Sharp SM510 Innards

This (right) is a block diagram of the SM510 4 Bit Microcontroller made by Sharp.

Almost all of these microcontrollers are very very similar of this era, they are probably based around the TMS1x00 series' design.

There's some ROM, some RAM which is accessed via an index register pair (BM:BL on this processor - it's a pair because you need more than 4 bits usually !), and some input and output ports.

Most of the connections here are to the LCD the processor is driving (a1-a16,b1-b16,H1-H4). a1-a16 and b1-b16 are the segments, and H1-H4 are common selection lines, allowing 32 x 4 segments in total , or 128 segments.

Other than that there are a few oddities. The 'W' register on the left is a Serial in Parallel out shift register written to by two instructions - write one/zero and shift  (WS and WR), these output on the S-Lines on the left.

There is a divider which can be tested (the DIV(15) bottom middle) which clocks at the Osc rate which is 32,768Hz, which as any good assembler programmer knows is 2^15Hz. Divide this by two 15 times and you get one Hz, which is ideal for a Watch or Clock device which is what this chip is for, one pulse a second.

R1 and R2 provide a 4,096 Hz tone which can be then modulated under program control. The later versions of this (see the SM511 datasheet) have automatic melody generation and even later ones have automatic sound effect generation. But on the SM510 you have to modulate it by hand. Oh joy.

ACL is the reset, K1-K4 are inputs that can be read into the Accumulator, and B and BA are inputs that can be tested individually by an instruction. A lot of these early microcontrollers have instructions that effect I/O directly - this one has (as an example) ATR which copies the 2 LSBs of the Accumulator into R1 and R2. Modern MCUs almost all have memory mapped I/O so instead of having WS and WR you'd have a register to write to to update the W register (and therefore S1-S8)

It's quite slow ; it clocks at 16,384Hz normally, a 61us instruction cycle which is even slower than a TMS1000 (50,000 Hz).  But if you are developing a watch all you need to do is wake the processor up every second (this MCU has a sleep mode), update the LCD display and go to sleep again.

However against this is the fact that you don't need to refresh the display. The display RAM (top right) which is between $60 and $80 is directly mapped onto the an,bn,hn lines, so to light an LCD segment all you have to do is set a memory bit, which is a single instruction. The MCU does all the work for you.

(This is probably partly responsible for the slower speed. The MCU will have to do 2 things at once - drive the LCD and run the Processor code, and dual ported Memory is expensive. It's more likely that the RAM access is locked for one or the other)

Having done some messing with the MB Microvision, this is much easier. Coding for the Microvision is a bit like coding for the 2600, in that you have to generate the display and refresh it at about 20 or so fps, it is not fun to put it mildly if you have the TMS instruction set which is very basic. With a Microvision (a 16x16 grid) you write 16 bits of row data, then 16 bits of column data, any pixel which is on in both the row and column lights up (actually it goes dark :) ) so this means usually you write a row at a time, with one column bit set, then you do the next one, round and round you go for ever.