What was the non-GAAP operating profit for the first quarter of 2021 for Silicon Labs
ss launching our transformation into a pure play leader of intelligent wireless connectivity for the IoT market. We also announced organizational changes that will enhance our ability to capitalize on this exciting new opportunity in IoT. Matt Johnson, formerly Senior Vice President and General Manager of IoT has been promoted to President of Silicon Labs, where he will be leading our day-to-day business operations, as well as our product development activities. Daniel Cooley, formerly our Chief Strategy Officer, will now serve as our Chief Technology Officer, where he will continue to develop our technology roadmap and align our products and solutions to the market opportunity before us. Tyson Tuttle will continue as Chief Executive Officer to further evangelize our products and platform to the growing IoT customer base.
Please note that the divestiture will include a small ASIC product line currently classified in the IoT reporting category, which accounted for less than $3 million of revenue last year. Our comments today will focus on the consolidated operations of Silicon Labs. We expect to be in position to report on our continuing operations on an IoT only basis in the July call.
We are very pleased to announce this morning record revenue for the first quarter at $256 million, up 5% sequentially and 19% year-on-year, significantly exceeding the high-end of our initial guidance for the quarter. IoT revenue for the quarter was $158 million, above our expectations, up 7% from Q4, and 34% year-on-year. IoT Wireless led the way with 44% year-on-year growth in Q1, with strong double-digit growth across all our major wireless protocols. Infrastructure and Automotive revenue in Q1 ended at $97 million, up 2% sequentially and about flat year-on-year. Timing and Isolation both increased from Q4, offset by slight declines in other I&A product lines.
Turning to end markets, revenue from industrial, consumer and communications markets were all up in Q1. Automotive was down slightly, coming off a strong Q4 recovery. Distribution sales were 79% of total revenue for the quarter, and we ended Q1 with DSI at around 41 days, down from 47 days at the end of Q4 with strong POS to end customers, driving down the distributor inventory level.
Geographically, we saw particular strength in Q1 in APAC and Europe, with sales in the Americas down in the quarter. Total bookings were very strong again this quarter, and about 5% higher than Q4 bookings, which were already at an elevated level. Non-GAAP gross margin for the quarter ended better than expected at 59.1%, due to strength in product mix, as well as operational efficiencies with the upside in revenue. Non-GAAP operating expenses in the quarter were $103 million, up about 6% from Q4, primarily due to increased payroll taxes, variable compensation, and medical claims expenses. R&D expenses were $61 million or 24% of revenue in the quarter, and SG&A expenses were $42 million or 17% of revenue.
Non-GAAP operating profit for the first quarter was $48 million or 18.7% of revenue. Our non-GAAP effective tax rate for the quarter was 10.8%. Non-GAAP earnings ended at $0.91 per share. On a GAAP basis, gross margin was 58.9%. GAAP operating expenses were $128 million. Stock compensation expense was $14 million, and amortization of intangible assets was $12 million, both in line with expectations.
Turning now to the balance sheet, we ended the quarter with cash and investments totaling $578 million. Our operating cash flow in the first quarter was $15 million, with significant cash outlays related to seasonally higher payroll taxes and increased levels of working capital. Accounts receivable was up on strong shipments in Q1, ending at $104 million. Our DSO increased slightly to 37 days. We have no known bad debt exposures related to our accounts receivable. The inventory balance grew in the quarter to $79 million, on anticipated growth in the business, with inventory turns declining to 5.3 times, which is still higher than our goal.
Though the overall supply chain in the semiconductor market remain
ss launching our transformation into a pure play leader of intelligent wireless connectivity for the IoT market. We also announced organizational changes that will enhance our ability to capitalize on this exciting new opportunity in IoT. Matt Johnson, formerly Senior Vice President and General Manager of IoT has been promoted to President of Silicon Labs, where he will be leading our day-to-day business operations, as well as our product development activities. Daniel Cooley, formerly our Chief Strategy Officer, will now serve as our Chief Technology Officer, where he will continue to develop our technology roadmap and align our products and solutions to the market opportunity before us. Tyson Tuttle will continue as Chief Executive Officer to further evangelize our products and platform to the growing IoT customer base.
Please note that the divestiture will include a small ASIC product line currently classified in the IoT reporting category, which accounted for less than $3 million of revenue last year. Our comments today will focus on the consolidated operations of Silicon Labs. We expect to be in position to report on our continuing operations on an IoT only basis in the July call.
We are very pleased to announce this morning record revenue for the first quarter at $256 million, up 5% sequentially and 19% year-on-year, significantly exceeding the high-end of our initial guidance for the quarter. IoT revenue for the quarter was $158 million, above our expectations, up 7% from Q4, and 34% year-on-year. IoT Wireless led the way with 44% year-on-year growth in Q1, with strong double-digit growth across all our major wireless protocols. Infrastructure and Automotive revenue in Q1 ended at $97 million, up 2% sequentially and about flat year-on-year. Timing and Isolation both increased from Q4, offset by slight declines in other I&A product lines.
Turning to end markets, revenue from industrial, consumer and communications markets were all up in Q1. Automotive was down slightly, coming off a strong Q4 recovery. Distribution sales were 79% of total revenue for the quarter, and we ended Q1 with DSI at around 41 days, down from 47 days at the end of Q4 with strong POS to end customers, driving down the distributor inventory level.
 Geographically, we saw particular strength in Q1 in APAC and Europe, with sales in the Americas down in the quarter. Total bookings were very strong again this quarter, and about 5% higher than Q4 bookings, which were already at an elevated level. Non-GAAP gross margin for the quarter ended better than expected at 59.1%, due to strength in product mix, as well as operational efficiencies with the upside in revenue. Non-GAAP operating expenses in the quarter were $103 million, up about 6% from Q4, primarily due to increased payroll taxes, variable compensation, and medical claims expenses. R&D expenses were $61 million or 24% of revenue in the quarter, and SG&A expenses were $42 million or 17% of revenue.
Non-GAAP operating profit for the first quarter was $48 million or 18.7% of revenue. Our non-GAAP effective tax rate for the quarter was 10.8%. Non-GAAP earnings ended at $0.91 per share. On a GAAP basis, gross margin was 58.9%. GAAP operating expenses were $128 million. Stock compensation expense was $14 million, and amortization of intangible assets was $12 million, both in line with expectations.
Turning now to the balance sheet, we ended the quarter with cash and investments totaling $578 million. Our operating cash flow in the first quarter was $15 million, with significant cash outlays related to seasonally higher payroll taxes and increased levels of working capital. Accounts receivable was up on strong shipments in Q1, ending at $104 million. Our DSO increased slightly to 37 days. We have no known bad debt exposures related to our accounts receivable. The inventory balance grew in the quarter to $79 million, on anticipated growth in the business, with inventory turns declining to 5.3 times, which is still higher than our goal.
Though the overall supply chain in the semiconductor market remain