SC16C652 datasheet(10/41 Pages) PHILIPS

10 / 41 page

Philips Semiconductors

SC16C652

Dual UART with 32 bytes of transmit and receive FIFOs

Product data

Rev. 04 — 20 June 2003

10 of 41

9397 750 11634

In the event that the receive buffer is overﬁlling and ﬂow control needs to be executed,

the SC16C652 automatically sends an Xoff message (when enabled) via the serial

TX output to the remote modem. The SC16C652 sends the Xoff1,2 characters as

soon as received data passes the programmed trigger level. To clear this condition,

the SC16C652 will transmit the programmed Xon1,2 characters as soon as receive

data drops below the programmed trigger level.

6.6 Special feature software ﬂow control

A special feature is provided to detect an 8-bit character when EFR[5] is set. When

8-bit character is detected, it will be placed on the user-accessible data stack along

with normal incoming RX data. This condition is selected in conjunction with

EFR[0-3]. Note that software ﬂow control should be turned off when using this special

mode by setting EFR[0-3] to a logic 0.

The SC16C652 compares each incoming receive character with Xoff2 data. If a

match exists, the received data will be transferred to the FIFO, and ISR[4] will be set

to indicate detection of a special character. Although the Internal Register Table

(Table 9) shows each X-Register with eight bits of character information, the actual

number of bits is dependent on the programmed word length. Line Control Register

bits LCR[0-1] deﬁne the number of character bits, i.e., either 5 bits, 6 bits, 7 bits or

8 bits. The word length selected by LCR[0-1] also determine the number of bits that

will be used for the special character comparison. Bit 0 in the X-registers corresponds

with the LSB bit for the receive character.

6.7 Hardware/software and time-out interrupts

The interrupts are enabled by IER[0-3]. Care must be taken when handling these

interrupts. Following a reset, if Interrupt Enable Register (IER) bit 1 = 1, the

SC16C652 will issue a Transmit Holding Register interrupt. This interrupt must be

serviced prior to continuing operations. The LSR register provides the current

singular highest priority interrupt only. It could be noted that CTS and RTS interrupts

have lowest interrupt priority. A condition can exist where a higher priority interrupt

may mask the lower priority CTS/RTS interrupt(s). Only after servicing the higher

pending interrupt will the lower priority CTS/RTS interrupt(s) be reﬂected in the status

result in data errors.

When two interrupt conditions have the same priority, it is important to service these

interrupts correctly. Receive Data Ready and Receive Time Out have the same

interrupt priority (when enabled by IER[3]). The receiver issues an interrupt after the

number of characters have reached the programmed trigger level. In this case, the

SC16C652 FIFO may hold more characters than the programmed trigger level.

Following the removal of a data byte, the user should re-check LSR[0] for additional

characters. A Receive Time Out will not occur if the receive FIFO is empty. The

time-out counter is reset at the center of each stop bit received or each time the

receive holding register (RHR) is read. The actual time-out value is 4 character time,

including data information length, start bit, parity bit, and the size of stop bit, i.e., 1

×,

1.5

×, or 2× bit times.

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