HOST INTERFACE (HI)
MOTOROLA
PORT B
5 - 31
program the RREQ bit to assert the external HREQ pin when RXDF is set. This informs
the host processor or DMA controller that the receive byte registers are full. These regis-
ters may be read in any order to transfer 8-, 16-, or 24-bit data. However, reading RXL
clears the receive data full RXDF bit. Because reading RXL clears the RXDF status bit, it
is normally the last register read during a 16- or 24-bit data transfer. Reset does not affect
RXH, RXM, or RXL.
5.3.3.7
Transmit Byte Registers (TXH, TXM, TXL)
The transmit byte registers are viewed as three 8-bit write-only registers by the host pro-
cessor. These registers are called transmit high (TXH), transmit middle (TXM), and
transmit low (TXL). These three registers send data to the high byte, middle byte and low
byte, respectively, of the HRX register and are selected by three external host address
inputs (HA2, HA1, and HA0) during a host processor write operation. Data may be written
into the transmit byte registers when the transmit data register empty (TXDE) bit is set.
The host processor may program the TREQ bit to assert the external HREQ pin when
TXDE is set. This informs the host processor or DMA controller that the transmit byte reg-
isters are empty. These registers may be written in any order to transfer 8-, 16-, or 24-bit
data. However, writing TXL clears the TXDE bit. Because writing the TXL register clears
the TXDE status bit, TXL is normally the last register written during a 16- or 24-bit data
transfer. The transmit byte registers are transferred as 24-bit data to the HRX register
when both TXDE and the HRDF bit are cleared. This transfer operation sets TXDE and
HRDF. Reset does not affect TXH, TXM, or TXL.
5.3.3.8
Registers After Reset
Table 5-5 shows the result of four kinds of reset on bits in each of the HI registers seen
by the host processor. The hardware reset is caused by asserting the RESET pin; the
software reset is caused by executing the RESET instruction; the individual reset is
caused by clearing the PBC register bit 0; and the stop reset is caused by executing the
STOP instruction.
5.3.4
Host Interface Pins
The 15 HI pins are described here for convenience. Additional information, including tim-
ing, is given in the DSP56002 Technical Data Sheet (DSP56002/D).
5.3.4.1
Host Data Bus(H0-H7)
This bidirectional data bus transfers data between the host processor and the DSP56002.
It acts as an input unless HEN is asserted and HR/W is high, making H0–H7 become out-
puts and allowing the host processor to read DSP56002 data. It is high impedance when
HEN is deasserted. H0–H7 can be programmed as general-purpose I/O pins (PB0–PB7)
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For More Information On This Product,
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