16-20
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Host Side Registers
16
When the Host DMA direction is from DSP-to-Host, the contents of the selected register,
RXH or RXL, are enabled onto the Host data bus when the HACK pin is asserted.
If the Host DMA direction is from Host-to-DSP, the contents of the selected register are
enabled onto the Host data bus when the HACK pin is asserted.
If the Host DMA direction is from Host-to-DSP, the selected register is written to TXH or
TXL from the Host data bus when the HACK pin is asserted.
The size of the Host DMA word to be transferred is determined by the Host mode 0
(HM0) bit. The HI8 register selected during a Host DMA transfer is determined by a 2-bit
address counter, preloaded with the value in HM1 and HM0. The address counter
substitutes for the Host Address inputs, HA1 and HA0 during a Host DMA transfer. The
Host Address input HA2 is forced to 1 during each Host DMA transfer. The address
counter can be initialized when the INIT bit is set. After each DMA transfer, the address
counter is incriminated to the next register.
When the address counter reaches the highest register (RXL or TXL), the address counter
is not incriminated but is loaded with the value in HM1 and HM0. This allows 8- or 16-bit
data to be transferred in a circular fashion and eliminates the need for the DMA controller
to supply the Host Address HA2, HA1, and HA0 pins. For 16-bit data transfers, the DSP
interrupt rate is reduced by a factor of two from the Host Request rate.
HM1 and HM0 are cleared by DSP reset.
Note:
When operating in 8-bit HDMA mode, the HLEND affects the location (upper
or lower byte) of the 8-bit value in the DSP register (HRX/HTX). For example,
in Host-to-DSP transfers, if HLEND = 0 the transferred data is placed in the
lower half of the HRX register. If HLEND = 1 the data would appear in the
upper half of the register.
Note:
Prior to initiating an 8-bit Host-to-DSP transfer the Host should ensure the
TXH/TXL registers at address 6 is initialized to 0 because the contents of this
register are transferred with the TXL/TXH register to the HRX register.
16.9.1.4 Host Flag 1 (HF1)—Bit 4
The Host Flag 1 (HF1) bit is used as a general purpose flag for Host-to-DSP
communication. The HF1 bit can be set or cleared by the Host Processor, but cannot be
changed by the DSP core. The HF1 bit is reflected in the HSR on the DSP Side. The HF1
bit is cleared on DSP reset.