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7-19

Z16C30 USC

USER'S MANUAL

ZILOG

UM97USC0100

the corresponding bit in the Transmit Command/Status

corresponding TCSR bit has no effect on the IP bit and thus

will not cause interrupts. The setting of the IA bits in TICR

has no direct effect on the TCSR bits.

When software wants to change the IA bits in the TICR after

the register is first initialized, it should write only the LS byte

of the register rather than all 16 bits, to avoid inadvertently

changing a threshold setting in the MS byte.

7.11.4 Transmit Data Interrupts

This interrupt type has only one source, so there’s no need

for an IA bit for it. The interrupt logic sets the Transmit Data

IP bit whenever the number of empty character positions

in the TxFIFO is greater than the number programmed as

the “Transmit Data Interrupt Request Level”. If transmitted

data is to be handled by an external Transmit DMA chan-

nel, disable this interrupt by leaving its IE bit 0. (A later

section discusses IE bits.)

To program the Transmit Data Interrupt Request Level, first

write the “Select TICRHi=/INT Level” command ( 0110) to

the TCmd field of the Transmit Command / Status Register

(TCSR15-12). Then write the number of empty character

positions at which the channel should start requesting a

Transmit Data interrupt, minus one, to the MSByte of the

Transmit Interrupt Control Register (TICR). For example, if

the channel should request a Transmit Data interrupt when

its 32-byte TxFIFO has only four characters left in it, write

hex 60 to TCSR15-8, then write decimal 27 (hex 1B) to

TICRI5-8.

RxCUp

14 13 12 11 10 9 8 7 6 5 4 3 2 1 015

RxCDn

TxRDn

TxCDn

CTSUp

DCDDn

RCC

Under

TxCUp

RxRDn

RxRUp

TxRUp

DCDUp

CTSDn

DPLL

DSync

BRG1

BRG0

Figure 7-14. The Status Interrupt Control Register (SICR)

/RxC

14 13 12 11 10 9 8 7 6 5 4 3 2 1 015

RxCL/U TxRL/UTxCL/U

/CTS

DCDL/U

RCC

Under

L/U

/TxC

RxRL/U /RxREQ /TxREQ

/DCD

CTSL/U

DPLL

DSync

L/U

BRG1

L/U

BRG0

L/U

Figure 7-15. The Miscellaneous Interrupt Status Register (MISR)

It is good programming practice to follow these two steps

with writing a “Select TICRHi=FIFO Status” command to

the TCSR, to protect the Request Level from inadvertent

modification when other parts of the software change the

IA bits in the TICR.

Code that writes or reads the Transmit Data Interrupt

Request threshold must ensure that no interrupt will occur

between the time it writes the “Select TICRHi=/INT Level”

command to the TCSR, and when it writes or reads the

value in the TICR, if such interrupts can lead to other code

writing a different Select command (for the FIFO Fill level

or DMA request threshold) to the TCSR.

Note that a Purge Tx FIFO (or Purge Rx and Tx FIFO)

command will typically make a channel immediately set its

Transmit Data IP bit. This will, in turn, make the channel

start requesting an interrupt on its /INT pin if:

■ it hadn’t been doing so,

■ the channel’s IEI pin is high,

■ its TD IE and MIE bits are 1, and

■ its TD IUS and all higher-priority IUS bits are 0.

As with all USC interrupts, a Transmit Data interrupt service

routine must explicitly clear the Transmit Data IP and IUS

bits by writing to the Daisy Chain Control Register (DCCR)

as described later; the bits aren’t cleared by simply writing

data into the TxFIFO.

UM009402-0201

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