[7:4] = 8) set to 32. Both have the

interrupt receive threshold (TLR[7:4] = D) set to 52.

UART1 begins transmission and sends 52 characters, at which point UART2 will generate

an interrupt to its processor to service the RX FIFO, but assume the interrupt latency is

fairly long. UART1 will continue sending characters until a total of 60 characters have

been sent. At this time, UART2 will transmit a 0Fh to UART1, informing UART1 to halt

transmission. UART1 will likely send the 61st character while UART2 is sending the Xoff

character. Now UART2 is serviced and the processor reads enough data out of the RX

FIFO that the level drops to 32. UART2 will now send a 0Dh to UART1, informing UART1

to resume transmission.

Fig 7.

Software ﬂow control example

TRANSMIT FIFO

PARALLEL-TO-SERIAL

SERIAL-TO-PARALLEL

Xon1 WORD

Xon2 WORD

Xoff1 WORD

Xoff2 WORD

RECEIVE FIFO

PARALLEL-TO-SERIAL

SERIAL-TO-PARALLEL

Xon1 WORD

Xon2 WORD

Xoff1 WORD

Xoff2 WORD

UART2

UART1

002aaa229

data

Xoff–Xon–Xoff

compare

programmed

Xon-Xoff

characters

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