View source for SIO1 Transmit Start Timing

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---
title: SIO1 Transmit Start Timing
type: test
summary: Measures when SIO1 reports TX ready and TX idle after a byte is written at baud reload values 1 to 8.
category: Hardware tests
info: Machine = SCPH-5501
info: Program = sio1probe3
---

## Overview

The program sets [SIO1](/hardware/sio/) to 8 data bits, no parity and 1 stop bit at baud factor 1, and tries each reload value from 1 to 8. Code built in RAM and run from a warm instruction cache resets root counter 2, writes SIO_BAUD (which restarts the baud timer), runs j NOPs, writes 55h to SIO_DATA, runs k NOPs and reads SIO_STAT and root counter 2. The count k rises from 0 until STAT bit 2 (TX idle) is set. Interrupts are off and nothing is connected to the port.[^out]

Each row covers one reload and one j from 0 to 3. It gives the k at which STAT bit 0 (TX ready) and bit 2 (TX idle) change and the counter value at that k in system clock cycles.[^out]

## Results

Bit 0 (TX ready) was clear at k = 0 in every row, read 8 or 9 cycles after the counter reset. It was set again 9 to 16 cycles after the reset.[^out]

Bit 2 (TX idle) was set at a time that depends only on the reload value. Each extra NOP of j moved the k at which it was first seen down by one, so the time from the BAUD write stayed the same.[^out]

Table: Counter value when SIO1 reports TX idle

| Reload | TX idle at (cycles) |
| --- | --- |
| 1 | 19 to 20 |
| 2 | 30 |
| 3 | 42 |
| 4 | 50 to 54 |
| 5 | 61 to 66 |
| 6 | 72 |
| 7 | 83 |
| 8 | 94 |

From reload 2 to reload 8 each step of the reload added 8 to 12 cycles, and 11 cycles in most steps.[^out]

## Runs

### 9 October 2026 link-sio1probe3

Raw output: [00SIO3.txt](/tests/raw/20261009T0000Z-5501a-link-sio1probe3/sio1probe3/00SIO3.txt)

```
===== 00 SIO3 SIO1 TX start phase at x1: STAT bits 0 and 2 (as 1, 4, 5) at k NOPs after the DATA write | BIOS 19951204
k:bits@timer2 (bits: 1 TX ready, 4 TX idle); timer 2 from its reset before the BAUD write
  reload 1 j 0: 0:0@8 4:1@9 14:5@19 | idle from k 14
  reload 1 j 1: 0:0@8 3:1@9 13:5@19 | idle from k 13
  reload 1 j 2: 0:0@8 2:1@9 12:5@19 | idle from k 12
  reload 1 j 3: 0:0@9 2:1@10 12:5@20 | idle from k 12
  reload 2 j 0: 0:0@8 5:1@10 25:5@30 | idle from k 25
  reload 2 j 1: 0:0@8 4:1@10 24:5@30 | idle from k 24
  reload 2 j 2: 0:0@8 3:1@10 23:5@30 | idle from k 23
  reload 2 j 3: 0:0@9 2:1@10 22:5@30 | idle from k 22
  reload 3 j 0: 0:0@8 7:1@12 37:5@42 | idle from k 37
  reload 3 j 1: 0:0@8 6:1@12 36:5@42 | idle from k 36
  reload 3 j 2: 0:0@8 5:1@12 35:5@42 | idle from k 35
  reload 3 j 3: 0:0@9 4:1@12 34:5@42 | idle from k 34
  reload 4 j 0: 0:0@8 5:1@10 45:5@50 | idle from k 45
  reload 4 j 1: 0:0@8 4:1@10 44:5@50 | idle from k 44
  reload 4 j 2: 0:0@8 3:1@10 43:5@50 | idle from k 43
  reload 4 j 3: 0:0@9 6:1@14 46:5@54 | idle from k 46
  reload 5 j 0: 0:0@8 6:1@11 56:5@61 | idle from k 56
  reload 5 j 1: 0:0@8 5:1@11 55:5@61 | idle from k 55
  reload 5 j 2: 0:0@8 4:1@11 54:5@61 | idle from k 54
  reload 5 j 3: 0:0@9 8:1@16 58:5@66 | idle from k 58
  reload 6 j 0: 0:0@8 7:1@12 67:5@72 | idle from k 67
  reload 6 j 1: 0:0@8 6:1@12 66:5@72 | idle from k 66
  reload 6 j 2: 0:0@8 5:1@12 65:5@72 | idle from k 65
  reload 6 j 3: 0:0@9 4:1@12 64:5@72 | idle from k 64
  reload 7 j 0: 0:0@8 8:1@13 78:5@83 | idle from k 78
  reload 7 j 1: 0:0@8 7:1@13 77:5@83 | idle from k 77
  reload 7 j 2: 0:0@8 6:1@13 76:5@83 | idle from k 76
  reload 7 j 3: 0:0@9 5:1@13 75:5@83 | idle from k 75
  reload 8 j 0: 0:0@8 9:1@14 89:5@94 | idle from k 89
  reload 8 j 1: 0:0@8 8:1@14 88:5@94 | idle from k 88
  reload 8 j 2: 0:0@8 7:1@14 87:5@94 | idle from k 87
  reload 8 j 3: 0:0@9 6:1@14 86:5@94 | idle from k 86
```

## References

[^out]: Hardware test run 20261009T0000Z-5501a-link-sio1probe3: [sio1probe3/00SIO3.txt](/tests/raw/20261009T0000Z-5501a-link-sio1probe3/sio1probe3/00SIO3.txt).