Add data about the UP5k CMOS driver

UP5k-Driver-Characteristics.md

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+The I-V curve of the UP5k LVCMOS driver (running at 3v3) were measured by @smunaut for both sink and source.
+  
+# Measurement setup
+
+The FPGA was connected to a Keithley 2400 SMU that was configured to either sink/source current and measure the resulting voltage.
+
+* For sink measurement, the FPGA was actively driving the output low and the SMU was setup to source sweep from 0 to 44 mA.
+* For source measurement, the FPGA was actively driving the output high and the SMU was setup to sink sweep from 0 to -50 mA.
+
+Measurements were done on 4 different pins, twice each.
+All measurements were done on the same FPGA, but just checking the short circuit current on multiple FPGAs showed little variations, so data should be fairly typical.
+
+# Results
+
+## Graphs
+
+![Sink I-V curve](https://people.osmocom.org/~tnt/stuff/ice40/up5k_driver/up5k_sink.svg)
+![Source I-V curve](https://people.osmocom.org/~tnt/stuff/ice40/up5k_driver/up5k_source.svg)
+
+## RAW data
+
+RAW CSV files are available for :
+* [Sink](https://people.osmocom.org/~tnt/stuff/ice40/up5k_driver/up5k_sink.csv)
+* [Source](https://people.osmocom.org/~tnt/stuff/ice40/up5k_driver/up5k_source.csv)