Project Euler is a popular site among enthusiasts of mathematics and programming problems. A very common template for its problems goes something like:
Let F(n) be some function expressed in words. You are given that F(100) is some relatively small number. Find F(10^7).
Often the best approach to such problems is to write a brute-force solution which gets the correct answer for the given data to verify that you understand the problem. Then examine the order of growth of this brute-force solution, say O(n^2), to determine what the best approach is, maybe O(n*log(n)), given that Project Euler says that all of its problems are solvable in under a minute.
This approach works well for software because it's easy to code up a brute-force solution and play around with it. It's not so useful for hardware for a similar reason: it isn't so easy to build hardware prototypes.
Nevertheless the MicroUK101 described last time was an attempt at a test-bed for those chips in the UK101 which weren't testable with Arduinos: the 6502 CPU and 6850 ACIA. A working MicroUK101 would verify that those chips were working too.
Unfortunately when assembled, it didn't work! Now what? Last time I tried to strip it back to the bare essentials required to test a 6502: a power supply, reset circuit and CPU clock. When this didn't work as expected, I had assumed that the 6502 had gone bad. This assumption was incorrect.
Time passed and recently I bought an OWON vds1022i USB oscilloscope and decided that an easy first task would be to fix the MicroUK101. Very quickly I discovered that the clock circuit wasn't oscillating at 4MHz as it ought to: according to the scope it was oscillating at 40MHz!
The simpler problem required another simpler problem: build the clock circuit on a bread-board and try to make it work there before making any changes to the PCB version. The problem turned out to be the 680R resistors in the clock oscillator: these did not provide the stability required, at least with the choice of crystal and 74LS04 I had on hand. Upping the resistors to 4k7 fixed the issue and with it the MicroUK101.
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| MicroUK101 by Grant Searle and Steve Gray |
Incidentally if I were making changes to Steve Gray's PCB, I would drop the power switch and RS-232 interface, using only an FTDI adaptor to power the board and talk to it.
