README: detail Scale widget precision limit.

DRIVERS.md

@@ -444,15 +444,24 @@ On Adafruit displays, combinations that don't produce mirror images are:
 
 #### Use with uasyncio
 
-Running the SPI bus at 60MHz a refresh blocks for 83ms. If this is acceptable,
+Running the SPI bus at 60MHz a refresh blocks for 83ms (tested on a Pi Pico at
-no special precautions are required. This period may be unacceptable for some
+standard clock frequency). If the blocking period is acceptable, no special
-`uasyncio` applications. Some may use lower SPI baudrates either for electrical
+precautions are required. This period may be unacceptable for some `uasyncio`
-reasons or where the host cannot support high speeds.
+applications. Some may use lower SPI baudrates either for electrical reasons or
+where the host cannot support high speeds, and some platforms may run Python
+code at a different speed.
 
 The driver provides an asynchronous `do_refresh(split=4)` method. If this is
 run the display will be refreshed, but will periodically yield to the scheduler
 enabling other tasks to run. This is documented [here](./ASYNC.md).
 
+The amount of data for SPI transfer for a 240x240 display is  
+240x240x16 = 921.6K bits  
+At a 60MHz baudrate this equates to  
+240x240x16/6e7=15.36ms  
+This suggests that about 80% of the latency results from the Python code. An
+option may be to overclock.
+
 ###### [Contents](./DRIVERS.md#contents)
 
 # 4. Drivers for sharp displays

README.md

@@ -752,7 +752,7 @@ Method:
  * `value=None` Set or get the current value. Always returns the current value.
  A passed `float` is constrained to the range -1.0 <= V <= 1.0 and becomes the
  `Scale`'s current value. The `Scale` is updated. Passing `None` enables
- reading the current value.
+ reading the current value, but see note below on precision.
 
 ### Callback legendcb
 
@@ -796,6 +796,12 @@ between consecutive large ticks and legends is divided by 10. This means that
 the `tickcb` callback must return a string having an additional significant
 digit. If this is not done, consecutive legends will have the same value.
 
+### Precision
+
+For performance reasons the control stores values as integers. This means that
+if you set `value` and subsequently retrieve it, there may be some loss of
+precision. Each visible division on the control represents 10 integer units.
+
 ###### [Contents](./README.md#contents)
 
 ## 3.7 Class Textbox