Psion Organiser II Top Slot (D)  Interfacing.

Introduction

Now we have learned the basic structure of the top slot and its relation to the other Organiser slots, we can attempt to design an interface for it.

In this part of the paper we shall discuss the rules and restrictions that must be observed, and the conventions used when interfacing to the Top slot. We shall also discuss external wake up and power supplies in a deeper fashion, and lastly we shall design a sample hardware interface and assembler program to drive it.

As previously explained the top slot shares common data lines, control lines (except slot select), and supply voltage with slots B and C. (See fig 2,1)

Rules & Restrictions.

No voltages should be present on the top slot when the slots are turned off (i.e. SVCC is at 0 volts) as permanent damage can occur.

Conventions.

A set of conventions have been drawn up by Psion to aid the design of hardware and software which will access the Organiser Top slot.

Standard Control line states.

A conventional table of control line states has been defined, to help with top slot interfacing:

Changing Control Lines

To help prevent problems when changing Control lines a routine has been defined. This is as follows:

• Ensure SS3 is High.
• Change line.
• Change direction of data bus if needed.
• Drive SS3 low.
• Drive SS3 high before changing any other lines.
  

External Switch On.

The AC line which has its origin in the top slot can be used to as an external switch on, when the AC line is driven low it has the same effect as an ON/CLEAR. However software can be used to detect if the ON/CLEAR key has been pressed if a differentiation's
needs to be made.

NB. AC being driven low does not switch on the SVCC power line.

External Power Supply.

An external power supply to the Organiser should observe the following rules:

• It should Supply into the VB pin of the Top slot.
• It should be diode isolated to prevent reverse Voltage 'Leaking' into it when it is switched off.
• It must supply a greater voltage, than the battery voltage plus diode drop, and thus prevent any drain on the battery.

The information in this data sheet is accepted for use entirely at your own risk. The Psion Organiser II team will not be held responsible in any way for your use of this information.

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