A unique circuit design using  conventional MOS technology 

has been  found to yield  a  sequential read-only analog 

memory  which  can  generate the x- and y-axis deflection 

voltages for  the stroke-by-stroke  synthesis of 

alphanumeric  characters

Cheng, Edmund K-I

Mead, Carver A.

Caltech Authors - Main

SESSION 11: Optoelectronics 
WAM 2.5: Single-Chip Cursive Character Generator 
Edmund K-l Cheng and Carver A. Mead 
California Institute of Technology 
Pasadena, CA 
A UNIQUE CIRCUIT  design using conventional MOS technol- 
ogy has been  found to yield  a  sequential read-only analog 
memory  which  can  generate the x- and y-axis deflection voltages 
for  the stroke-by-stroke  synthesis of alphanumeric  characters. 
Each  character is synthesized from  48 strokes. To generate 
the Ax and Ay voltages for  each  stroke, consider the capacitive 
voltage divider with a voltage step  input  in Figure l(a). If 
C % C1, the  output voltage is approximately  proportional to C1. 
If more  capacitors C1, C2, C3, . . . are connected to C, and are 
pulsed with a  sequence of uniformly-spaced voltage steps VI, 
V2, V3, . . . respectively - ( b )  in  Figure 1 -- C acts as  a current 
integrator and  the  output voltage is a staircase with step sizes 
proportional  to C1, C2, C3, . . . The staircase generated by this 
method is monotonic,  but if two sets of this  capacitive network 
are used, and  their  outputs are connected  to a  differential  am- 
plifier, then  the  output voltage can be  an  arbitrary discrete time 
function which can have both positive and negative slopes. 
The principle  illustrated by Figure 1 has been  used  as the 
basis for a MOS design shown schem_atic_ally-in Figure 2. Here 
the  capacitors C1, C2, C3, . . . and C1,  C2,  C3, . . . are realized by 
&e gate-to-channel  capacitance of MOS transistors, while C and 
C are off-chip capacitors (-50  pF).  The  Ax,  Ay  and z voltages 
are due  to charges induced  in  the channel of the  program tran- 
sistors by voltage steps  at their gates. The  amount of induced 
charge is governed by  the  gate area,  which is programmable. 
Since Ci and Ci are both  present,  only  their  difference is avail- 
able  as output voltage. This cancels out such uncontrollables as 
variations in  the  photo-etching process and  other edge-effects. 
The z capacitors are  fed  into  current amplifiers,  which triggers 
a S-R flip-flop. 
voltage steps to appear on  the drive lines which are made of gate 
material. When the CHARGE/START line is in  the “1” state, 
all the drive  lines are precharged close to VDD. When it goes to  
the “0” state, all the drive lines are left floating except  the first, 
which gets discharged. This, in  turn, causes the  second to dis- 
charge, and so on. The  time interval  between steps is the  sum of 
the propagation times of the line and gates, about 50  - 100  ns. 
The character generator chip is shown in block  diagram form 
in Figure 4. The self-contained 125 x 140 mils 16-pin MOS chip 
accepts 7-bit ASCII code, and  contains 32 characters. Each 
@-stroke character can be generated in under 5 ps. Additional 
groups of 32 characters  can be  added by merely paralleling addi- 
tional chips. Character  encoding on the  chip  is accomplished in 
one mask at  the  diffusion  step,  and a straightforward mask gen- 
Figure 3 shows the MOS circuitry to generate  a sequence of 
eration  procedure  for programming new fonts has been 
developed. 
Characters formed  by  an N-channel chip operating  on a 
single 5V supply  are  shown in Figure 5. A photograph of the 
chip is shown  in Figure 6. 
C- 7‘ 
“,--I I 
C- 
(b )  1 - 
FIGURE I-Capacitive voltage divider is shown in (a); a 
simplified circuit for generating  programmable  staircase is in 
(b). 
c l  r - -  Cr. .  
FIGURE 2-Schematic for the generation of x and z voltages. 
FIGURE 5-CRT picture of the  character set. 
+CHARGE [START 
FIGURE  3-Charge  and  discharge circuit for the drive lines. 
MULTIPLEXER 
1 , 
DECODER 
FIGURE 4-Block diagram of the character  generator chip. 
FIGURE  6-Photograph of the character  generator  chip. 


Single-Chip Cursive Character Generator

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Single-Chip Cursive Character Generator

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