NSAM265SR Datasheet, PDF (12/58 Pages) National Semiconductor (TI) – CompactSPEECH Digital Speech Processors

2 0 Functional Description (Continued)
TL EE 12378–4
FIGURE 2-1 Recommended Power-On Reset Circuit
2 3 THE SERIAL MICROWIRE INTERFACE
The CompactSPEECH supports the MICROWIRE synchro-
nous serial communication protocol The communication
protocol used by the CompactSPEECH is an extension of
this protocol scheme The microcontroller is the protocol
master and provides the clock for the protocol The Com-
pactSPEECH supports clock rates of up to 400 kHz This
transfer rate refers to the bit transfer the actual throughput
is slower due to byte processing by the CompactSPEECH
and the microcontroller
Communication is handled in bursts of eight bits (one byte)
In each burst the CompactSPEECH is able to receive and
transmit eight bits of data After eight bits have been trans-
ferred an internal interrupt is issued for the Compact-
SPEECH to process the byte or to prepare another byte for
sending In parallel the CompactSPEECH sets (1) the
MWRDY signal to signal the microcontroller that it is busy
with the byte processing Another byte can be transferred
only when the MWRDY signal is asserted (0) by the Com-
pactSPEECH When the CompactSPEECH transmits data it
expects to receive the value 0xAA as an echo after each
transmitted byte The CompactSPEECH reports any status
change by asserting the MWRQST signal (0)
If a parameter of a CompactSPEECH command is bigger
than one byte the Most Significant Byte (MSB) should be
transmitted first by the microcontroller If a return value is
bigger than one byte the MSB is transmitted first by Com-
pactSPEECH
Note Although the CompactSPEECH does not enforce a lower limit on the
bit transfer rate the time between bytes within the same command
must not exceed 2 ms
2 3 1 Signal Description
The following signals are used for the interface protocol
Input and output are relative to the CompactSPEECH
Input Signals
MWDIN
MICROWIRE Data In Used for input only for transferring
data from the host to the CompactSPEECH
MWCLK
This signal serves as the synchronization clock during com-
munication One bit of data is transferred on every clock
cycle The input data is available on MWDIN and is latched
on the clock rising edge The transmitted data is output on
MWDOUT on the clock falling edge The signal should re-
main low when switching MWCS
MWCS
MICROWIRE Chip Select The MWCS signal is asserted (0)
to indicate that the CompactSPEECH is accessed Assert-
ing the MWCS causes the CompactSPEECH to start driving
the MWDOUT with bit 7 of the transmitted value Negating
the signal resets the transfer-bit counter of the protocol so
it can be used as a synchronization between the Compact-
SPEECH and the microcontroller
To prevent false detection of access to the Compact-
SPEECH due to spikes on the MWCLK signal use this chip
select signal and toggle the MWCLK input signal only
when the CompactSPEECH is accessed
Output Signals
MWDOUT
MICROWIRE Data Out Used for output only for transferring
data from the CompactSPEECH to the microcontroller
When the CompactSPEECH receives data it is echoed back
to the microcontroller on this signal unless the received
data is 0xAA In this case the CompactSPEECH echoes a
command’s return value
MWRDY
MICROWIRE Ready When active (0) this signal indicates
that the CompactSPEECH is ready to transfer (receive or
transmit) another byte of data This signal is deactivated (1)
by the CompactSPEECH upon each byte transfer comple-
tion It remains deactivated while the CompactSPEECH is
busy reading the byte writing the next byte or executing the
received command (after the last parameter has been re-
ceived) MWRDY is asserted by reset
For proper operation after a hardware reset this signal
should be pulled up
MWRQST
MICROWIRE Request When active (0) this signal indicates
that new status information is available MWRQST is deacti-
vated (1) after the CompactSPEECH receives a GSW (Get
Status Word) command from the microcontroller After re-
set this signal is active (0) to indicate that a reset occurred
MWRQST unlike all the signals of the communication pro-
tocol is an asynchronous line that is controlled by the Com-
pactSPEECH firmware
2 3 2 Signal Use in the Interface Protocol
After reset the MWRQST signal is activated (0) and the
MWRDY signal is activated (0)
The MWRQST signal is activated to indicate that a reset
occurred The EV RESET bit in the status register is used
to indicate a reset condition
The GSW command should be issued after reset to verify
that the EV RESET event occurred and to deactivate the
MWRQST signal
While the MWCS signal is active (0) the CompactSPEECH
reads data from MWDIN on every rising edge of MWCLK
CompactSPEECH also writes every bit back to MWDOUT
This bit is either the same bit which was read from MWDIN
(in this case it is written back as a synchronization echo
mechanism after some propagation delay) or it is a bit of a
value the CompactSPEECH transmits to the microcontroller
(in this case it is written on every falling edge of the clock)
When a command has more than one parameter return-val-
ue the parameters return-values are transmitted in the or-
der of appearance If a parameter return-value is more than
one byte long the bytes are transmitted from the most sig-
nificant to the least significant one
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