Target Design Considerations
	Target Design Considerations

Since the SerialICE Kernel requires minimal ROM and RAM, the main issue is how the
SerialICE Controller will communicate with the Target.

There are three basic choices:

  »  An On-chip SerialICE Port
  »  An On-board Commercial SIO
  »  An Off-board Commercial SIO

Whether the the SIO port is implemented onchip or offchip, it must be
mapped into the target CPU's address space and connected to one of
the processor's interrupt request inputs. The base address may be
chosen to suit individual system requirements.

If there are not enough unused interrupt request inputs to devote
one solely to the SIO, it is possible to implement a software
sharing scheme that would permit the application and the SerialICE Kernel
to use the same interrupt request input.

On-chip SerialICE Port

The on-chip SerialICE Port offers the highest potential performance at the lowest
cost of board real-estate. In most cases the small die area required will
be insignificant. This is the preferred solution for new designs.

The silicon area needed for such a simple function is very small (well
below 1mm2), and, since the interface is serial, the pin count overhead
is also very low. 

The reference onchip SIO design can support raw baud rates up to
1.25Mbit/sec; allowing for communications overhead, the maximum data
transfer rate during program downloads is around 50K Bytes/sec. This
makes it possible to work with large download files. 

For such an onchip implementation, it will usually be most convenient
to derive the clock rate for the SIO from the CPU clock, through a
frequency divider. For systems in which the clock rate must be
flexible, it may be supplied from an offchip source; however, this
requires an additional package I/O pin.

The fast serial connection between the SerialICE Manager and the debug
target must provide good signal integrity. In general, long cable
runs should be avoided, since transmission line effects may severely
affect the operation of the link at high clock rates.

Recommended Configuration

  »  An 8-bit header within 2" of the SIO pins (pinout shown below). If
this close proximity is not possible, then 47R series terminating resistors
should be placed on all outputs from the target.
This is important, since it affects the integrity of the high-speed
signals running between the target system and the Pod. If the
board traces running between the connector and the SIO device are too
long, they will cause transmission-line effects which could make the
interface's operation unreliable.
The recommended connector presents 2 rows each of 4 pins, on the
industry standard 2.5 mm (0.1") pitch IDC connector.  These are listed
below.

Polarizing PinVCC
GroundSerial Input (to target)
GroundSerial Output (from target)
GroundSerial Clock (to target)

  »  A cable pod containing RS422 transceivers.
Examples include the DS3695A devices offered
by National Semiconductor Corp. These may be powered by the VCC feed from
the 8-pin header. The pod should be connected to the target via
8-way ribbon cable of no greater than 6" in length.

  »  Good quality twisted pair data cable should be used for the
connection between the pod and the SerialICE Controller.

On-board Commercial SIO

If it is not possible to integrate the SIO on-chip, a commercial SIO
may be mounted on the Target. There is of course a penalty in board
real-estate for this solution. But it may be used if board area is not 
critical.

Off-board Commercial SIO

In this solution a small daughter card containing a commercial SIO is
connected to the Target via a small number of pins. This removes most
of the  board real-estate penalty, as it requires only the space needed
for a small (approximately 25 pin) connector to connect an 8-bit
peripheral to the CPU bus.

In this way, standard production units carry only the minimal overhead
of a small connector, yet can be used as development platforms at any
time simply by plugging in the SIO card. In extremely cost sensitive
applications, the debug connector position can simply be left open -
reducing the overhead to a small amount of board area.

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